GTM roll cage modifications and race car build log

[crash]

I'm just going to think out loud here for a minute, so please, if my train of thought is incorrect, then point it out...

2800 LBS car

Roughly 40/60 split front to rear weight distribution.

I'll get conservative and estimate 1400 LBS in rear.

Two tires, so weight is divided by two...700 LBS.

Data shows over 2 Gs of lateral loading.

Therefore back up towards the 1400 LBS in dynamic loading. Probably higher as when side loads increase the inner tire does less work(usually) than the outer tire.

Granted that upper and lower arms take most of this load. Still, one would be "hard pressed" to develop a test rig that will apply 1400 LBS of load to the rear suspension.

Now consider the fact that we used to be able to see it move by using our legs to push on the wheel...

Conclusion? The rear suspension is going to move and flex in a dynamic way under the loads we are now producing.

Also I believe it is imperative to look at the rear suspension system dynamically and to maybe start with something closer to 1/8 toe in, but also to see where it goes when loaded. A 1/16 toe in is pretty light, especially considering that would be 1/32 per side.

In general, the cars I have raced have been very sensitive to toe adjustments. The GTM appears to be no different, and a wider tire just exaggerates the situation because of the distance between the hub and the outer edge of the bigger tire. Moving the upright away from the tire centerline may even exacerbate this issue.

[kabacj]

I'm just going to think out loud here for a minute, so please, if my train of thought is incorrect, then point it out...

2800 LBS car

Roughly 40/60 split front to rear weight distribution.

I'll get conservative and estimate 1400 LBS in rear.

Two tires, so weight is divided by two...700 LBS.

Data shows over 2 Gs of lateral loading.

Therefore back up towards the 1400 LBS in dynamic loading. Probably higher as when side loads increase the inner tire does less work(usually) than the outer tire.

Granted that upper and lower arms take most of this load. Still, one would be "hard pressed" to develop a test rig that will apply 1400 LBS of load to the rear suspension.

Now consider the fact that we used to be able to see it move by using our legs to push on the wheel...

Conclusion? The rear suspension is going to move and flex in a dynamic way under the loads we are now producing.

Also I believe it is imperative to look at the rear suspension system dynamically and to maybe start with something closer to 1/8 toe in, but also to see where it goes when loaded. A 1/16 toe in is pretty light, especially considering that would be 1/32 per side.

In general, the cars I have raced have been very sensitive to toe adjustments. The GTM appears to be no different, and a wider tire just exaggerates the situation because of the distance between the hub and the outer edge of the bigger tire. Moving the upright away from the tire centerline may even exacerbate this issue.

Agree good points.

My idea for side load was to use a bottle jack against the concrete wall and one side of the rim and a dial indicator on the on the edge. All this while the tested wheel was on a turn plate. I can use my friends trailer tongue weight scale to measure the load.

I'm sure I could generate tons of force. Actually break stuff if I'm not careful. The only goal of the exercise was to see if setup A is more stiff then setup B.

But yes it's a dynamic system with lots of variables and a side load test would only be information not a conclusion.

You think adding more rear toe would help? I'm game for that.

I'm thinking stiffer is better to try to limit the dynamic nature of the toe link setup. If it's at all possible to measure. That would be helpful.

My goal is to have a bag of tricks to pull from next weekend at the track.

I'll make the rear as stiff as I can without major surgery. Then I would love to have some tuning tricks to try.

This discussion has been very helpful.

John

[crash]

Agree good points.

My idea for side load was to use a bottle jack against the concrete wall and one side of the rim and a dial indicator on the on the edge. All this while the tested wheel was on a turn plate. I can use my friends trailer tongue weight scale to measure the load.

I'm sure I could generate tons of force. Actually break stuff if I'm not careful.

I'm sure you will find the limits of tire adhesion to the floor pretty quickly!

[tachman]

Mike and John,
My concern on the deflection issue was chiefly toe changes (and more offset force moments trying to yaw the car) from accel and decel, not as much from lateral load, that could occur with increased track and scrub radius. Side load would cause toe change (and camber loss) more from aframe bushing deflection since the load is from the tire contact patch and also from bump steer with the vertical movement from roll.

