Gt40 suspension configuration and setup

[EGLITOM]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

I think Jac already did that...

Don´t think so.

my opinion,
Yes use the "soft shock stop" before the "hard" spring coil. But if your spring is to soft and therefore the car is overriding it permanently, the soft spring stop will be overloaded as well and than it basically results in the same load to the other suspension components as with a coil bound spring.

Both components need to be designed correctly first and than the suspension component like A arm than need to be designed according to the resulting load.

Again also i fully agree that this resulting load is very depending on driver , road condition, how the car is used (abused) and therefore designing a failsafe component is a big challenge, especially when you have to take in account also economic parameters ( what is your clientel willing to pay for).

Just take F1 with almost unlimited budgets and engineering ressources. How often do you hear of broken suspension components ( just have the picture in mind of the car which broke both front a Arms at braking into a corner). They have all the datas, the software, the knowledge to do it x times better than any of us, but still they fail to succeed, because they take other parameters into the calculation ( like weight, aerodynamics an so on) which compromises the design of the suspension component.

I once testdrove a brand new Audi A4 3.0 Quattro. At parking the car in front of the dealership i slightly hit the boarder of the parking space ( did not damage tire or rim) but the lower wishbone broke. After analysis we coud see that it already had a crack before ( oxidised aluminium). So either that was a manufacturing issue or someone have damaged it before.

TOM

 

[Seymour Snerd]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

I'm not entirely sure of the relevance of what one particular spring rate would be....

That's OK. Here's the relevance. Tom said

most of our beloved GT40s replicase are undersprung because of the comfort desire of all us old farts driving them. Face it, they are racecars and its supsension is concepted like a racecars, short strokes, heavy springs, at least on the front, The rear is much more forgiving

and I was looking for a quantity to attach to that qualitative statement.

 

[Cliff Beer]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

Dear Cliff

if i understand you correctly than you talking about the spring taking a set ( which most springs will do to more or less extend, depending on the quality and springlayout). Practically speaking , this set will not be depending on the preload, it will be depending on the total load put on the spring and its capability to withstand. What you end up is a spring with a different unload lenght ( shorter) and more relative spring winds on this new length ( spring winds per inch will increase) and therefore you get a spring with a lower spring rate.

TOM

Tom,

Thanks, but that's not the point I'm trying (and failing I guess...) to make.

The point I'm trying to make (and which may be a key factor in the bent wishbone that Carlo experienced here) is the following.... If a guy adds a bunch of pre-load to the spring by winding up (ie. compressing) the adjustment nuts supporting the spring perch, then the spring just has that much less working travel. As such, it's just that much more likely to bottom out the spring and thereby transmit a large shock load into the wishbone, particularly if the car is being used hard on say a track day. It could just as easily be a pot hole causing this in daily driving however. The effect of this is made much worse if the bump stop has been removed for some reason.

Why would a guy wind up (compress) the spring like that? To change the ride height of the car or get the four corner balance optimized most often.

As you probably know, the standard drill to try to avoid the above situation is to increase the spring rates (ie. change the springs for stiffer springs) or increase the compression damping, preference typically being increased spring rates rather than damping. As an alternative, sometimes it's possible to use a different length spring in substitution with a corresponding change in spring perch position so that the working travel is maintained, but due to the maximum length of the shock/spring it's not always possible. But that's just an aside really.

 

[EGLITOM]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

Dear Cliff,

Thats exactly the missinformation most people have.

Some of it is true what you say. Under a certain condition some spring coils will bind up but not by adding preload only.
Just think about it. As long as the shock still has some way to move out, turning up the lower spring perch will only change the level of the car. The spring still gets the same load and will not change his length, only it will be positioned higher in relative position to the shock.
What you say is true, if one turns up the lower spring perch to a point that the shock can no further extend anymore ( because it is bottomed out). But as soon this point is reached the height of the car can not be changed any more and any further turn up will only further compress the spring ( and finally lead to coil bind ). But this a absolute no go scenario anyway, because than the shock is way to short.


