Ride Height, Rakes and Turkeys

[nrider42]

New to formula vee. I am installing an adjuster on by protoform p2 because I want to. What is a goal for ride height and do you run rake? Are there any specific guidelines about the turkey leg positions when the car is in a neutral position? The problem expands quickly when I start thinking about suspension travel. I guess I am just wondering if the community has settled on any general setup constraints. This could be a big topic, I just felt like chatting about it.

Thanks,

Nate

[Daryl DeArman]

New to formula vee. I am installing an adjuster on by protoform p2 because I want to. What is a goal for ride height and do you run rake? Are there any specific guidelines about the turkey leg positions when the car is in a neutral position? The problem expands quickly when I start thinking about suspension travel. I guess I am just wondering if the community has settled on any general setup constraints. This could be a big topic, I just felt like chatting about it.

Thanks,

Nate

My goal was to run the front as low as I could on my FV without bottoming. Smoother tracks I could run lower.

As to rake, the rear ride height is dictated by desired camber in rear.

If your turkey legs are level at ride height bumps will be harsher, hitting any curbs with the front wheel will be more upsetting.

[Amon]

Check with Ray Carmody in New York as he use to be involved in building and setting up these cars.

[S Lathrop]

This question of what position is best for the front trailing arms (Turkey Legs) is something that I think needs to studied carefully.

When I set up a FV years ago I went for horizontal.

Just now, I did some very quick and dirty sketching and I think if I were setting a car up today, I would go for a horizontal or a bit higher. The reason is that as you go lower, the angular change of the trailing arm and thus the rotation of the spring increases with each increment of ride height change. This results in a rising rate front suspension. And rising rate suspension is something I avoid because it does not help tire grip. And rising rate suspension on one side implies that in droop the rate declines as you roll, thus the roll is increased a bit because of the change in the spring rates from the inside to the outside of the car. What actually happens is that the front jacks up in this situation.

This issue may not show up on any single lap but over the course of a race, the little bit of rising rate you get running the front low will increase tire stress and ware.

This advise is not something I learned running FVs. I has come from decades of working with FF and F2000 cars. It may be something that you can ignore in a FV.

[Daryl DeArman]

This question of what position is best for the front trailing arms (Turkey Legs) is something that I think needs to studied carefully.

When I set up a FV years ago I went for horizontal.

Just now, I did some very quick and dirty sketching and I think if I were setting a car up today, I would go for a horizontal or a bit higher. The reason is that as you go lower, the angular change of the trailing arm and thus the rotation of the spring increases with each increment of ride height change. This results in a rising rate front suspension. And rising rate suspension is something I avoid because it does not help tire grip. And rising rate suspension on one side implies that in droop the rate declines as you roll, thus the roll is increased a bit because of the change in the spring rates from the inside to the outside of the car. What actually happens is that the front jacks up in this situation.

This issue may not show up on any single lap but over the course of a race, the little bit of rising rate you get running the front low will increase tire stress and ware.

This advise is not something I learned running FVs. I has come from decades of working with FF and F2000 cars. It may be something that you can ignore in a FV.

I was more concerned about what happens when you hit a curb. When the turkey leg is horizontal and you encounter a bump the rotation of the turkey leg is forcing the tire forward into the face of the bump. If the turkey leg is angled slightly downward (beam end slightly higher) the opposite happens. I would rather have the turkey leg angled slightly from horizontal at ride height and raise the beam mount on the chassis to get the ride height where I want it.

If you DO set the ride height with the lower leg horizontal at least you don't transition through the horizontal during the bump travel.

[S Lathrop]

I was more concerned about what happens when you hit a curb. When the turkey leg is horizontal and you encounter a bump the rotation of the turkey leg is forcing the tire forward into the face of the bump. If the turkey leg is angled slightly downward (beam end slightly higher) the opposite happens. I would rather have the turkey leg angled slightly from horizontal at ride height and raise the beam mount on the chassis to get the ride height where I want it.

If you DO set the ride height with the lower leg horizontal at least you don't transition through the horizontal during the bump travel.

With the trailing arm horizontal, the wheel will move forward about .085 in. for 1 inch of bump. While this does not seem like a lot, if it happens fast, then that also will hinder tire grip along with the rising wheel rate of the springs. Double whammy.

I had not even considered that issue. But I certainly think is is important to take into account. Thanks for the input.

[Matt Clark]

Something to consider... if you simply lower the car by slamming the nose & having a lot of angle in the turkey legs, and not drop the rear of the car too (via actually raising the engine/trans with new mounts), castor will change & can cause some rather discerning shimmy, especially under braking.
I run my turkey legs level, and overall chassis rake is then a resultant of whatever rear camber I run.