To test for toe change from accel or decel the force could be applied from front or rear, rather than from the side, to the outside of a wheel or test fixture to generate a moment around the ball joint axis. (Ratchet strap to wheel?) This would show if the toe link or the bushings are causing the issue. We also found hub bearing deflection was an issue before going to the Z06 hubs. BTW I repeated the sitting on the floor leg test and noted much less movement than in the test done in '09 before the upgrades.

A test putting side load ahead or behind the hub might better test contact with another object.

JimH

[LS MAN]

The weird thing is how this wiggle shows up. After a left hand turn and a full throttle 600 yard short shoot in a straight line. When I lift that's when it wiggles hard right. At worst I move over half a car width. I figure I keep the rear loaded through the corner and down the strait then when I lift it does it's weird thing.


John

John, this may not be a rear end problem. Most rear steer instability caused by rear toe, or rear bump steer problems are felt under throttle load, not off throttle.
This may be a problem in the front end. With the throttle applied, the torque reaction of the rear suspension geometry is lifting the front end, when you lift off the throttle, weight is transferred back to the front end. Sounds like your car may be reacting to this forward weight transfer.
It may be possible that there is some static cross weight, more loading on the RF than the LF, (or LF loaded more the RF with toe-in). also could be some preload in the front swaybar causing this.
Have you scaled the car to check your corner weights? make sure you disconnect the sway bar when checking the corner weights. Also a difference in the compression settings of the front shocks, could be a weak shock, (check your gas pressure in shocks too).
Could be a front toe-in problem also, are you running front toe-in or toe-out?
Hope you get her figured out, keep up the good work, very impressive.

[kabacj]

Mike and John,
My concern on the deflection issue was chiefly toe changes (and more offset force moments trying to yaw the car) from accel and decel, not as much from lateral load, that could occur with increased track and scrub radius. Side load would cause toe change (and camber loss) more from aframe bushing deflection since the load is from the tire contact patch and also from bump steer with the vertical movement from roll.

To test for toe change from accel or decel the force could be applied from front or rear, rather than from the side, to the outside of a wheel or test fixture to generate a moment around the ball joint axis. (Ratchet strap to wheel?) This would show if the toe link or the bushings are causing the issue. We also found hub bearing deflection was an issue before going to the Z06 hubs. BTW I repeated the sitting on the floor leg test and noted much less movement than in the test done in '09 before the upgrades.

A test putting side load ahead or behind the hub might better test contact with another object.

JimH

Thats also a good point Jim. Ill test as many things as I can to see if I can identify if its the toe link or the bushings that are causing the issue.

I also got the Z06 hubs from the get go in order to have the least bearing run out possible.

and ratchet straps are another good way to load up the wheel and look for deflection of the wheel or suspension itself.

[kabacj]

John, this may not be a rear end problem. Most rear steer instability caused by rear toe, or rear bump steer problems are felt under throttle load, not off throttle.
This may be a problem in the front end. With the throttle applied, the torque reaction of the rear suspension geometry is lifting the front end, when you lift off the throttle, weight is transferred back to the front end. Sounds like your car may be reacting to this forward weight transfer.
It may be possible that there is some static cross weight, more loading on the RF than the LF, (or LF loaded more the RF with toe-in). also could be some preload in the front swaybar causing this.
Have you scaled the car to check your corner weights? make sure you disconnect the sway bar when checking the corner weights. Also a difference in the compression settings of the front shocks, could be a weak shock, (check your gas pressure in shocks too).
Could be a front toe-in problem also, are you running front toe-in or toe-out?
Hope you get her figured out, keep up the good work, very impressive.

Hey Ted,

Thanks for the feedback. I tried swapping the rear shocks side to side but ill also try the fronts. I verified that both compression and rebound are the same. I would agree with you that the weight transfer could be a problem, but I also have little wiggles on the exit of the corner as the rear end unloads. It feels just like if you hit a slick spot then it catches and its fine again. This problem could be the front, but I dont get any feedback from the steering wheel and the car rotates from the rear like a slide but does not turn from the front like you would expect if the front was the issue.

