First time I drove Ron's car, a couple weeks ago, it was not very enjoyable to say the least. There is a great canyon road next to Ron's house, but pretty rough, with tar strips across it. I was having trouble just driving the car at normal speed, yet alone aggressively. The car was real twitchy, just going straight, & would almost jump off the pavement on the rough parts. After about 2 miles, I had enough & we turned around & went back to the shop.
1st thing we did was check the front toe in. There is much debate on this pretty simple subject. I have personally driven 4 GTMs - 3 street versions, & the PDG race car. On 3 of them, we have changed the toe from 1/8" toe in to 1/16" toe out. My opinion is this is the best adjustment to make, especially if you are castor limited. There are opinions that this is a really bad idea, but personally it has worked 3 out of 3 for me, including the race car. The race car has run for the last 3 season with this setting. We spent 10 min. making the adjustment, & went right back out for a back to back. Ron couldn't believe the difference this simple adjustment made, you could actually drive the car, but it still was bad over the bumps. My first thought was the front end was over-sprung/under-damped.
We installed the front sway bar kit & went back out. Directional stability & steering response was noticeably better, but the ride quality, especially on this particular road was still not acceptable.
Decided we needed do do more work.
Saturday Ron came over, & this is where we started:
So we make adjustments to balance the chassis out by using the spring adjusters to raise or lower each spring. The idea is if your ride heights are where you want them, you adjust up & down on each spring adjuster to balance the weight distribution, but not change the ride heights.
A couple notes. - if the spring rate is higher don't adjust as much, whereas if the spring rate is lower, you have to adjust more.
Example - front rate 450# - rear spring rate 550# - you would adjust the rear spring slightly less, & the front spring slightly more. The higher the difference in rate, the more the difference in adjustment.
So -- we raise the LF, by adjusting down against the spring that puts more load on that corner, but also raises the ride height at that corner of the car. We balance that by lowering the RF & LR. That brings the ride height back down to compensate the rise in the LF. But we also need to raise the RR to replace the lost height in the LR, RF. That is the way you can balance out the chassis & keep your ride heights even.
You CANNOT change weight distribution by this method! That is you can't make the front of the car heavier, or the rear lighter. to do that you have to physically move weight around, ie move the battery from the front to the rear etc. When you drive your car alone you are changing the weight distribution , especially to the left. On a race car we would set the car with the driver in the seat & move things like the fuel tank, dry sump tank or ballast to the passenger side to balance the car out.
Also note that I refer to adjusting "down" or "up" on the adjusters. What I mean is "up" means tighten the spring perch adjuster against the spring to raise that corner of the suspension, In actuality that means screw down on the front & Up in the rear. The difference is that the shocks are upside down in the front & right side up in the rear.
So we now have the chassis balanced out, the sway bar installed, & have adjusted the toe out.
Next we wanted to take a pretty big swing at the chassis set-up.
Richard & Jim lent us the shocks off of the PDG race car to try. Went from the #450 springs & yellow Konis to - putting the #350 front springs on the Bilstein race shocks. & rebalanced the chassis corner weights. We are using the Medium sway bar.
Now a little fine adjustment
You can see with the softer spring I should have made a bigger adjustment, Probably a turn & 1/2
Now the fun begins!
I drove it for a loop through the countryside & the changes made a pretty big improvement. The car really settled down & was more stable & way more fun to drive.
Then Ron took it out -
Ron got on the power a little too much & the rear end stepped out about 45 deg, & did 3 big tank slappers - but he did a great job of keeping it out of my neighbors yard.
I have had Gtms this sideways including the race car & believe me you usually will lose the thing & go all the way around backwards, with potentially bad results.
I think we are really going in the right direction, but there is more work to be done.
Here you go Stig - shock extended length is 15.75" - collapsed is 11.75" to the bump rubber. Keep the info on shocks coming.
We need to check the suspension vs shock travel at both ends of the car at some point to see where the shock bottoms/tops out. Another test to add to the list, along with C/G height.
Here's the link to the forum/thread I was thinking of. David continues to be active on this thread and it continues to grow. Some really good info over there.
