Very Basic Rules Of Thumb Regarding GT40 Rear Suspension

[Bob Childress]

Hello Mr. & Mrs. GT40 and All the Ships at Sea,

I know there is a lot of “technical muscle” resident in the members of the Forum and I need a little help. I would like to understand more about how the GT40 rear suspension is supposed to work. Is anything here wrong, dangerous, or just stupid? Any comments, clarifications, or corrections to the follow statements would make my day.

A. The shock is supposed to limit the travel of the suspension both up and down since no chassis mounted bump stops are practical on our cars.

B. The spring on the shock is supposed to always have some compression even when the shock is at maximum length (although the compression does not have to be very much, slight compression is ok).

C. At rest, when the car suspension is carrying the static weight of the car, and the car is at its correct ride height, the available suspension travel upwards should be about twice the distance of the available suspension travel downwards. The exact ratio here is not as critical as the concept that, especially for pavement based race cars, sufficient suspension compression travel upwards is critical and the distance unloaded suspension can drop should restricted.

D. If the shocks only have 2.5 inches of available range of movement before reaching the bump stops, the shock should be mounted so that when the car’s suspension is carrying the static weight of the car, and the car is at its correct ride height, the shock can compress about 1.5 inches and extend about 1.0 inch. The compression of the bump stops must be added to this upward suspension travel distance.

E. The rubber bump stops are to limit the travel of the suspension to minimize or prevent damage. They are not intended to be a routine part of the cars working suspension. They are only for unusual bump events that are not frequently encountered.

F. If the shock does compress enough to reach the rubber bump stops, the compression rate of the bump stop is intended to be as smooth and proportional as possible without further disrupting the control of the car. To this end, shock absorber manufacturers have calculated the required length, shape, and density of the bump stops supplied with there products. Reducing the length of the manufacturer supplied bump stop would probably make the event more abrupt and non-linier and compression may not be sufficient to actually slow the suspension to a stop before other parts are impacted and damaged which defeats the purpose of the bump stops.

I know this is pretty basic stuff but thanks for your help.

Bob

 

[John W]

I'm no expert, but my answers would be:

a) Yes.

b) Yes, its called preload. On adjustable platform shocks you can adjust this preload to alter the rideheight.

c & d) Yes, although I've never seen any fixed figures for ratios. Its normally adjusted by spring rates and preload, and may vary for different tracks.

e) bump stops will limit suspension travel, but are actually to try to stop damper damage by crashing the rod into its endstops, or damaging the seals.

f) I am not sure of the design of the bump stops but would guess they have some proportional compression design. Note that the damper supplier doesn't know the weight of your car, or whether your going to be jumping off ramps and always hitting the stops, and unless they knew this couldn't design them to suit your actual application.
I'd therefore suggest the bump stops are a general design.
Shortening the bump stops could result in damp[er damage, yes.
I would suggest if you have to shorten the bump stops either you have the wrong length dampers, your trying to run an incorrect ride height for the suspensiuon as it was designed, or your damper pickup-points are incorectly placed if you have insufficient travel.

As I say, I'm no expert. These are just my thoughts based on experience from playing with cars & bikes over the years.

Cheers,
John.

 

[Bob Childress]

Hi John,

I understand you comments, totally reasonable and prudent for sure. I also thought items C and D would be the most difficult to resolve. Admittedly there are actually a lot of suspension differences between the GT40 replica brands. Still I would think that all GT40s are constrained by common gross measurements like the body, fenders, tires, and ride height similarities.

Nevertheless, if we assume the shocks and springs are correct for a car, how does anyone (including the designers) choose the distance for suspension travel up and suspension travel down from the static resting position? Does anyone know the ratio of the up to down movement for their make of GT40? How would you go about getting this information?

Bob

 

[Bob Putnam]

<<C. At rest, when the car suspension is carrying the static weight of the car, and the car is at its correct ride height, the available suspension travel upwards should be about twice the distance of the available suspension travel downwards. The exact ratio here is not as critical as the concept that, especially for pavement based race cars, sufficient suspension compression travel upwards is critical and the distance unloaded suspension can drop should restricted.>>

Why? If the suspension bottoms out on rebound, there is an immediate drop to zero tire load and a corresponding drop in traction. There's no advantage to limiting travel when the spring is still loaded.

 

[John W]

Sorry Bob no idea.

its an interesting question, and not one I've really given a lot of thought to before...

I can see it could be very complex though, worrying about spring rates, compression/rebound damping, springs not becoming coil-bound, etc. I bet it would take a lot of trial and error, lots of road testing, and many hours trying different settings.