Im running toe in on the front 1/16th.

At least I was last time I checked it. After I put the rear back together a full alignment and corner weight will be done.

Your suggestion is a good one. Ill add the front check out to my list of things to verify and also to troubleshoot at the track.

I know the car will do well if I can just run it full out. Thanks for the suggestion.

John

[fastthings]

I have felt that wiggle in a Vette, I didn't like it. I can't wait till you figure this out.

[Fraser D]

John,

My setup is stock FF5/Corvette with the exception of poly bushes on the control arms only and aftermarket brake disks.
last Friday I had the car on the local CAT scales for registration purposes and with full stock corvette tanks weighed in at:
Total 2540lbs
Front 960lbs
Rear 1580lbs
Ride height is set at 4 1/2"
Tires are 275 front and 335 rear, (all are 888’s)
I don't have my current alignment figures with me.
When I had my car at the track I was experiencing very similar issues that you are describing with the car becoming unsettled when getting off the gas and on the brakes at over 140mph so I am very interested in this discussion.
I have an appointment with the alignment shop this weekend and a track day next Wednesday and am willing to work with you on some of the suggested setups to see what works.

[LS MAN]

Hey Ted,

Thanks for the feedback. I tried swapping the rear shocks side to side but ill also try the fronts. I verified that both compression and rebound are the same. I would agree with you that the weight transfer could be a problem, but I also have little wiggles on the exit of the corner as the rear end unloads. It feels just like if you hit a slick spot then it catches and its fine again. This problem could be the front, but I dont get any feedback from the steering wheel and the car rotates from the rear like a slide but does not turn from the front like you would expect if the front was the issue.

Im running toe in on the front 1/16th.

At least I was last time I checked it. After I put the rear back together a full alignment and corner weight will be done.

Your suggestion is a good one. Ill add the front check out to my list of things to verify and also to troubleshoot at the track.

I know the car will do well if I can just run it full out. Thanks for the suggestion.

John

John, how are you setting the rear toe-in? It is critical to understand that rear toe settings are each wheel set to the chassis centerline, not just the total toe in. The fact that the car pulls only to the right may indicate that the rear toe vector is not pointing to the center of the car. We string each rear wheel to the center of each front hub, so that if you want 1/8" rear toe in, each side must have 1/16", and the measurement of the string line at each front hub must be the same, so that the rear end is not dog-tracking.
Also try 1/16" - 3/32" toe-out in the front, surprising how much more stable it is.. Both the PDG GTM & our SLC race with front toe out.
Hope this helps,

[kabacj]

John, how are you setting the rear toe-in? It is critical to understand that rear toe settings are each wheel set to the chassis centerline, not just the total toe in. The fact that the car pulls only to the right may indicate that the rear toe vector is not pointing to the center of the car. We string each rear wheel to the center of each front hub, so that if you want 1/8" rear toe in, each side must have 1/16", and the measurement of the string line at each front hub must be the same, so that the rear end is not dog-tracking.
Also try 1/16" - 3/32" toe-out in the front, surprising how much more stable it is.. Both the PDG GTM & our SLC race with front toe out.
Hope this helps,

Thanks for the info Ted. I spent a few hours measuring the centerline and marking it on the car for later use. Then stringing the car and setting the wheels to dead zero verifying that measurement and then setting toe from there.

I'll try toe out. I'm actually happy with stability but maybe I don't know what I'm missing


Thanks again.

John

[kabacj]

John,

My setup is stock FF5/Corvette with the exception of poly bushes on the control arms only and aftermarket brake disks.
last Friday I had the car on the local CAT scales for registration purposes and with full stock corvette tanks weighed in at:
Total 2540lbs
Front 960lbs
Rear 1580lbs
Ride height is set at 4 1/2"
Tires are 275 front and 335 rear, (all are 888’s)
I don't have my current alignment figures with me.
When I had my car at the track I was experiencing very similar issues that you are describing with the car becoming unsettled when getting off the gas and on the brakes at over 140mph so I am very interested in this discussion.
I have an appointment with the alignment shop this weekend and a track day next Wednesday and am willing to work with you on some of the suggested setups to see what works.