So again the amount of total load to the spring for going coil bind is not different than without preload. ( as long as your shock has still some negative way to bottom out in rideheight)

Increasing compression dampening to avoid this. I know that people are doing this , but this is not a very good solution for a spring dimensioning issue and will defenitely compromise the sensitivity of a cars suspension. The tire than would need to take over some of the dampening function and in competition will heat up more and have more tire wear.

TOM

 

[EGLITOM]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

That's OK. Here's the relevance. Tom said and I was looking for a quantity to attach to that qualitative statement.

to be honest, i´m suprised by this values of the mk11. Front could be ok, but the rear seems to be quite soft, especially taking in account the weight of the big block engine.

The RCR 40 geometrie is very close to the one of original cars . And i ended up with a 660 lbs/inch front and 570 lbs/inch rear. This said i have a swaybar setup adding to that in cornering and increasing the wheel rate by app 50% in front and 20% in the rear.

If you check this post, you will find the file with the calculations and setup informations.

http://www.gt40s.com/forum/gt40-tech-chassis-brakes-tires-wheels/38553-swaybay.html#post387947

is it to hard and uncomfortable that way? No i don´t perceive it like this. When i drive on our bad backcountry roads against my friends ( most of them driving 996 or 997 GT3 RS´s) i´m able to go faster than they, because there P´s start to spring whereas my car is still able to compensate.
TOM

 

[Seymour Snerd]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

to be honest, i´m suprised by this values of the mk11. Front could be ok, but the rear seems to be quite soft, especially taking in account the weight of the big block engine.

Maytbe the weight of the engine is the reason. I assume the car in question is Claude Nahum's (would someone please post the picture?). If so, the placard presumably reflects the car's current spring rates. I vaguely recall his being informally quoted as saying that compared to a Mk I the weight of the engine in back made the Mk II a bit of a handful. Since Claude actively campaigns the car he may have changed the rates to suit his own preferences. Thus perhaps he has softened the rear to help keep the back "in line."

Or perhaps they were all that way for that reason.

 

[Cliff Beer]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

Dear Cliff,

Thats exactly the missinformation most people have.

Some of it is true what you say. Under a certain condition some spring coils will bind up but not by adding preload only.
Just think about it. As long as the shock still has some way to move out, turning up the lower spring perch will only change the level of the car. The spring still gets the same load and will not change his length, only it will be positioned higher in relative position to the shock.
What you say is true, if one turns up the lower spring perch to a point that the shock can no further extend anymore ( because it is bottomed out). But as soon this point is reached the height of the car can not be changed any more and any further turn up will only further compress the spring ( and finally lead to coil bind ). But this a absolute no go scenario anyway, because than the shock is way to short.


So again the amount of total load to the spring for going coil bind is not different than without preload. ( as long as your shock has still some negative way to bottom out in rideheight)

Increasing compression dampening to avoid this. I know that people are doing this , but this is not a very good solution for a spring dimensioning issue and will defenitely compromise the sensitivity of a cars suspension. The tire than would need to take over some of the dampening function and in competition will heat up more and have more tire wear.

TOM

Thanks Tom, I appreciate the information and explanation but I'm struggling to understand.

For me it's very simple. The top of the shock/spring assembly is attached to the chassis and the bottom is attached to the lower reverse wishbone, nearish to the outboard end. The thing in the middle which supports the car is the spring, supported on the bottom by an adjustable spring perch (with locking nut). Adjusting the position of the spring perch changes the height of the car - adjusting the spring perch "up" moves the shock body (and lower attachment point) down relatively speaking. There's a maximum length to the shock/spring assembly when un-loaded (set by the max piston travel). What I'm calling "pre-load" is the amount to which the spring is under tension when the shock/spring is unloaded via winding up of the adjustable perch position.

If the perch position is wound up (ie spring compressed more) then there's just that much less working travel to the spring. The reduction in working travel can be a big problem where the spring is fully compressed (on a compression bump, say) and made worse if the bump stop is not present.

