GTM roll cage modifications and race car build log

[kabacj]

Cool, any video of the 150+ runs? Did you race, or was it a track day? Did you like your alignment canges? Who did you want to keep up with, and did you do it?

Fun stuff keep it up John

Hi Gene,

Yep i did grab some video of 150+. Unfortunately I was hampered by my oil pressure. On shorter tracks I never see the extended high RPM running that I do quite a bit of at VIR. As a result anything over 5500 RPM i was blowing oil out the rear main seal. Kind of annoying, but since lowering the pressure on my dry sump is an involved job I just short shifted.

I did make many alignment changes. The alignment that I liked best was -1/16 of total toe in up front and - 3/8 of total toe in out back. I am going to work on setting up the car with more caster as well as attempting to tune the bump steer.

The new wing was great and I also really liked the 800 lb springs in the rear.

It was a race weekend. VIR is a long and complicated track to learn. I hope to get back there in May for more track time.

I will find some video and post it.

[Plebeian]

May I ask what times you were running at VIR?

[kabacj]

May I ask what times you were running at VIR?

Hi Plebeian

Unfortunately I was not able to turn any fast laps this weekend. The combination of my oil pressure issues with my dry sump pump set at too high pressure spraying oil on the exhaust if I ran above 5000 rpm. The hole in my CV boot during the race Saturday smoking up the cabin and nearly getting me black flagged then taking me out of contention on Sunday. Then the setup and adjustments I needed to make to my new cable shift setup and all of the alignment changes I was making on Saturday held me back.

This was a test and tune weekend where I had hoped I had guessed correctly on all my winter changes. Some were great. Like the stiffer spring rates and revalved shocks. Some were wrong like the aggressive toe. I made good progress but I'm not there yet.

This is a reality forum. sometimes things go well and we have storybook results. Some times Lady Luck has something against us and we get blanked. I would not enjoy this if it was easy .

Based on the times set by guys I run with normally I should be setting times around 2.00 flat.

I want to get back to VIR in May for hyper fest. We are going to need to wait for that to see what the GTM can do.

[fastthings]

If you wouldn't mind, could you describe the difference in the feel you got from the toe changes. Also what so you think the stiffer rear spring did for you, I think you did it to keep the car from compressing the shock so much. Did it give you a different feel, enterence or exit?

Messy weekend, life in the fast lane. Your a fun guy, I know that didn't bother you a bit.

[kabacj]

If you wouldn't mind, could you describe the difference in the feel you got from the toe changes. Also what so you think the stiffer rear spring did for you, I think you did it to keep the car from compressing the shock so much. Did it give you a different feel, enterence or exit?

Messy weekend, life in the fast lane. Your a fun guy, I know that didn't bother you a bit.

Ha I'd rather have everything go like clockwork, but I make mistakes and things happen. So I need to fix them that part is fun.


Lets start with a giant disclaimer. I have found that every setting you make on a GTM changes how the car feels at speed. We will call “at speed” to be +100mph. I know from talking to lots of guys who build factory five cars that many of us do a “quick alignment” oh that looks about right.. and go out and drive. Nothing wrong with that until you start going fast on a track or a top speed run at a sanctioned event.

Then your car that was fine to dive at 70mph on the highway picks up some weird handling behavior that people make up reasons to describe. Unlike buying an OEM factory built sports car and hopping it up and going as fast as you can. Factory Five give you a GTM setup for the street, developing the GTM for racing conditions just like the paint and assembly is up to you. If you want to drive your GTM at high speed under racing conditions you need to setup the car for that purpose. Team PDG did that. I am learning how to do that. Hopefully we can make it easier if you want to do a bit of this yourself. Every setting is important and relates to the others. If you are driving on the street at speeds that will not get you thrown in jail you will be fine with the setup from the manual. OK with that said.


I was going to change the toe at home based on Crashes experience and recommendation , but I ran out of time getting everything packed for my first trip to the track.