If you have a car you can determine the ratio by measuring the damper fully extended (car jacked up with wheel unsupported), the length of the damper rod vs the damper body (not all the rod may fit in the body so possibly length from top of body to underside of bump stop), and then the static sag length (to get the rest position).
How you would go about this in the first place though, god knows.

All we need now is for someone who knows to enlighten us /ubbthreads/images/graemlins/smile.gif

Cheers,
John.

 

[Bob Childress]

Bob and John, thanks for your critiques.

I guess this does smack of a theoretical exercise, but it is really just a practical issue I am trying to answer.

Depending on the tire diameter and the ride height (or ground clearance) of the car, the static resting position of the suspension and the ratio of the available compression and extension in the shock absorber will change.

So, assuming a car is correctly set up and otherwise has no other restrictions, the only question I am trying to answer is: From the static suspension resting position, about how much of the shock absorbers stroke needs to be available for compression before touching the bump stops? Too little would result in hitting the bump stops too often. Too much would reduce the distance that the suspension can drop and just turns the question around to one about how much drop is needed?

Like John said, probably the only way to independently get this information is to do a lot of track testing. I do not have that resource. But, I do think forum member who have good handling cars could easily provide a range of numbers. Our cars are similar enough that I believe the numbers can be compiled and a practical guess made from those.

All I really need to know is, with the car at rest, what is the distance from the top of the shock absorber cylinder to the bottom of the bump stop on the shaft of the shock?

Bob

 

[John W]

Bob,

knowing that distance is meaningless without also knowing at least the spring rate, its length, and the damper settings.
Having the same length of movement but with softer springs will do you no good.

I suppose if you knew the corner weight you could try to adjust accordingly, but that's just back to trial & error again...

Cheers,
John.

 

[Harry Hudson]

Hi folks.

After reading the posts on GT40s.com for the past two or three years I have finally joined.

I have spent the past 15 years, on and off, building my KVA C type GT 40. Being a KVA I haven’t had the benefit of any manufacturers help and have thus had to design and make or modify almost every component. I don’t mind doing that as, for me, it is part of the enjoyment of owning one of these great cars, although it does get frustrating to be continually building and stripping the car every time a newly designed part needs test fitting

I have always had an interest in suspension design and, for what it’s worth, would like to add my comments to Bob Childress’s post.

A) My first post and I’m disagreeing already!

If we separate the combined Spring/Shock unit into its two components then, in my opinion, the function of a Shock is to quickly damp out the oscillations of the Springs. These undamped oscillations can cause major instability to the vehicle if they are allowed to continue too long.

It is the job of the Springs to limit the travel of the suspension. Stiffer Spring rates will give less travel and vice versa.

B) The Springs do not always have to have some compression (preload). I agree with John W on his adjustment comment and would add that it is preferable, if possible, to design the suspension system so that the Spring/Damper assembly does not have any preload as this makes it much easier, and safer, when fitting the Springs to the Dampers.

C) I agree with your comment Bob, but would add to your final sentance that the distance the suspension can drop (or bump) should be restricted by the spring/damper assembly reaching its max. (or min) length not by any suspension component e.g. ball joint etc. running out of movement and going solid. The instantaneous shock loads this will generate can cause catastrophic suspension failure, with consequences to horrific to contemplate. The original front suspension that came with my KVA was so poorly designed it would have done this as it had only 1inch of droop movement at the road wheel whereas the bump movement was over 8 inches, which would have had the tyre rubbing the body away long before it had run out as bump travel.

The above only applies to the ultimate travel of the suspension e.g. when jacking the car up. The suspension bump and droop travel during normal use should never reach these limits.

D) Again I agree, and your bump to droop ratio seems about right. My 40 has yet to see the road but I have designed my suspension with a similar ratio. If I find that the static ride height I plan to use (5 inches ) needs to be changed this will change the dampers bump to droop ratio. In that case I may need to change the location of spring/dampers upper mounting point. If it needs too much adjustment I may even need to go back to the drawing board, which brings me to your main point about the distance between the top of the shock absorber cylinder and the bottom of the bump stop.

As John W summarized so neatly, I’m afraid it is much more complicated than that. The total usable stroke of the damper is found during the design stage, which begins with a chosen suspension frequency. I have designed my 40 for Road and occasional Track Day use, and am using a rear frequency of, from memory, 105Hz. All the other design aspects of the suspension, including damper bump length stem from this and brings in such items as sprung and unsprung weights, the mechanical advantage of the suspension, the position of the roll axis etc. etc.