Here's the link to the forum/thread I was thinking of. David continues to be active on this thread and it continues to grow. Some really good info over there.
Thanks Mike. I'll read through it tonight. I appreciate it!
The Stig
Some say, that I only know two facts about ducks, (both being wrong); and that if I could be bothered, I could solve the "da Vinci Code" in 47 seconds...
All I know is that I'm called "The Stig". GTM #0081
This is GREAT information. You're going to be a hero to a bunch finished GTM owners who are trying to get the twitch out of these cars. I couldn't tell if you have the bumps steer kit on your car but if you do can you give us a little insight on how you have that set as well? Oh and Ron nice recovery they are tough to bring back once they start to go.
Keith
Have you come accross or found a reason why a GTM might be overly sensitive at high speeds and on the brakes from high speed ? to the point where it feels scarry even just driving in a straight line ... think that is a toe setting ? What do you guys have your rear toe at ?? in or out ? when you toe out the front ?
Thanks !
First off, before you guys go out and have your chassis set up, lets wait until we dial it out first. There's a lot of factors involved in setting up your chassis.
Originally I had my chassis set to ZO6 specs. I found that the car was very twitchy. Any little bump in the road would potentially send me off in another direction.
All of the suspension settings we have done was in Ted's garage. No fancy 80,000 dollar alignment rack were used. Just old fashioned racing techniques. Ted and I will walk you guys through it step by step. First of all set your ride height. My car is at 4.5 front and 5.0 in the rear. If you have a lot of bad roads or speed bumps in you area set the car a little higher. The biggest factor was the toe setting. It made a big difference, I mean huge difference! More videos to come.
First off, before you guys go out and have your chassis set up, lets wait until we dial it out first. There's a lot of factors involved in setting up your chassis.
Originally I had my chassis set to ZO6 specs. I found that the car was very twitchy. Any little bump in the road would potentially send me off in another direction.
All of the suspension settings we have done was in Ted's garage. No fancy 80,000 dollar alignment rack were used. Just old fashioned racing techniques. Ted and I will walk you guys through it step by step. First of all set your ride height. My car is at 4.5 front and 5.0 in the rear. If you have a lot of bad roads or speed bumps in you area set the car a little higher. The biggest factor was the toe setting. It made a big difference, I mean huge difference! More videos to come.
Happy Building, Ron
a lot of good info in this thread. Thanks guys. Ron when you measure the 4.5 and 5 in. of ride height from what point are you measuring each?
Have you come accross or found a reason why a GTM might be overly sensitive at high speeds and on the brakes from high speed ? to the point where it feels scarry even just driving in a straight line ...
Well, IRON MAN apply illustrates where we are at so far. When you can't even drive straight down the road, without hanging on for dear life, we need to focus on directional stability first, before we need to address developing the cornering performance.
Castor defiantly helps here, no question, but what I have found on these cars so far is that the GTM responds to toe adjustments acutely. I don't think that bump steer is affecting direction stability much at all, & you really have to have large discrepancy in the pavement to have bump steer affect the car in a straight line. Now in hard cornering, bump steer plays a bigger role. Yes, defiantly be careful if you adjust your suspension, but as bad as these things can be, you probably won't make it much worse. Remember if you start jacking a bunch of castor in, your road feedback may diminish, & your steering effort will increase quite a bit in parking lot situations. 6-8 degs, & your will probably want power steering. I prefer the feel of the manual steering, so I use the least amount of castor I can get away with. The nice thing is, you can adjust these cars to find what you like best.
Fred, I believe Ron's car has 750s on the rear, & you bring up a good point.
If we look at the scale #s we come up with 41.1% front - 58.9% rear - weight bias.
Now with 350# F & 750# R spring rates, how is the same shock suppose to control such different weight & spring loads - they cant.
Another very important geometry that is key - is the motion ratio of the suspension - that is the ratio between the movement (displacement) of the wheel vs the movement of the shock/spring. because the shock mounts inboard of the spindle, & is mounted at an angle, when the wheel moves 1", the shock moves less, so does the sway bar.