So, net, what I'm saying is "pre-loading" the spring (via winding up the perch) reduces spring travel and that increases the liklihood of the shock reaching full compression on a bump thereby potentially bending other suspension components. Am I missing something?

Thanks Tom.

 

[Bob Putnam]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

<<Spring coil
Front - 505 lb
Rear - 336 lb>>

Perhaps they've increased the rate in the front to reduce suspension-rise-induced aero lift, and they've put a larger anti-sway in the rear for more adjustability.

I suspect that they keep the overall suspension rates pretty low for driver comfort and chassis longevity. It's a long grueling race, after all.

 

[Jac Mac]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

<<Spring coil
Front - 505 lb
Rear - 336 lb>>

Perhaps they've increased the rate in the front to reduce suspension-rise-induced aero lift, and they've put a larger anti-sway in the rear for more adjustability.

I suspect that they keep the overall suspension rates pretty low for driver comfort and chassis longevity. It's a long grueling race, after all.

Front coil over lower attachment point is inboard from the lower ball joint..therefore front spring travel is a percentage of wheel travel & requires higher rate spring..

Rear coil over attaches directly to the lower w/bone-upright therefore its travel is directly related to wheel travel..

I realise that coil over installation angle has an effect on this but leave that angle out of the equation as it is similar for front/rear & makes its easier to understand the difference in reqd spring rates... lets assume both bottom A-arms are 10" long..therefore front coil over attachment point might be around 7.50" out from inner chassis point...sooo actual wheel rate would be about 75% of the quoted 505 lb = ~378 lb, not that far away from the direct coupled [100%] rear rate of 336 lb is it

 

[Howard Jones]

What Jac said. Listen to Jac.

After all if the load keeps going up something has to fail. A lower bent a-arm is about the safest way to fail. If the shock bottoms out it usually breaks off a mount and you have a flopping around lower a-arm with the tire and wheel attached. That can be a real hard thing to steer.

If the spring runs out of travel then something else must absorb the load. The tire will until its fully compressed and blows out. Again, hard to drive like that.

Or if the tires hold the wheel center can fail. Again, not good.

Wahts left is chassis pieces. Pickup points should hold up the longest but you never know until something breaks. The lower a-arm is more than likely the correct thing to bend first. At least the wheel remains more or less in place.

The purpose of suspension is to suspend the load, bottomed out locked solid isn't suspended, and I can't think of a way to say it clearer except to maybe try placing a pillow on the side of your head and bang it into the wall. Now remove pillow, repeat........... spring................no spring.

 

[Jac Mac]

?..looks like one of the forum moderators has tried to shift this thread...not my doing, but no problem if you want it to kick off further discussion..

 

[Randy V]

Yeah Jac, I've been trying, but the bloody software is kicking my butt..

 

[EGLITOM]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

For me it's very simple. The top of the shock/spring assembly is attached to the chassis and the bottom is attached to the lower reverse wishbone, nearish to the outboard end. The thing in the middle which supports the car is the spring, supported on the bottom by an adjustable spring perch (with locking nut). Adjusting the position of the spring perch changes the height of the car - adjusting the spring perch "up" moves the shock body (and lower attachment point) down relatively speaking. There's a maximum length to the shock/spring assembly when un-loaded (set by the max piston travel). What I'm calling "pre-load" is the amount to which the spring is under tension when the shock/spring is unloaded via winding up of the adjustable perch position.

If the perch position is wound up (ie spring compressed more) then there's just that much less working travel to the spring. The reduction in working travel can be a big problem where the spring is fully compressed (on a compression bump, say) and made worse if the bump stop is not present.

So, net, what I'm saying is "pre-loading" the spring (via winding up the perch) reduces spring travel and that increases the liklihood of the shock reaching full compression on a bump thereby potentially bending other suspension components. Am I missing something?

Thanks Tom.

Dear Cliff

the issue in you not fully understanding is the following.