I went out for my first run with -3/16 on each side up front and rear toe at - 1/4 on each side I progressively took toe out of the front till I stopped at - 1/16 of total toe in.

As you take toe out of the car it changes direction even more quickly. It also feels like it tracks around the radius of corners a bit better. I was happy with the front so I left it alone and started changing the rear toe.

As I removed rear toe, I think I started feeling bump steer. I set my bump steer before I ever drove the car. I also did not know the ride height that I should tune the bump steer around . I plan on measuring and resetting the bump steer at my current ride height.
The changes in how the car felt were not drastic as I removed rear toe in, but I ended up most happy with – ½ inch of total toe. Note that under 130 mph the difference between -1/2 and -1/4 of total toe was not as large as the decrease in toe up front.

The stiffer springs:

The stiffer spring and stiffer damping package benefit weight transfer. The setup I was running last year did not have enough control of body roll and front to rear weight transfer. I left this change to the experts at Performance Shock who recommended changes based on the motion ratios of the suspension and my feedback.
I was locking the front inside tire on decreasing radius turns like turn 1 at Lime Rock and I had to wait an extra heartbeat for the car to settle after making very sharp low speed chicane type turn . Unless you are running the exact same anti roll bar package, shock damping settings the springs are not going to get the same behavior, but it makes sense that factory five sell the gtm with a spring package that is firm but not harsh.

How does it feel with the stiffer setup?

I have more front grip and less body roll. Both good things. The tracks I run on are pretty smooth, so I didn't notice that the ride was more harsh.

It's fun to change how the car behaves using these tuning tools. Since you are almost done with your car you can spend weeks tuning your setup to your use case and what you like or just set it up like the manual suggests and drive it. For the street either way works and both are fun.

John

[mikespms]

Hey John,
Thanks for sharing your experience and results it is very helpful. How is your rear tire wear with that much toe,did you check tire temps?
Waiting for the new track video if not just the sound track.
Mike

[kabacj]

Hey John,
Thanks for sharing your experience and results it is very helpful. How is your rear tire wear with that much toe,did you check tire temps?
Waiting for the new track video if not just the sound track.
Mike

Hi Mike. I do get noticeable tire wear in the middle inside of the contact patch from the toe setting. The wear is not significant enough to be a problem however since the tires heat cycle to the point that grip is gone before they wear out.

I did not check tire temps this weekend because Friday the track was damp and Saturday I was chasing other things.

I have scheduled 2 test days last week in April to spend more time on the alignment setup as well as suspension and aero tuning.

I think 1/2 inch of total toe is a bit too much. It feels good, but I also think that if I reset the bump steer around my current ride height I may be able to back the rear toe out. For those of you that are new to bump steer it's how the toe changes as the wheel travels up or down from ride height.

My goal is to get 1/16th of total toe in per inch of compression in the rear and 1/32 of toe out per inch of compression up front from bump steer.


I plan to view my video this weekend. Hopefully I can find some good bits. I will also document the alignment and what settings I had and what I change them to.

John

[fastthings]

Thanks for doing all that typing John, I do alot of suspension tuning with a simulator, so I like hearing what people feel from a susp change. I completly understand your disclaimer about driving at speeds, versus highyway driving. I live in a real cool place, lots of places to go nuts. This car will be capable of killing me at any givin moment. It must be tuned to drive on country roads at speed, very dangerous. And I do live 10 minutes from a 2.5 mile road corse. It will be a long learning curve, but what ever, I'm looking forward to it.
Sounds like the rear bump steer continues to be an issue, and like you say one thing affects another. I wonder, does the 1/2 inch of rear toe feel good because it nolonger bump steers, the bad part about too much rear toe would be temps, and maybe a little too loose into the corners.

[kabacj]

and like you say one thing affects another. I wonder, does the 1/2 inch of rear toe feel good because it nolonger bump steers, the bad part about too much rear toe would be temps, and maybe a little too loose into the corners.