Hey Dave,

Sounds like fun. What track are you planning on visiting?

As you can see from the experts the GTM is very sensitive to alignment.

here is the setup I used last year that worked well.


Left Front Right front
caster -3 caster -3
camber -2.8 camber -2.7
toe -1/16 toe -1/16




Left Rear Right Rear
camber -1 camber -1
toe -1/16 toe -1/16


for this time I am going to try toe out as Ted suggests.

I will also try a little more rear toe as Mike suggested.

I will also remove .25 degrees of camber from the rear. I found that I was getting a little too much heat and wear on the insides of the rear tires.

Also note that i did not see any ill handling until I started to really carry some G load. On my best lap last year I pulled 1.6 Gs.This past weekend I was pulling 2Gs

Im not saying that you are not having the same problem, just saying that I would first make sure the alignment is good then start chasing other things. Like I said I had a very stable car going faster then all or most all of the guys at the track last year. It was only going up against full on race cars that brought out my current issue.

You are also running a big torquey turbo motor so maybe you are showing the same toe link flex. No sure. I would go with the alignment as the first thing to concentrate on however.

You should try to get some rear suspension video if you can. Im going to rig up several camera angles before I go to the track in case I dont solve the problem.

John

[fastthings]

Hey Dave,

Sounds like fun. What track are you planning on visiting?

As you can see from the experts the GTM is very sensitive to alignment.

here is the setup I used last year that worked well.


Left Front Right front
caster -3 caster -3
camber -2.8 camber -2.7
toe -1/16 toe -1/16




Left Rear Right Rear
camber -1 camber -1
toe -1/16 toe -1/16


for this time I am going to try toe out as Ted suggests.

I will also try a little more rear toe as Mike suggested.

I will also remove .25 degrees of camber from the rear. I found that I was getting a little too much heat and wear on the insides of the rear tires.

Also note that i did not see any ill handling until I started to really carry some G load. On my best lap last year I pulled 1.6 Gs.This past weekend I was pulling 2Gs

Im not saying that you are not having the same problem, just saying that I would first make sure the alignment is good then start chasing other things. Like I said I had a very stable car going faster then all or most all of the guys at the track last year. It was only going up against full on race cars that brought out my current issue.

You are also running a big torquey turbo motor so maybe you are showing the same toe link flex. No sure. I would go with the alignment as the first thing to concentrate on however.

You should try to get some rear suspension video if you can. Im going to rig up several camera angles before I go to the track in case I dont solve the problem.

John

Can't wait to see the video. The one of the suspensoin, the one showing off your new shifter, the one when your passing cars on the track, the one pulling 2gs. LOL

[crash]

Left Rear Right Rear
camber -1 camber -1
toe -1/16 toe -1/16



I will also try a little more rear toe as Mike suggested.

John

If you are running an eighth total rear toe in, that is probably good. We were running a sixteenth total and so that was only 1/32 per side when we were having issues. Too much rear toe will also make the car unstable. It is a balancing act. I think you know this, but I would only do one change at a time so that you know what change is having what effect.

[Fraser D]

Hey Dave,

Sounds like fun. What track are you planning on visiting?

As you can see from the experts the GTM is very sensitive to alignment.

here is the setup I used last year that worked well.


Left Front Right front
caster -3 caster -3
camber -2.8 camber -2.7
toe -1/16 toe -1/16




Left Rear Right Rear
camber -1 camber -1
toe -1/16 toe -1/16


for this time I am going to try toe out as Ted suggests.

I will also try a little more rear toe as Mike suggested.

I will also remove .25 degrees of camber from the rear. I found that I was getting a little too much heat and wear on the insides of the rear tires.