So when you see me working the shock on the bench, remember the car has a mechanical leverage so - as an example, the wheel moves 1" the shock/spring will only move .6" So that says you have a spring rate of 350, your wheel rate is only 210# - same goes for the shock. Jim measured the ratios, so will get them & share here.
When I calculated the sway bar rates, I had to take this into consideration.
As you can see we all have a long way to go, but with feedback from everyone here, I know we can make huge improvements, & make your cars way more enjoyable to drive.
I used the same points that the build manual said to use. Behind the front wheels where the wheel well sheet metal makes a 90 degree turn look rearward you will see were the belly skins mate. The back I used the 2" square tubing just forward of the rear wheels.
I used the same points that the build manual said to use. Behind the front wheels where the wheel well sheet metal makes a 90 degree turn look rearward you will see were the belly skins mate. The back I used the 2" square tubing just forward of the rear wheels.
Ron
I'm still looking for the 4" tube. This is a cut and paste from the recently released e-manual.
"Make sure that the car is at the correct ride height before the alignment procedure is done. Ride height should be 4½” at the front and 4½” at the back measured to the bottom of the 4” round tube with the normal number of people/weight in the car."
R/s
Vidal
CURRENT STATUS: Interior Rework and Bodywork.
GenII GTM #331. Delivered (23/9/10)
BUILD LOG AND WEBSITE: http://gtmbuild.weebly.com/ .
Hey GTMKris, there should be race shops around that have scales, & may be able to help you
kbentzel, not sure if Ron has the bump steer kit, will check with him. A good subject to address for sure
IRON MAN, We set our toe at 1/16 out front - 1/8 - 3/16 IN rear. Guys make sure you don't have toe out in the rear - that's evil!
As far as stability under braking - the sway bar really helps, as will the shocks - but try the toe out in the front first, see if you like it. There is also the brake bias that we will talk about soon, as I suspect that most guys may not have adjusted it.
Also, check your tire pressures - I forgot this---- Always check your pressures first! Try 24 front - 26 rear. Don't be afraid to try different pressures & feel the differences.
Setting your toe - in is easy. We will demonstrate soon - Here is a link for toe adjustment plates: http://www.summitracing.com/parts/LNG-79500/
Pretty cheap to buy, or you can make your own - or just use 2 straightedges.
With you going a complete 180 on the toe settings it has really messed with my head trying to picture the difference in the driveability of the car ! It would seem that when you are going in a straight line you are always going to have some steering reaction from just having your hand on the steering wheel and correcting to keep it in a straight line. Each little pull on my car seems to create some extra pull from somewhere... now if the toe is in and Im correcting left... the car will lean on the right tire thats toed in making it turn left a little extra ... which seems like thats what Im feeling.
By making the toe out, correcting left will make the car lean on the tire thats probably going in an almost straight line.... which would keep it from "pulling extra" ... so with that said.. whats the point of ever having toe in ??? Would it help a car that has too much play in the steering or something ?
Hey Keith, thanks for the info - I think I screwed up on the rear toe specs. We use 3/32" -1/8" toe in - not 3/16". Good to see 1/16" work for you.
It just sucks when you get old!
The 7500 is a great shock and as you have found, they recently came out with a dual adjustable 7500. At $500 each, thats a great deal. Im thinking of them for the MK4 build. One thing I would like to know is what the compression adjustment affects. The rebound is bleed and affects slow shock speed(below 3-4 IPS). If the compression is the same, thats going to be a killer shock for the money!
Def check your compressed and extended length at ride height. Sometimes non remote res shocks are longer than normal to allow for nitrogen space.
The 7500 is a great shock and as you have found, they recently came out with a dual adjustable 7500. At $500 each, thats a great deal. Im thinking of them for the MK4 build. One thing I would like to know is what the compression adjustment affects. The rebound is bleed and affects slow shock speed(below 3-4 IPS). If the compression is the same, thats going to be a killer shock for the money!
Def check your compressed and extended length at ride height. Sometimes non remote res shocks are longer than normal to allow for nitrogen space.