Yep. I'm not sure why I like 1/2 inch of total toe better than 1/4 in the rear, but I am sure that since I did not purposefully set the bump steer it's probably not optimal. That's why I am getting the right tools and doing the setup properly.

I have not had trouble with tire temps or the car getting loose so it's not a major deal. I am probably giving up some grip and we don't want to do that

John

[fastthings]

Well, what your doing is really cool, I wish you lived over my way.

[kabacj]

Well, what your doing is really cool, I wish you lived over my way.

Gene I think I would rather live by you. With all those beautiful smooth roads (not covered with crater sized potholes)and that great track around the corner you have the best of both worlds for a GTM.


You are going to need to have the nose of your hood wrapped with chip guard however. I would hate to see that beautiful black paint ruined on the racetrack. In one year the nose of my car started turning red as there were more chips than painted area. The other area that gets chipped is the rocker area in front of the rear tire.

[kabacj]

Working on improving my setup with the new shock and spring package. I very happy with the stiffer springs and added additional damping each run trying to limit the number of changes I was making per run.

I was getting excited that I had some newfound grip in the first session on Sunday am. With ambient temps around 45 and dew off line the track was a little challenging.

Last lap I decided to run it in a bit deeper. Well I got a little off line and lots of under steer. I was clearly not making it around the corner but I wanted to slow as much as possible before I left the track. All ended well as I did not ruin my new splitter.

On the down side I locked up the inside front tire a bit too long. It's a very soft compound .



I finished another lap on the tire and it was ok but low speed it had a nice thump thump.

Ok fine I can't use it for the track, but I wanted to try an auto cross. Should be fine for that. Not going too quickly and nothing to hit. On further inspection I think it's junk.


There is a soft spot/ bubble. Man that sucks. Oh well at least the other tire is still new with only one heat cycle . I can replace the flat spotted tire and the tires should have similar life.

[fastthings]

Damn, that sucks, glad you didn't tear it up. I remember a thread about a guy trying to get the willwood adjuster to adjust enough to the back. I went out and took mine apart, it did seem that the master on the left, I think, always got more preasure. It just so happens I have the front using the left master. I have not tested mine yet, I hope I can dial up too much rear bias.

Have you been able to get your brakes to lock up front or rear, lock to lock, with your adjuster? I wonder if you will be dialing in a little more rear bias, because of that. I love it that you share the bad with the good. Ken Block would have grabed his E-brake.

[kabacj]

I have not tested mine yet, I hope I can dial up too much rear bias.

Have you been able to get your brakes to lock up front or rear, lock to lock, with your adjuster? I wonder if you will be dialing in a little more rear bias, because of that. I love it that you share the bad with the good. Ken Block would have grabed his E-brake.

Hi Gene,

Race car development is not easy. Especially since I have never done it before I will make lots of mistakes. I learn the most when I make mistakes so I think you all should learn with me.

I did not get to adjust my brakes this time at the track. My fronts lock first and have for most of the time I have been taking it to the track. Front lock up is better in the rain. I can out break most folks pretty easily so I have not spent much time getting the most out of the system. However I am sure I can make the car easier to drive and brake even harder if I get everything setup just right.

I am going through the registration process so I can work on things like brake balance on the street both in the dry and the wet. I will get adjustments in the ball park then do the fine tuning at the track where aero and other factors are in play.

The Willwood pedals and manual adjuster do not have a massive range of adjustment. It seems like we will need to set the major adjustment using the threaded rods that push down on the master cylinders then use the Willwood cable adjuster and the balance bar to tune the balance front to rear. I will report back on how that goes.

Its far easier to deal with a car that locks the front tires first. In the past I have had the rear bias such that I could rotate the car using the brakes. That was interesting, but I backed off from doing that since I figured it would be better to err on the side of caution until I get other major systems setup the way I wanted.

I'll let you know how it works out.

