Possible Suspension Arm Failure, Racing - Tornado GT40

[Seymour Snerd]

I think there is a pic of a gold MK11 at le mans? in the gallery somewhere with a placard beside it with the spring rates on it

Just looked, couldn't find it. Might it be somewhere else?

 

[Jac Mac]

Have been looking myself, I know I have posted on another thread in the past, cannot remember what it was in relation to though! will post it if /when I find it, have checked all my own saved stuff as well.

 

[Randy V]

I'm not entirely sure of the relevance of what one particular spring rate would be unless you knew the rest of the setup and the track it was configured for. I can't tell you how many times I changed spring rates for various tracks all on the same car. Right hand turn bias vs left hand, high bank and smooth vs. flat and technical, high speed vs. medium speed, aero no aero, etc.. Would we not have expected the same differing setups of these original cars and teams?

 

[EGLITOM]

Tom,

Yes, technically speaking you're exactly right - the amount of pre-load doesn't affect the point of coil bind. However, practically speaking you (and anyone who knows the minimal basics) knows that it does. Why? When a guy winds up the adjuster on the spring perch so that there's less working travel in the spring (commonly done to lower the car....), and re-uses the same spring, then the likelihood of hitting the point of coil bind is much higher. That's how it works practically speaking.

I think everyone understands that. And I think you understand I wasn't suggesting otherwise.

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

 

[EGLITOM]

I'm not entirely sure of the relevance of what one particular spring rate would be unless you knew the rest of the setup and the track it was configured for. I can't tell you how many times I changed spring rates for various tracks all on the same car. Right hand turn bias vs left hand, high bank and smooth vs. flat and technical, high speed vs. medium speed, aero no aero, etc.. Would we not have expected the same differing setups of these original cars and teams?

Exactly,

thats what i pointed out also. One have to find the springs for his driving conditions. Of course if you are not fighting for the last 10th , the spring rates for a givven car will be not to far of from one car to the other.

Hey Randy, think this is a interessting discussion, but don´t want to completly destroy Carlos´s Thread. What about asking an admin (wink) to relocate those post to a new tread?
TOM

 

[Fred W Ballinger]

I think there is a pic of a gold MK11 at le mans? in the gallery somewhere with a placard beside it with the spring rates on it

I have the pic saved. It says

Spring coil
Front - 505 lb
Rear - 336 lb

not a typo, they are non round numbers

Cheers

Fred W B

 

[Randy V]

Exactly,

thats what i pointed out also. One have to find the springs for his driving conditions. Of course if you are not fighting for the last 10th , the spring rates for a givven car will be not to far of from one car to the other.

Hey Randy, think this is a interessting discussion, but don´t want to completly destroy Carlos´s Thread. What about asking an admin (wink) to relocate those post to a new tread?
TOM

Good idea Tom. I tried to do that once already this AM and failed. Will attempt again after more coffee.. :stunned:

 

[Keith]

don´t want to completly destroy Carlos´s Thread.

I think Jac already did that...

 

[EGLITOM]

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]

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]

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]

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]

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]

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]

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]

<<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]

<<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

 

[Randy V]

I am closing this thread for now and would ask you all to carry your chassis and suspension setup discussion to this thread where I have finally managed to capture many of these posts to.

http://www.gt40s.com/forum/gt40-tec...8719-gt40-suspension-configuration-setup.html