David
Thanks Dave,
I am working with a company called RE Suspension, here in Mooresville. They apparently do a lot of work with/for the NASCAR shops in the area. The tech that I am working with did mention that the shock lengths are usually a bit longer with standard shock bodies, versus remote reservoir shocks.
Ted gave the extended and compressed lengths of the KONI shocks that comes with the GTM kit. But I am starting to think that I need the shock length (at ride height) in order to get a feel for how much compression I need to account for as the car sits.
I am also concerned about the shock body length, since I am installing the "RamLift Pro" hydraulic lift kit. When that lift is activated, it extends the shock by a couple of inches, in order to help clear speed bumps and driveway entries.
If the "droop" is 2" then I would imagine that I will be Ok. But if it's only an inch, I would give up the other inch of extention in ground clearance. And I also don't want the lift to put repeated (negative) pressure against the shock body, rods, and internal components.
I need to go ahead and order the shocks, but I don't have my car to measure the corner weights. Would it be advisable to use the weights that Ron and Ted posted previously in this thread? I know they wouldn't be exact, but I would imagine that it would be a close comparison in weight.
Left Front - 572 # Right Front - 574 # Cross Weight - 1385 #
Left Rear - 811 # Right Rear - 809 # Cross Weight - 1381 #
What else would the technician need in order to set the shocks up for initial use?
Thanks for your help.
Ted / Ron, are you still planning to create a video for setting the toe at the front and rear? These videos are a huge help in understanding the set up process.
Last edited by The Stig; 06-09-2011 at 10:23 AM.
The Stig
Some say, that I only know two facts about ducks, (both being wrong); and that if I could be bothered, I could solve the "da Vinci Code" in 47 seconds...
All I know is that I'm called "The Stig". GTM #0081
I have had good luck with setting the rear bump steer to produce loaded tire toe-in when the car rolls under cornering loads. This combined with static toe-in produces a very "stable" feeling car. The opposite is also true….if the rear loaded tire is experiencing toe-out under cornering loads (either from bushing deflection or bump steer) the car will feel “wickedly unstable”. Maybe someone with a completed car can check it out.
Ciao,
Joel
Working ever so slowly on GTM #269, Twin Turbo SBC, Ricardo, Kit arrived April 5, 2009
I do not have the bump steer kit. That's probably the only thing I didn't get.
Having fun, Ron
What additional benefit does the Bump steer kit give you in the front suspension set up? It looks as though you completely turned you're cars handling around and made it much more predictable, by the addition of the sway bar kit, and Shock & Alignment changes.
The Stig
Some say, that I only know two facts about ducks, (both being wrong); and that if I could be bothered, I could solve the "da Vinci Code" in 47 seconds...
All I know is that I'm called "The Stig". GTM #0081
What additional benefit does the Bump steer kit give you in the front suspension set up? It looks as though you completely turned you're cars handling around and made it much more predictable, by the addition of the sway bar kit, and Shock & Alignment changes.
When the suspension travels, the length of all the arms, in the X axis, parallel to the ground, changes. Some change differently than others. Bump steer is simply that the tie rods for the steering change slightly differently than the upper and lower control arms with respect to the X axis. This causes the upright to "steer" slightly when the suspension goes through it's travel. The steering effects can either toe a car in or out, or sometimes both when the suspension goes through it's travel. The "bump steer kit" simply tries to put the mounting point of the outer tie rod end in a location that minimizes the bump steer throughout the suspensions travel. I have never seen a "perfect" suspension that did not experience at least small amounts of bump steer. You can minimize it, but there will always be some. This is due to the fact that the inner mounting point on the steering rack is neccessarily not a fixed point and therefore the tie rods will always make a slightly different arc than the suspension arms and their fixed mounting points.
The original GTM setup from the factory came with 350 in the front and 450 in the back. Some time later FFR decided to update the the springs combinations and asked the builders to move the rear springs to the front and install the new 750 to the rear. FFR did change their Koni shocks to the newer updated version. We have note seen or tested them.
We at Quick Racing Products are in the processes of trying different shock and spring rate for the best ride possible. We intend to have a performance street and race suspension packages available soon. Regarding the anti sway bar setup, Ted has designed the sway bar system and they are made in house just for the GTM.