John

[crash]

Maybe just personal preference, but the guys always set the PDG GTM up so that it goes out with too much rear brake. I simply can't stand a pushy car, so I will usually end up turning in about 2-3 turns of front brake during the first session.

Again, the nice thing is that the adjustments do work and the car is pretty sensitive to them.

[kabacj]

Again, the nice thing is that the adjustments do work and the car is pretty sensitive to them.

Speaking of how responsive the GTM is to adjustments. When I was changing front and rear toe at the track I was surprised how different the car feels with minor toe changes.

That got me thinking about how I never really set the bump steer after I got the GTM on the track.

I figured I would measure by disconnecting the shocks and using a jack and toe plates to cycle the car through the full suspension travel.

The problem I had was taking measurements by myself I got a little too much variation. I needed a better way so i got this tool. I love tools!



With this guy I can measure to the nearest 10,000th of an inch. That's far more than we need for alignment, but that's ok. I love precision measurements.

Turns out I have too much bump steer.

You can see in the graph I get over 1/32 of change per inch of suspension travel 1/32 inch of change is the upper limit. I need to try to reduce it.



This is just one wheel. I'm going make sure both wheels are the same. Then start adjusting.

John

[crash]

As I have mentioned before, chassis setup is a DYNAMIC thing. Once you understand what is happening, then you can make static changes and understand what effects those changes are having, but first you must get everything setup the way it needs to be dynamically. This is good that you are going down this path. It is a requirement for making the GTM work at speed.

If you would like another "tool" I can not recommend highly enough the purchase of Suspension Analyzer from Performance Trends. The most significant part of this program that I use is the fact that it is a DYNAMIC program that shows you how everything interacts with each other. It is one of my secrets to building cars correctly...but I guess not a secret anymore!

http://performancetrends.com/SuspAnzr.htm

[kabacj]

As I have mentioned before, chassis setup is a DYNAMIC thing. Once you understand what is happening, then you can make static changes and understand what effects those changes are having, but first you must get everything setup the way it needs to be dynamically. This is good that you are going down this path. It is a requirement for making the GTM work at speed.

If you would like another "tool" I can not recommend highly enough the purchase of Suspension Analyzer from Performance Trends. The most significant part of this program that I use is the fact that it is a DYNAMIC program that shows you how everything interacts with each other. It is one of my secrets to building cars correctly...but I guess not a secret anymore!

http://performancetrends.com/SuspAnzr.htm

thanks very much Crash. That will be very helpful. It's always about having the right tool for the job.


John

[fastthings]

That is a cool tool John, I should get one. I know damn well I would pay that much in labor for someone to set up all four corners.
How do you mark ride hight with the wheel off, seems like with the wheel off it would be hard to know exactly where 0 is. Is it a guess, do you measure from the body? And we know that from 0, full up and full down is not a useable range. How much travel are you allowing for up and down before you quit trying to solve a bump steer condition.

I also wonder if I need to get some corner scales, have you set corner weights yet. I think I need to do this first. These are kind of spendy thoough.

[mikespms]

Hey Gene,
I have an inexpensive bump steer gauge I got from speedway motors,It works good I Was able to get the front bump steer very close to zero about .005" in 3" of travel .Here is a link to the instructionshttp://static.speedwaymotors.com/pdf/910-82700.pdf . Since you have to unload the springs to set the bump steer the scales and corner weight get done after your alignment and ride height is set.

John,

I've been replacing my upper control arm and playing with the alignment but before I finalize the alignment I am going to check and set the rear bump steer. It is recommended to set your bump steer so that it toes out if you can't eliminate it. Bump steer kits for c5 corvettes cost about $200.00 the front kit should work on the rear.http://www.summitracing.com/parts/ba...model/corvette

[kabacj]

That is a cool tool John, I should get one. I know damn well I would pay that much in labor for someone to set up all four corners.