Also note that i did not see any ill handling until I started to really carry some G load. On my best lap last year I pulled 1.6 Gs.This past weekend I was pulling 2Gs

Im not saying that you are not having the same problem, just saying that I would first make sure the alignment is good then start chasing other things. Like I said I had a very stable car going faster then all or most all of the guys at the track last year. It was only going up against full on race cars that brought out my current issue.

You are also running a big torquey turbo motor so maybe you are showing the same toe link flex. No sure. I would go with the alignment as the first thing to concentrate on however.

You should try to get some rear suspension video if you can. Im going to rig up several camera angles before I go to the track in case I dont solve the problem.

John

John,

Thanks for the numbers and I will see how close I can get to them but I think that I will be struggling to get 3deg caster on the fronts as I have the stock FF5 control arm pickup points at the moment.
I will be running at MSR Houston for a private event track day.
http://www.msrhouston.com/about/facility-maps
This is the same track that I ran at last year in the rain.
The goal is to run some form of data logging to help understand a bit more of what is happening in regards to my driving as well as the cars handling.
If I can make it happen I will send you the data as well.

[kabacj]

Can't wait to see the video. The one of the suspensoin, the one showing off your new shifter, the one when your passing cars on the track, the one pulling 2gs. LOL

Ok Gene, ill have cameras running. If i can sort out this handling you will get a good show.

John,

Thanks for the numbers and I will see how close I can get to them but I think that I will be struggling to get 3deg caster on the fronts as I have the stock FF5 control arm pickup points at the moment.
I will be running at MSR Houston for a private event track day.
http://www.msrhouston.com/about/facility-maps
This is the same track that I ran at last year in the rain.
The goal is to run some form of data logging to help understand a bit more of what is happening in regards to my driving as well as the cars handling.
If I can make it happen I will send you the data as well.

If you have an iphone and a good mount, harries lap timer is a great app for gathering video and lap times. It actually has much of the same data logging as much more expensive systems. It really kills the battery on your phone but beyond that its great. I usually run both harries and my stack datalogger just to have a second data source.

John

[kabacj]

After all of the great advice from folks on the forum I formulated a plan purchased some metal and went to town.

My goal was to weld the toe link mount reinforcement without taking the transaxle out.

Sure it will only take an hour to take it out and re install, but a little inconvenience is worth the hour saved right? More on that later.

The toe link mount is attached to a single diagonal frame tube.

When force is applied to the mount it tends to twist the frame tube. Since
it's steel it's pretty flexible. You will not see a problem on the street, but sticky tires on the track overload the mount.





I figured if I can triangulate the toe link mount that would be ideal. I was also looking to stiffen the lower suspension mounts as the way I figure it they get the majority of the side load from tire and upright and the way the brackets are attached are prone to twist as well.

My solution was to run two pieces of 3/4 square tube.

One between the toe link mount and the forward lower a arm mount. This will cancel one axis of twist at the lower A arm as well as one at the toe link.



Then put a brace between the x brace under the transaxle and the toe link mount.

The x brace under the transaxle is a little weak to take a side load. It's also difficult to. Weld to the x as their is not enough landing area for a tube from either side.

I fabricated up braces from 1/8 inch plate.




Then welded those then installed the braces.





Oh and while I was at it I boxed in the toe link mount and strengthened the attachment to the brace going to the lower a arm mount.




What a pain it was to weld this all while the transaxle was installed. I was welding on my back, upside down on the bottom of parts overhead and wedged in the wheel well on others. Of course I'm stubborn as hell and refuse to give up on welding with the transaxle installed.

I actually did one weld hanging over the rear qtr panel head down by the axle with the tig pedal on a shelf. Silly, yes. Should I get a finger operated tig torch. Maybe. But why not just figure out how to make what you have work?

So I proved the point to myself. I can weld in suspension braces with the transaxle installed.

But guess what. Transaxle is coming out next weekend so I can weld the other side in an hour.