Tom

 

When I was purchasing shocks, the company I talked with said as a general rule of thumb, at rest with all its weight on the wheels, the shock should be approx 40% compressed. Never mind that I dont 100% trust the guy as he has been promising me my parts for the last 2 years and I only have half of them.

 

[Bob Childress]

Hello Tom and Eric, and Hello Again John,

Tom - If you have been working on a KVA for 15 years then I know I am hearing from a true admirer of GT40s. My wife asks me when I will be done with my car. I tell her if I am lucky, never. I have never been to Essex but I was told to stay at Hadleigh Castle even though the rooms are a bit breezy.

Your comment about item (A) is a fair caught. I am so focused on the position issue that I did not describe the primary purpose of a coil over.

Eric – I have also heard the 40% number and that for racing cars it should be reduced. (I know waiting for parts is tedious and I hope you receive yours soon.)

John and Tom, both of you have reminded me that to accurately answer this question really dependents on a lot of other specific details. Nevertheless the calculations needed would all be based on GT40 cars with very similar shape, dimensions, curb weight, weight distribution, tire and fender well clearance, and ride height. And, for the most part, I will bet most owners use similar shocks and spring rates recommended or supplied by each chassis manufacturer.

All I am hoping for is that a few forum members volunteer this measurement. Some basic idea of the range and central tendency of the measurements would be helpful to anyone doing a scratch build or revision. If ten owners with well sorted cars report only 1 inch of compression before the bump stop is touched, that would surprise me since it intuitively seems to be a very small range of available movement. If they reported 4 inches that would also be surprising. Unfortunately, <font color="black"> </font> I think that both of these extremes are routinely to be found.

I guess another way to get a handle on this issue is to ask for the measurement from a real MK1 or the calculated number from someone doing a chassis design. Or maybe Goram or Russ will drop in. I wish I was at a club meeting or event where I could see a dozen cars and get this information directly. Maybe a GT40 club will take pity on me and secretly donate a few numbers in the private mail.

Bob <font color="blue"> </font> /ubbthreads/images/graemlins/crazy.gif

 

[John W]

Sorry Bob,
I thought you were trying to get to the bottom of how these figures were arrived at, rather than asdking for someone to tell you what measurements their car gives.

I can't help you with that (not that I think I've helped much above either /ubbthreads/images/graemlins/smile.gif )

Good luck with your quest.

Cheers,
John.

 

[Chris Duncan]

""A. The shock is supposed to limit the travel of the suspension both up and down since no chassis mounted bump stops are practical on our cars.""

correct, the coilover shock is the bump stop. A separate stop is not pratical nor necessary. Any items that have a dual purpose save weight. Why have that rigid upper shock mount if you can't use it as a bump stop also.

""B. The spring on the shock is supposed to always have some compression even when the shock is at maximum length (although the compression does not have to be very much, slight compression is ok).""

correct, at least some compression to keep things from rattling around at full droop. High rate springs may have less, some lowering kits have small lower rate springs just to achieve this, but have never seen this on a kit car.

""C. At rest, when the car suspension is carrying the static weight of the car, and the car is at its correct ride height, the available suspension travel upwards should be about twice the distance of the available suspension travel downwards.""

ride height at 33 to 40 percent from full extension is about right.

""The exact ratio here is not as critical as the concept that, especially for pavement based race cars, sufficient suspension compression travel upwards is critical and the distance unloaded suspension can drop should restricted.""

not really restricted, I wouldn't want any less than 33 percent, although you could probably get away with it. You want to keep the tire on the road through a dip, road level change or extreme body roll.

""D. If the shocks only have 2.5 inches of available range of movement before reaching the bump stops, the shock should be mounted so that when the car’s suspension is carrying the static weight of the car, and the car is at its correct ride height, the shock can compress about 1.5 inches and extend about 1.0 inch. The compression of the bump stops must be added to this upward suspension travel distance.""

Your bump stops are unusually long, I would sort of ignore them in your equation. What ever the total shock travel is I would start at 33 percent of that. Looks like it might be 5" so about 1-3/4" compressed.

""E. The rubber bump stops are to limit the travel of the suspension to minimize or prevent damage. They are not intended to be a routine part of the cars working suspension. They are only for unusual bump events that are not frequently encountered.""

As long as your bump stops are I would say that's incorrect. I would think they'd hit in everyday driving. Aldan's bump stops measure about 3/4". Yours are sort of like a variable rate spring.