The Cheap way to do this:

at ride height measure from the tire sidewall to the body. I put a line on the sidewall and body with a sliver sharpie so I could have to exact points to measure to.
Remove or disconnect the shocks put the wheels back on then using a jack under the car lower the car back to ride height.

I was able to put the jack under the transaxle so I could still run the tape measures under the car without them hitting the jack. I have the beefy transaxle mount i made so not sure how well this would work with the standard one. if its a problem just rig up the jack so you can still lift the car from the center line keeping it level side to side but not have it in the way of the tape measures.
Take toe measurements every 1/4 inch. Lots of measurements will average out any error you get from the tapes and toe plates.

How do you mark ride hight with the wheel off, seems like with the wheel off it would be hard to know exactly where 0 is. Is it a guess, do you measure from the body? And we know that from 0, full up and full down is not a useable range. How much travel are you allowing for up and down before you quit trying to solve a bump steer condition.

1 and 1/2 inch above and below ride height is a reasonable range to address bump steer. With my shocks and springs I dont use more travel than this even when crashing over the curbs. Not sure how the stock setup will behave, but if you have addressed the change in that range i think you are going to be fine anyway.

I also wonder if I need to get some corner scales, have you set corner weights yet. I think I need to do this first. These are kind of spendy thoough.

Yes scales are expensive. Before I had them I simply set the car up so it was level with me in the car measuring at the 4 corners of the frame in front and behind the wheels. If you can add a little rake to the car (front of the frame a little lower then the rear) that will help you, but its not critical. Just make sure the car is level side to side with you in it.
When I got the scales just recently I measured the corner weights and I was close enough with this method to not cause issues on the track.


If I were you I would get the bump steer gauge before the scales. I think you have far better investments to make in the setup before you buy scales. Those are the type of thing you can check once and be all set. Any NASA event has scales that I am sure they would let you use to setup the car. you can roll on take a measurement then adjust your spring perches roll on again and you will be plenty close enough for the street. Consider the fact that a passenger massively changes the corner weights of your car and on the street you just dont notice it. Even on the track, unless you are driving at the limit, it does not matter.

If your toe settings are way off you will feel those on the street. I have really screwed mine up when I got things misaligned rushing at the track and its just the car will turn better left than right....not like your car is going to suddenly spin off the road and crash you into the woods.

hope that helps.

[kabacj]

Hey Gene,
I have an inexpensive bump steer gauge I got from speedway motors,It works good I Was able to get the front bump steer very close to zero about .005" in 3" of travel .Here is a link to the instructionshttp://static.speedwaymotors.com/pdf/910-82700.pdf . Since you have to unload the springs to set the bump steer the scales and corner weight get done after your alignment and ride height is set.

John,

I've been replacing my upper control arm and playing with the alignment but before I finalize the alignment I am going to check and set the rear bump steer. It is recommended to set your bump steer so that it toes out if you can't eliminate it. Bump steer kits for c5 corvettes cost about $200.00 the front kit should work on the rear.

Hi Mike,

Factory Five tell me that we want 1/16 of total toe in per inch of compression travel in the rear and 1/32 of total toe out in per inch of compression in the front.

Agree with you a cheap bump steer setup is the way to go.

[fastthings]

Thanks guys

[kabacj]

Over this weekend I took literally hundreds of measurements of the rear suspension motion.

My goal was to map out how adjusting the toe links using shims changes the behavior of the whole suspension.

I use a bump steer kit that replaces the corvette toe link.

This is a photo from last year but it shows the part.


The key is the longer stud that fits into the tapered hole in the C5 upright. This allows adjustment of the rate of toe change. Not a big deal to worry about at highway speeds. If you are going 130+ you have one wheel turning in at a higher rate than the other you might have what some people falsely claim is aero instability. It's just your rear tires steering the car as the suspension travels up and down. I can see how people make the mistake. People that race the corvette do this same mod. A stock corvette is far less stiff than a GTM so the compliance of that setup may mask setup flaws.