Assume the following. The lower spring perch is in a givven position; with the cars weight on it, the shock is compressed by lets say 1,5". Lets also assume the compressed spring lenght is than 7" and the uncompressed spring length is 8,5". In this situation we would have no preload on the spring if you lift the car, because the shock will move out 1,5" and therefore the spring will extend to its uncompressed lenght of 8,5". Putting the car down again and therefore loading the spring with the cars weight will again compress the spring by 1,5" ( you could also say the car is preloading the spring).
Now you want to increase the ride height by on inch. WHat you do is winding up the lower spring perch by one inch. The cars height will be changed accordingly , but the spring will still have a compressed lenght of 7", because the load has not changed. Only the shock will be now compressed 0,5" only from full out position. The spring itself will still have the same additional load needed to go coil bind.

Nest step :

Instead of winding up only 1" you wind up 2" instead.
Means the top spring perch is only able to move up by 1,5" than the shock is out of travel. the additional 0,5" of winding up the lower spring perch will compress the spring by the same amount to a length of 6,5". The spring is no preloaded 0,5" more than it would be only by the cars weight itself. BUt here starts the understanding of this preload effect. YOu now would see no compression movement of the shock als long as the additional load is not more then the equivalent to this 0,5" preload . Only if the load is higher a compression movement would start.

in other words, a givven spring always needs the same force to go coil bind, preloading does not change the amount of total force needed. Even if you would compress the spring almost fully the force would not change.

let say the spring in our example has a spring rate of 200 lbs/Inch and has a fully compressed lenght of 5". This spring would need a force of 800 lbs to fully compress it from its 9" uncompressed length. If you would now compress this spring to a lenght of 6" ( in our case. by the load of the car of 300 lbs = 1,5" and additional 1,5" = 300 lbs by winding up the lower spring perch to far so the top spring perch bottoms out and the spring will be compressed additionaly). This spring still needs an additional load of 500 lbs to go coil bind. it will not move for the first 300 lbs of this additional load, because this is the amount of preload given and will start to move in with any load above this 300 lbs with the rate of 1 inch per 200 lbs.

Hope this makes it clearer

TOM

 

[EGLITOM]

Re: Possible Suspension Arm Failure, Racing - Tornado GT40

Front coil over lower attachment point is inboard from the lower ball joint..therefore front spring travel is a percentage of wheel travel & requires higher rate spring..

Rear coil over attaches directly to the lower w/bone-upright therefore its travel is directly related to wheel travel..

I realise that coil over installation angle has an effect on this but leave that angle out of the equation as it is similar for front/rear & makes its easier to understand the difference in reqd spring rates... lets assume both bottom A-arms are 10" long..therefore front coil over attachment point might be around 7.50" out from inner chassis point...sooo actual wheel rate would be about 75% of the quoted 505 lb = ~378 lb, not that far away from the direct coupled [100%] rear rate of 336 lb is it

Exactly, but this wheelrate seems to be on the soft side for me.

Would be interessting to find out the wheelrates of succesfully racing GT40s. As most of midengine cars are prone to understeer ( even more if they are equipped with a Plates style differential lock, which also locks in push and not only in pull) i think one need a more loose rear to compensate for that. Therefore the rear should be stiffer than the front.

My car is set up with a 20% higher wheelrate rear than front ( but also equipped with a torsion style LSD, which allows to run into the turn completly unlocked). So far i could not detect any severy understeering tendencies.

Would be very interessted to share others setups and experiences. Russ, Chris, Ian, your input would be highly valuable.

TOM

 

[Bob Putnam]

The weight on the front is roughly 30% less than the rear, so the wheel rates should be approximately proportional to keep the natural frequencies similar. (Which they're not in this case.) (I did the geometry calculations before I posted.)

Usually street cars run a somewhat higher frequency in the rear to lessen harmonically-induced pitch. They balance roll resistance by compensating with a heavier front anti-sway bar, but that's not necessary on a race car.

So my comment is still relevant.

 

[Howard Jones]

When I originally set up my GTD I was thinking that the stronger spring went in the back so I put 450s on the back and 350s on the front. I went on my merry way and continued to balance the car with roll bar settings. All seamed fine and I was running about 2:18- 2:20 at my favorite track.