John

[Cobra 61]

John,
You are right about one thing. The X brace is weak in respect to the lateral load resistance. We did what you are doing 6 years ago. We then found that the X brace was cracking at the welds to the 1.5 box tube chassis. I did it slightly different. Instead of making the gussets that fit in the middle intersection (thinking the gussets were not going to be enough) I used a 4" x 8" plate. The reason I used the plate is because the intersection of the two pieces or "area" seamed to be small and "flexy" looking. My thinking was to increase the load resistance over a larger area. The same as you would look at door bars when crossed. The intersection of the two bars can become a "hinges of sorts and that was the result of changing the door bar rule to diaphragm or gusset the intersection to eliminate this. Also, the interpretation of "two bars required for door protection. The intersection area is still only one bar at that point. Same concept, one vert and other horizontal. This weekend, we will revisit this as we are increasing tire size anss a huge increase in grip. I would suspect we will experience some additional movement in this area again.

R

[crash]

This weekend, we will revisit this as we are increasing tire size adding a huge increase in grip. I would suspect we will experience some additional movement in this area again.

R

[kabacj]

John,
You are right about one thing. The X brace is weak in respect to the lateral load resistance. We did what you are doing 6 years ago. We then found that the X brace was cracking at the welds to the 1.5 box tube chassis. I did it slightly different. Instead of making the gussets that fit in the middle intersection (thinking the gussets were not going to be enough) I used a 4" x 8" plate. The reason I used the plate is because the intersection of the two pieces or "area" seamed to be small and "flexy" looking. My thinking was to increase the load resistance over a larger area. The same as you would look at door bars when crossed. The intersection of the two bars can become a "hinges of sorts and that was the result of changing the door bar rule to diaphragm or gusset the intersection to eliminate this. Also, the interpretation of "two bars required for door protection. The intersection area is still only one bar at that point. Same concept, one vert and other horizontal. This weekend, we will revisit this as we are increasing tire size anss a huge increase in grip. I would suspect we will experience some additional movement in this area again.

R

Thanks for the info Richard. I left a spot to attach the x brace to the beefy 1.5 inch tubes that form the main structure of the rear frame. This way when the toe link mount brace pushes down the other strut will be in tension.





Grip is good more grip is even better. Do you think we can get 2.5gs?

John

[kabacj]

I love data. Since everything i post here is relative and not terribly scientific I don't know that we can draw direct correlations, however a car is such a complex system you need to be far beyond my amateur level to really measure this type of stuff. Even the pros make mistakes. So now that my disclaimer is complete, I wanted to share what I think is some very cool data.

I measured the deflection of the rear rim at the 9 o'clock position using my dial indicator.



The goal was to compare the rim deflection between the modified side of the car and the unmodified.

The car is on jack stands. I repeated all measurements 3x going back and forth between each side of the car and averaged the results and rounded to the nearest tenth. I also measured how much I move the car itself since it is up on jack stands. During this exercise the max i could move the car is 4 thou while twisting the wheel. So assume I have a 4 thou margin of error. I took a measurement with the toe link in compression and tension. To apply force I simply twisted the tire with my arms as hard as I could yanking sightly. (not a lot of force, but its repeatable)

The un modified side of the car.

Toe link in compression -.09 inches of deflection
Toe link in tension .08 inches of deflection

Modified side of the car

Toe link in compression -.04 inches of deflection
Toe link in tension .02 inches of deflection

I was also interested in how much deflection I was getting at the center of the X under the transaxle.



As a control I tested the deflection when twisting the unmodified side of the car. I expect only the deflection due to me moving the car.

Unmodified side deflection under X

Toe link in compression .0005 inches of deflection
Toe link in tension -.0005 inches of deflection


Modified side of the car

Toe link in compression .03 inches of deflection
Toe link in tension -.025 inches of deflection

So my conclusion is that I can add or remove almost 1/8 inch of toe from the rear wheel while twisting it by hand on the unmodified side.

My modification has decreased that to between a 16th and a 32nd.

The transaxle is coming out and im going to brace it all up even more.

Not sure what all this means for the track , but its cool information.

John

[carbon fiber]

John, it sure seems like you're onto something, if you can move the toe 1/8" just by pulling on it. I'm sure i'm not the only one interested in seeing how this plays out.