The VIR high speed back strait has some bumps and dips that cycle the suspension through its travel. I noticed the car would shimmy from the rear end over these sections. Adding damping to the rear shocks lessened the shimmy. Hmm.

Now that I have mapped the rear suspension travel on my car with literally over 200 measurements and seen a thin .027 inch shim change the toe rate. Im reasonably sure that people who don't set their bump steer will not have equal rates per rear wheel.

Another trick i found was adding rollers under the bottle jack allowed for more accurate measurement. The rollers eliminate any binding introduced by the suspension Traveling in an arc connected to a static bottle jack.

[Fraser D]

John,
Great information!
I have experienced the rear become unstable when transferring the weight forward quickly at speeds 130+ or when the road gets a little lumpy. It is less noticeable when I roll off the throttle and feed in the brake.
Your thread is a huge help to me in making sure that I make the right upgrades and know how to measure changes for this great car. I have to say that out of the box, if set up correctly the GTM handles very well up to that point, but I don’t know many production cars that can easily exceed 130mph on a track.

[kabacj]

John,
Great information!
I have experienced the rear become unstable when transferring the weight forward quickly at speeds 130+ or when the road gets a little lumpy. It is less noticeable when I roll off the throttle and feed in the brake.
Your thread is a huge help to me in making sure that I make the right upgrades and know how to measure changes for this great car. I have to say that out of the box, if set up correctly the GTM handles very well up to that point, but I don’t know many production cars that can easily exceed 130mph on a track.

Glad you find the info helpful Dave. Since we are starting from a box of parts it's reasonable that we need to make the proper adjustments to get super car performance.

I got a few pics from my track day at VIR.

[Kempo]

Great info John. The car looks so COOL on those pictures!!!!

[OZIGTM]

John,
When you were putting the rear end through it's movements did you notice if the amount of anti-squat makes any difference with instability?
I know Crash said they have reduced the amount of anti-squat in the GTM-R.
Cheers,

Rohan

[kabacj]

Great info John. The car looks so COOL on those pictures!!!!

Thanks Hugo!

John,
When you were putting the rear end through it's movements did you notice if the amount of anti-squat makes any difference with instability?
I know Crash said they have reduced the amount of anti-squat in the GTM-R.
Cheers,

Rohan

Hi Rohan

I have not gotten to the point where i can adjust anti squat. The way I figure it I first need to optimize the Factory five design before I change it.
I think the key point to take away from this step in the process is that your bump steer or roll steer will not be the same unless you actually measure and set it. I made the mistake of assuming that if I set both sides with the same number of shims my bump steer would be equal. Not many alignment shops are even going to check this setting. Maybe a race shop would but that will cost you some labor.

In fact I needed to make thinner shims to actually match the rates from side to side.

While i think my measurements are a bit of overkill because I am measuring 1,000ths of an inch. I firmly believe that your calculations are only as good as your least accurate measurment.

Once I test out this batch of changes on the track I'll see if anti squat is a concern for me.

I'm far from an expert, but i have a feeling that once you dial in the basics of proper toe, camber and bump steer for the GTM. The next round of changes are more about what you like as a driver. Like I said I'm not sure of that, but you can be sure I will report what I find. .

[johngeorge]

John, put some stripes on that thing! plastidip!

[kabacj]

John, put some stripes on that thing! plastidip!

100% right John. I need to do the plasti dip thing while I finish the body mods. I just need a little warmer temps to get that done. I'm up north by you today and it's sleeting! It's the winter that never ends.

[flickery8]

Damn, that sucks, glad you didn't tear it up. I remember a thread about a guy trying to get the willwood adjuster to adjust enough to the back. I went out and took mine apart, it did seem that the master on the left, I think, always got more preasure. It just so happens I have the front using the left master. I have not tested mine yet, I hope I can dial up too much rear bias.