Then Bud had a Olin shock guy look at his car from a professional point of view and recommended stiffer springs in the front compaired to the rear. I wondered about this for a while but didn't change my car. As I continued to learn to drive I began to brake the car much harder and and put a lot more power down earlier. Thing's didn't feel as good as before. I was getting a bit more power over steer that I didn't think felt like lack of grip just that the car wasn't transferring weight to the rear very well. As I played with ride height (more front, less rear) it got a little better. Also under very hard braking the front of the car seamed to dart around, not real bad just not nice and straight like before, when I was going slower.

Then the light came on! I put the 450s, in the front and the 350s in the rear, went back to the original ride height settings and picked up 2-4 sec a lap. Very stable under braking now and I can just put the peddle right on the floor off the corners now and it just goes nice and straight like I have wider tires and more grip. On the longest straight I am now forced to use 5th because I am running out of revs in forth. At another track I am again using 5th for the first time on both long fast sections of the track because am getting onto the straights much faster than before.

The thing is, it feels easier to go faster now to the point that I would like to add power (50-60HP) if I was going to continue to run this car on track.

I can't quote the math but I know what feels good and these cars like stiffer springs in the front. No doubt about it.

 

[Jac Mac]

According Ronnie Spains book..
MK1 weight distribution ~46% F / 54% R @ 2000 lb
MKII weight distribution ~38% F / 62% R @ 2682 lb.[MKIIB 2505 lb ]

 

[EGLITOM]

Howard,

Thanks for posting this valuable input. Now it would be interessting what this setup is equaly to in terms of wheelrates.
Would be great if you could take the chart from the link attaced , fill in your datas and post it. Think this would be a great help for all those GTD´s out there.

http://www.gt40s.com/forum/gt40-tech-chassis-brakes-tires-wheels/38553-swaybay.html#post387947

@ Jac Mac

Thats my weight distribution ( car with about 20 gls fuel, plus me on board)

FRONT REAR
Weight 1092 lbs 1360 lbs
Distribution 45% 55%
Wheelrate 534 lbs/inch 659 lbs/inch (incl swaybar rate)
Wheelratebalance 45% 55%

THanks
TOM

 

[Howard Jones]

Those of you with GTD's should have a look at Derek Bells website. Check out his ideas on GTD suspension geometry design problems. Mainly you will need to correct the rears camber gain issue.

My GTD uses the bottom A-Arms but I have made new fronts so I can adjust camber and caster easily. At the rear I made new top links, again to adjust camber, and moved the top inboard pickup point per D Bells recommendations. If I ever get to it I will make new trailing links with rod ends on them instead of the standard bushings.

I am using QA-1 shocks and a home made rear anti roll bar that can be adjusted. At the front I cut off the hole and welded on a three hole tab so that I can adjust it also.

So there's not much standard on my GTD

I just looked for the website I am talking about. I can't find it. Maybe one of the old timers about here can help.

I'll take some pictures in the next few days of my GTD.

 

[Mike Pass]

A couple of things popped into the remains of my brain.
I was always under the impression that spring preload was when you put a longer spring on the shock that there was space for. i.e. putting a 10" spring onto a shock which had 9" between the perches. In this way the spring was preloaded. This was done so that a soft spring would take the weight of the car at a sensible ride height. The spring would need to be compressed by spring compressors so that they could be installed.
On the GTD the issue on the front suspension is not so much the camber being out but that the KPI or king pin inclination is too shallow. This can only be corrected by changing the uprights (and by implication the wishbones also). There is also an issue with the lack of Ackerman on the standard setup (actually has anti-ackerman) which causes tyre scrub at high steering angles and which can only be corrected by changing the uprights. The Mk3 Granada/Scorpio uprights can be fitted by using a "mushroom" in the top socket where the Macpherson strut used to go and making up a new set of wishbones with appropriate swivel joints.
Cheers
Mike