[Fugly_Old_Cowboy]

stands to reason any reduction in chassis/suspension flex should improve handling...

[kabacj]

After a few hours of careful fitting and welding I have triangulated the rear toe link mounts

First I added a strut to tie in the toe link mount and the lower forward A arm mount.

This should stiffen both the toe link and the lower A arm mounts.

Next I used that strut to box in the toe link mount.






I used the boxed in section of the toe link mount to land the forward brace.

Here is an overhead shot of the whole layout.




Now that I'm done for this round of re enforcement. It was time to take some measurements.

What I found shocking was I could feel the rear wheel is stiffer. I guess that's because it is!

I was able to get the toe link itself down to 2 thou of deflection. So the bolt that attaches the toe link to the frame itself is only moving 2 thousands of an inch under load. Thats pretty good.

It's notable however that I was only able to decrease the flex at the wheel to a point. As the frame got stiffer the wheel did as well but flex would only lower the rim deflection to a point.

Since I have the 10 micron runout hubs I'm told were used by Pratt and Miller on the c5r and would last a season on a pro level race car. The toe links have high quality rod ends and tight tolerance attachment to the upright and frame.

I attribute most of the flex to the bushings.

Even though I have pfadt poly bushings I think there is still a bit of give especially in the first few lbs of side load as the frame / suspension takes a set. Then I think the chassis stiffness will shine as the increase in load will not increase the wheel deflection in the same way as it did before the frame upgrade.

Since you would never achieve track level side loads on the street, the stock frame and poly bushings are fine for street use an even occasional track days. As loads increase at race pace the next weakest link is exposed.

I just need to put it all back together this weekend and I'm ready for track testing and my first super unlimited race next week at Thompson Motorsports park in CT. This will be interesting as it's a new track.

Lots of folks will be learning lines. It's too bad I will not have the same track layout to test the car, as I could flex the frame every lap in the same corners time after time.

Actually just realized something. The quick shifting sequential most likely also added to the chassis load.

I could keep the suspension loaded as shifts were so quick I moved my shift points so I could stay on the gas longer.

It's always the same story. The more you improve the more you need to improve.

Sure is fun though!

John

[fastthings]

Looks good, John.

Couple years ago I took a ride in a buddies Vette. That thing would do such a weird wiggle in the rear when he let off the gas after a hard pull, or going over a bump. I didn't like it, I kept picturing myself in the GTM, when it was done, and thinking no way in hell hell I have a car that does that. I said something to him, and he said, they all do that. Long story short I have been worried about that spot since day one.

It would be way too easy to add a support, to not do it. I'll copie what your doing I susspect, except for the 2 bars going to the rear. How important do you feel those 2 rear bars are.

Pretty cool man,
thank you

[kabacj]

How important do you feel those 2 rear bars are.

Hey Gene

You will not have any trouble with a wiggle on the street even after a hard pull. I could not generate the instability unless I was at race pace on hot tires. Even if you were totally nuts it almost impossible to drive that hard on the street

I didn't even have it on the track till now.

The rear bars removed two thousands of flex measured under the X brace. I think you only need those on a race car.

John

[carbon fiber]

You said "almost impossible" right?

[kabacj]

You said "almost impossible" right?

Ha yep. This is the internet. You gotta caveat everything.

I can say this for sure. I could not duplicate the behavior on the street. When I was trouble shooting the problem I tried. The track has a large network of access roads and parking lots. No one will hastle you if you quickly test somthing.

I could never cause any instability no matter how hard I tried. On the track where there is better grip and triple digit speeds after I got up to race pace. That's where you see this behavior.

Took me a year on the track to even get up to the level where I could make it happen. I was top one or two fastest guys every weekend when the other drivers were not racers. I did not see the problem.

If you are planning a track car then stiffen it up. One or two track days a year on street tires it's optional. If you will never go on the track you are fine.

That's my opinion anyway.

John

[carbon fiber]

Thanks for the info. I do plan on some track time, and i'll probably get addicted... VIR isn't too far away.