Have you been able to get your brakes to lock up front or rear, lock to lock, with your adjuster? I wonder if you will be dialing in a little more rear bias, because of that. I love it that you share the bad with the good. Ken Block would have grabed his E-brake.

John,
The first year I drove my car on the track I won a TOYO hat for going off track more than anyone else. Some of this is inexperience. However, for those reading your great story here, my problem was trail braking. Any braking with wheel turned resulted in me going the wrong way down the track (I still have grass and hay everywhere). I do think I have to much rear bias as the front pads look new when replacing the rear. Do you find your car to be hyper sensitive to wanting to come around with any trail brake? This could even apply to agressive street driving where you have posts and curbs instead of soy beans like we have here.

[kabacj]

John,
The first year I drove my car on the track I won a TOYO hat for going off track more than anyone else. Some of this is inexperience. However, for those reading your great story here, my problem was trail braking. Any braking with wheel turned resulted in me going the wrong way down the track (I still have grass and hay everywhere). I do think I have to much rear bias as the front pads look new when replacing the rear. Do you find your car to be hyper sensitive to wanting to come around with any trail brake? This could even apply to agressive street driving where you have posts and curbs instead of soy beans like we have here.

Hey fickery

Yes sounds to me like you have too much rear bias. I have found that trail braking actually is a benefit to transfer weight forward and aid front grip mid corner.

You should be replacing rear pads half as often as you change fronts. Definitely not more often.

It's possible to get lots of braking done with the rear only as you have shown, but as you also have experienced when heavy rear bias will hurt your directional control when trail braking. The big tires out back and the amount of slip in your differential will determine how much your car pushes or slides the front tires mid corner.

If you lock the rear tires mid corner, yep then you are going backwards. I personally like when the front tires lock first as that is the easiest to control and allows me the highest mid corner speed. Rear lock up in the rain is even worse.

I would screw the rod that goes from the rear master cylinder all the way into the brake pedal assembly. Try that. On the street in a safe place. We don't want to find out you only have rear brakes and the fronts have a problem.
Then start screwing the rod on the front master cylinder out of the assembly both things moving the brake bias forward. Once you have max stopping force then the fronts locking you are at a good starting point to fine tune the setup.

I put a square of duct tape on each rod attaching it to the balance bar on the pedal assembly just to be sure the rods don't vibrate in or out into a new adjustment.

Oh just remembered that you did need to trim one of the rods on the master cylinders as part of the build. Maybe you missed that step? I don't have the build manual handy. Maybe that has something to do with the imbalance. If somebody can check which rod is trimmed that would help.



John

[eseethal]

Sorry to disagree, I think this is a misunderstanding of the balance bar concept.
See the Wilwood instructions.
The bias is only adjusted with the balance bar and not with the rods to the master cylinders. The rods only are adjusted to compensate for the different travel of the MCs. If the balance bar itself does not give you enough adjustment, switching to different master cylinders is the way to go.

Hey fickery
I would screw the rod that goes from the rear master cylinder all the way into the brake pedal assembly. Try that. On the street in a safe place. We don't want to find out you only have rear brakes and the fronts have a problem.
Then start screwing the rod on the front master cylinder out of the assembly both things moving the brake bias forward. Once you have max stopping force then the fronts locking you are at a good starting point to fine tune the setup.
John

[kabacj]

Sorry to disagree, I think this is a misunderstanding of the balance bar concept.
See the Wilwood instructions.
The bias is only adjusted with the balance bar and not with the rods to the master cylinders. The rods only are adjusted to compensate for the different travel of the MCs. If the balance bar itself does not give you enough adjustment, switching to different master cylinders is the way to go.

Hi Eseethal.

I don't disagree with you. In this case however the assumption is that the current bore master cylinders work with the calipers in use. I have not heard of anyone unable to adjust bias so it is balanced front to rear.