On my car I have the OEM suspension bushings with the old Koni shocks. I have the 750 on the rear and it's very predictable and not much more I will do with it except new shocks. I have the custom Bilstein racing shocks with 350 springs in the front. What a difference it made. I'll let Ted talk about chassis setting.
I'll be honest with you guys and gals. When I first drove the it I did not like it at all. It was twitchy almost to the point I was asking myself what the hell did I buy. I had the suspension setup with standard ZO6 specifications. With Ted's help It's bad to the bone. I love the car. I'm amazed on how much the car has improved. FFR you have a winer!
Vidal - 4" round tube? - sounds like a Roadster, not a GTM, they must have made a mistake.
IRON MAN - Everyone seems to think that the "ultimate" capabilities of this car is the most important thing - How much power does it have? - How fast will it go? - How big are the tires?
Don't get me wrong, those are the reasons for building a "supercar" for sure. The problem is we spend 99% of the time just driving down the road. This is where the car come up short - it just needs a lot of development in driveability.
I have felt the same feeling as you described on all the street versions I have driven.
Toe settings have different effects when you are cornering under load, & when you are just going straight down the road. What we are focusing on is directional stability - just driving straight down the road, with no cornering load. This is where irregularities in the pavement wreck havoc on the GTM. When 1 front tire catches a ripple, crack, or a seam in the road, is where the toe out really helps keep the car from following that influence from the road. I know it sounds backwards, but try it, see if you like it. We have tried setting the toe straight up, but found it is better just slightly toed out in the front.
Don't be afraid of trying different settings, that way you can feel the differences. Tire packages, wheel offsets can have a big influence also, just find what your car likes.
I added info in the previous post on toe adjustment.
I would have to agree with Ted that "the proof is in the pudding", and "don't be afraid to try something."
That said, be VERY careful with suspension adjustments. Toe out, historically, generally speaking, usually equates to a "darty" car that turns in EXCELLENT(hence the reason for putting toe out into a race car) but can be a bit difficult to handle under power in a straight line.
As I have said before, you MUST be looking at things dynamically as well as statically when it comes to suspension adjustments. If you don't have the equipment or computers to simulate the motion of your suspension and what certain settings will do for you, then the next best thing is to just adjust and try.
My guess is that if Ron doesn't have the bump steer kit, and this hasn't been checked, then something interesting is happening when the nose lifts under accelleration.
As Ted mentioned, there is really no special equipment involved with this. Matter of fact, I have seen people use old beam scales that they got from feed stores to do car scaling. As long as the capacity is high enough they will work.
Scaling/alignment/ride height settings on a car is something I do before EVERY race event. Not sure you need to even get to the point of scaling your car, if it has the correct components and the adjustments are set correct, but it won't hurt either. Just don't think you HAVE to scale your car to make it work right...you don't. I believe that Ted's bar kit, along with some spring, and maybe shock adjustments are definitely some good additions to the equation.
So far, I still have the yellow Koni's. But I am quietly looking at different coil-over options that will work with the RamLift Pro lift kit.
This is a great thread to get some Real info in terms of set ups that really work, and not speculation.
Great Save Ron!
Last edited by The Stig; 05-23-2011 at 09:38 PM.
The Stig
Some say, that I only know two facts about ducks, (both being wrong); and that if I could be bothered, I could solve the "da Vinci Code" in 47 seconds...
All I know is that I'm called "The Stig". GTM #0081
Ted & Ron,,,
Great info, thank you. OK so a few questions to help me level set. What were the initial settings on the front and rear suspension? Stock Bushings or poly? 750 Lb springs in the rear? What is the rate for the ARB? If you can share it, is that Speedway Engineering bar?
If it fair to say that if the yellow Koni is not enough for 350-450 Lb springs in the front it would be the same for the rear at 750lbs?
In your opinion, would 7 degrees of front caster make a difference in moving to toe-out vs toe-in on the front.
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Chassis Set - Up
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Originally Posted by crash
This is a cut and paste from the recently released e-manual.