[kabacj]

While I was welding the rear frame I saw this as the perfect time to weld in some supports for a rear anti roll bar.

I cut some plate and welded it in




Then added a little gusset in back. Here it's mostly welded.





Then you can simply add any stock or after market c5 rear anti roll bar. This is nice because there are quite a few inexpensive options.

I'm starting with the stock corvette rear arb. Since I only have 4 working hours before I need to load up the car and head off to Thompson CT for the inaugural race on a brand new race track. I might not make the bar adjustable just yet.

This is the setup we are carrying over from the c5.







3 days at the track with lots to test.


John

[mikespms]

Hey John,

That mount looks good and strong,you are going to need to space the sway bar mount back about 3/8" to clear the rear cross member.And a mounting tab for the control arm.

[kabacj]

Hey John,

That mount looks good and strong,you are going to need to space the sway bar mount back about 3/8" to clear the rear cross member.And a mounting tab for the control arm.

Good call Mike. Just last night I was debating on how to address the clearance issue and mounting point on the lower control arm.

One option was to flip the rear ARB over and cut off modify and re align the mounting holes on the arb.

This allows me to make the bar adjustable and spaces the bar away from the frame.



I was also considering welding on an aluminum tab directly to the lower A arm. Only problem there is that means pulling the whole thing out and taking the bushings out.

It will take high amperage to weld the thick plate on to the cast A arm. That will heat up the whole A arm very quickly. I think I would roast the bushings.

I'm so used to chopping things apart I imidiately go to that as a solution.

Your idea is better. Retaining the stock bar geometry is a big plus.

Thanks for posting you solution. Now I have a shot of getting the ARB sorted for the track this weekend.

John

[JCHRacer]

Looking good as always John. I would think twice about welding on the aluminum control arms. I don't know about the corvette stuff in particular but most aluminum control arms are forged and heat treated and actually have much better properties than cast. Welding would reduce the properties significantly.

[crash]

Looking good as always John. I would think twice about welding on the aluminum control arms. I don't know about the corvette stuff in particular but most aluminum control arms are forged and heat treated and actually have much better properties than cast. Welding would reduce the properties significantly.

What he said.

Joel...I'm looking for some serrated plates to use for adjustable mounts. I can't remember if you used those in your suspension setup or not, but I have not been successful in locating a source for them yet. If anyone else might have any ideas, that would be appreciated also. These parts are typically used on panard bar attachment points and such. I have a setup on my brake pedal assembly that I may be able to get a picture of for reference if people don't know what I am talking about, but they are a trick part, and a great solution to allow wide adjustability. Any help in locating a source would be greatly appreciated.

Sorry for the detour, I just don't see Joel posting much anymore and hoped he might check back here.

[kabacj]

Thanks for the feedback Joel and Crash. I have avoided welding the A arms so far with the same concern, but there is so little quality information on the A arm construction available that I was considering it. Your point taken however and I will just make a nice tab.

Crash what about these?





http://www.etrailer.com/Accessories-...ese/58024.html

[crash]

Yep. That's the idea. I need them in strips though so I can machine them to my specs as far as slots and attachments.

[crash]

I just did another search and found the Urbine Machine stuff. They are a little course, and I would prefer a 1/8th tooth step.

Would you like me to start another thread John?

[kabacj]

I just did another search and found the Urbine Machine stuff. They are a little course, and I would prefer a 1/8th tooth step.

Would you like me to start another thread John?

Does not bother me if we go on tangents. It's good race car building info.

[KeithBoden]

Caught again! I have enough fun posting your pics...maybe I should spend some time in the garage instead of at work!

2014-06-23.png

[kabacj]

Caught again! I have enough fun posting your pics...maybe I should spend some time in the garage instead of at work!

Keith yet another great picture! I think you have supplied more great pics then any pros have gotten.

This shot was taken from a giant mound they left when they built Thompson. It does not make any sense till you go up there and view a race. It's a great vantage point. Thanks for the photo. I'll have a race report in the am.

John