If bias is way off my guess was that the rods were not adjusted properly. I need to dig out my manual. I'm pretty sure one rod needed trimming. If that step was missed. Maybe its impossible to get proper balance using the bar alone.

My other guess is that the front master cylinder is damaged . I had a torn seal in my clutch master cylinder that could have been damaged during installation.

One way or another something is wrong.

Thanks for the clairification.

John

[crash]

Just for the record, we changed our MC sizes on one of the MCs. I can't remember which one and to what size, but when we went to C5 front calipers all the way around I'm pretty sure that is when we changed the master cylinder sizes. The car really works better with larger rear brakes. We have since gone to a larger StopTech front caliper, but only because we wanted a larger pad to get more pad life out of the brakes for the 25 hour race. The C5 fronts all the way around is a pretty good, and economical, setup.

[kabacj]

Just for the record, we changed our MC sizes on one of the MCs. I can't remember which one and to what size, but when we went to C5 front calipers all the way around I'm pretty sure that is when we changed the master cylinder sizes. The car really works better with larger rear brakes. We have since gone to a larger StopTech front caliper, but only because we wanted a larger pad to get more pad life out of the brakes for the 25 hour race. The C5 fronts all the way around is a pretty good, and economical, setup.

I also use the stop tech 6 pot up front and ended up using their 4 pot out back. Stop tech engineers agree that the GTM can use more brake than the c5 stock setup.



Last time at the track my brakes were great! I was side lined by a hole in my upper cv boot.

I imagine it was hit by a sharp rock during my Saturday race. It's kind of weird to see a hole like this. The hole was invisible unless i deformed the boot. I wrongly assumed that grease was leaking out from the band clamp or metal to metal joints. I cleaned up the mess re clamped and l cleaned and made fresh RTV gaskets. Only to respray the engine with grease

Here was the culprit.

Invisible unless I dimple the rubber.

[eseethal]

John,
yes, I had to change my rear MC to a smaller one to get more rear brake bias with my Z06 big brakes. The balance bar is only for fine tuning the bias. And the length of the rods do not change the brake bias in any way, unless the adjustment it totally off. It only affects the balance bar position, with a moderate pedal effort the balance bar should be straight.
Compared to a Corvette the GTM has way more percentage weight in the rear so it normally needs more rear brake.

@flickery8: Having to much rear brake is indeed a little strange. Look for trapped air or a defective MC. You could easily measue brake temps with a infrared temp gauge. If the front disks are colder than the rear ones you will know there is less performance with the front brakes.

[mikespms]

Hey John,

Here is my 2 cents,I used the corvette abs and don't have a bias problem but in the process of researching the compatibility of the master cyls with the ebcm and adjustment of the balance bar I spent some time on the phone with wilwood tech service and got conflicting info from tech and finally they put me in contact with the engineer that developed their corvette brakes and according to him the 6 piston and 4 piston rear combo should use a 7/8" bore master for proper volume but that increases the pedal effort since we are not using a power buster,by using a 3/4" master the pressure will increase,the volume will decrease giving you a better pedal feel.Wilwood recommends a 3/4" master for their corvette system and you can use a 5/8"master to increase the pressure to lessen the pedal effort with the wilwood brakes but they don't recommend the 5/8"master with the corvette brakes because the lack of volume if you are using the corvette abs. That been said you could use a 3/4" and 5/8" master to adjust you bias but it was not recommended with the corvette brakes. As far as bias on the pedals assy it is done with the balance bar and it is what it is,you need to make sure that you have free play on your master cyl rods or your will be dragging you brakes.An adjustable valve is an option. On the clutch side you need to cut the rod or you will bend it, I change form the FF supplied 3/4" master to a 5/8" master for a better pedal feel and slave operation.

Mike

[crash]

Also note that pad material will make a HUGE difference, so if you are having an issue where the rears are locking up first and the pad material is different front to rear, the first thing I would do is make sure and change to the same material at both ends. This can have a dramatic effect on how the brakes operate.