""F. If the shock does compress enough to reach the rubber bump stops, the compression rate of the bump stop is intended to be as smooth and proportional as possible without further disrupting the control of the car. To this end, shock absorber manufacturers have calculated the required length, shape, and density of the bump stops supplied with there products. Reducing the length of the manufacturer supplied bump stop would probably make the event more abrupt and non-linier and compression may not be sufficient to actually slow the suspension to a stop before other parts are impacted and damaged which defeats the purpose of the bump stops.""

The springs should be stiff enough to avoid full bottom, the bump stop is just to cushion a full bottom event like a chuck hole. I wouldn't cut them if they came with the shock, they are part of the design.

When fitting coilovers that didn't come with the car it's a good idea to remove the spring (and bump stop if possible) and put the suspension through full shock travel. Look for clearance everywhere and check for suspension joint binding. Tire to chassis and body, wheel to suspension/ brakes, lines. suspension to chassis. In the front turn full lock both ways and simultaneous full shock travel. Suspension joints should bind after shock has bottomed out, not when. If you have joint bind right at full travel you may get breakage on an extreme event.

 

[Bob Childress]

Thanks, Kalun_D and John,

I can tell this thread is moving toward closure. Although I would still like to have a few actual measurements, the proportions suggested here and the other comment will definately help me with my job.

If anyone else has missing information that should be taken into account, please make a short post as I will probably make some suspension adjustments next week.

Bob

 

[Harry Hudson]

Bob

You asked for some sample bump distances so I have measured mine. It is 1 inch, at a ground clearance of 4.5 inches. You wanted measurements from “well sorted” 40s and as I mentioned before, my one is not quite finished. I don’t know if this dimension will change when I have put some road and track mileage on the car but I don’t think so as the mechanical linkage is designed on well proven principles and will almost certainly stay the same.

I must admit I was surprised that the distance was only 1 inch until I checked my design notes and was reminded that, with a Mechanical Advantage figure of 1.6, this gave a wheel bump distance of 1.6 inches i.e. a reduction in ride height to 2.9 inches before the bump stop would come into play. The bump stop is approx. another inch long so, under the most extreme conditions (which I hope I never experience!) the minimum, designed, ground clearance will be just over one inch.

A couple of other points before I finish, firstly, in my original post I mistakenly said that my rear suspension frequency was about 105 Hz. This should have been 105 cycles per min, which would be 1.75 Hz. 105Hz would have given a rather too firm ride!

Secondly, I wonder if one of the far more knowledgeable people than I on this site could explain to me why every book I have ever read on suspension design uses the term Mechanical Advantage when calculating the relative movements of the suspension. This is defined in Mechanical Engineering Text Books as Load/Effort whereas the correct term should surely be Velocity Ratio? It may seem a pedantic point but, if I am correct, why isn’t the correct phrase used?

Tom

 

[StuartB]

Bob, when reading other people figures always try remember that there`s a lot of differance between suspension travel and shock travel and a lot can be changed by `the angle of dangle` though we try to keep it sensible.
The shock is considered 100% effective when verticle, 98% effective at 10 degrees and 86% at 30 degrees, avoid going any further as it all gets a bit crap after that.

 

[Bob Childress]

Thanks again Tom and to you too Stewart,

I am feeling good about getting some actually rear suspension numbers offered here. And, I have also received a few direct emails with measurements. Several people have reported only 1 inch of shock absorber travel in compression. The judgments about 30-40% proportion and the effectiveness of a shock at varying angles both sound about right and are exactly the sort of comments I had hoped to get.

And, apparently, just like a few of you, I have been working the problem from both sides and have come to appreciate that the problem is very complicated.

On the suspension rise side there are constrains of clearance for the suspension to rise without touching the wheel wells, the range of free upwards suspension travel, and reduction in ground clearance as the suspension is compressed. Clearly the shock needs to be located to function within this range and to stop the travel of the suspension before it reaches any of these limits.

On the suspension droop or drop side, not much advise has been offered but my first impression is that, intentional or not, suspension drop is the stepchild of suspension rise. Whatever is left over and not used by rise becomes the drop.

If we leave the suspension rise or compression side of the problem, then how much rear suspension drop is reasonable or desirable? On some cars, maximum suspension travel downwards is limited by the trailing arm as it exits the chassis. The extension of the shock absorber stops the suspension before the trailing arm is touched. Is this a default standard? Would less drop be better? Do GT40s need more that 1 to 2 inches of drop?

Bob

 

[Bob Childress]

(A picture of the trail arm as it exits the chassis with the suspension at full drop.)