[nrider42]

Caster...hmmm...this is wonderfully complicated. Caster seems to be set by the angle that the beam mounts to the frame itself. I guess you could shim the beam?

Great posts on the geometry of the turkey leg. I need to draw some pictures. I think I will just tack the adjuster in and then run the suspension through its full range of motion and measure some things.

Thanks,

Nate

[Daryl DeArman]

Caster seems to be set by the angle that the beam mounts to the frame itself. I guess you could shim the beam?

Yes. A few aftermarket choices exist. Like this: https://vwparts.aircooled.net/Caster...r-shim-kit.htm

[Matt Clark]

Caster...hmmm...this is wonderfully complicated. Caster seems to be set by the angle that the beam mounts to the frame itself. I guess you could shim the beam?

Great posts on the geometry of the turkey leg. I need to draw some pictures. I think I will just tack the adjuster in and then run the suspension through its full range of motion and measure some things.

Thanks,

Nate

Yes. A few aftermarket choices exist. Like this: https://vwparts.aircooled.net/Caster...r-shim-kit.htm

We just stack 1/16" or 1/8" aluminum strips behind it.

As far as full range of motion, you have to also account for the spring twist once the car is on the ground. One beam I had an adjuster added to did not account for that, and I ran out of adjustment just sitting there static. Had to disassemble, cut it apart again & rotate it more.

[problemchild]

Some people don't worry about the turkey arm angle and pin the front to the ground with droop limiters, preload, and bump rubbers/packers. If you have lousy suspension geometry, but limit the travel, the effect of the lousy geometry is reduced, in both ends of the car. I am not promoting the concept but people are doing it. I like to get the turkey legs level and use limiters to minimize travel.

[nrider42]

What do you use for limiters? The horns are still on my beam can you utilize those?

[S Lathrop]

The best I have found are the bump stops made by Dynamics and available through Carl Haas Auto. They come in 2 diameters and 3 different spring rates. Stack them us and you can get almost infinite different spring rates.

I would use these on the front shocks. I would not use the stock bump stops.

Setting the front up to run on good quality bump stops is a very good approach to FV handling.

[Daryl DeArman]

What do you use for limiters? The horns are still on my beam can you utilize those?

Shock absorbers with 18metric crap-tons of rebound dampening work. I was extremely happy with my Performance Engineered Products pair. Not a name that will come up in many road-racing circles, but the man is legend in other disciplines.

[problemchild]

What do you use for limiters? The horns are still on my beam can you utilize those?

This is what I used. The vertical limiter rod slides freely through the horizontal rod end. I would often run spacers of different sizes under the top nut so I could quickly change the limiter settings.

Most people just put internal spacers in the shocks. That does not make for quick changes.

[Daryl DeArman]

I love simple, effective and easy to tune/adjust! ^^^^^^

[nrider42]

I agree...this looks like an elegant solution. I might borrow it. Thanks for sharing.

[jpetillo]

Daryl, I was just reading this thread now and saw your posts on the torsion arm angle and harshness of a curb hit. You mentioned that you lower the front end as much as you can. When you do that two things happen. The effective spring rate is increased as Steve Lathrop mentioned, but also the tire wants to move forward much more for a given vertical deflection than compared to when the turkey leg is parallel to the ground. (I'm assuming that you're not comparing to angling the turkey legs down.) Am I picturing/interpreting what you're describing properly?

I agree with Steve Lathrop about the overall effect will be to raise the front end during cornering when you start with the torsions arms pointed up, since it is a rising rate when run like that. I'm adding nothing to this argument - just noting a good observation by Steve.

The other issue as Steven Saslow brought up many years ago on the FV Interchange, when the legs are pointed up there is a anti-dive effect under braking. We did a calculation on it back then to show how significant the effect was. Another good observation on this topic by Stephen. John

[S Lathrop]

This thread is really tuning into a great FV front geometry discussion.

jpetillo is spot on about anti dive. I have done back to back testing of anti dive and rising rate bell cranks. I don't use either on any of my cars. The first geometry fix that Zink and I made to the Zink Z10 FF was to eliminate the front anti dive and that was in the mid 1970's.

Some time ago, I modeled the FV front suspension in Bill Mitchel's WinGeo program. I use Mitchel a lot and I have found it to be very enlightening. I had always heard that the front roll center of a FV was at ground level. Mitchel shows the front roll center to be 13 inches above the ground. That is close to the rear. This does explain why the front of the car wants to jack up in corners, very much like the rear, and why droop limiting works so well on the front of a FV.

Decades ago, I had built a system to limit front suspension travel. This was on the Zink Z12. Steve Oseth was testing the car at Atlanta. At the end of the day, he had changed the setup so it was limiting the droop only and he was at the limit of what I had made. The system was very close to what has been posted here. I never pursued the droop limit front beyond that car and that test. I was covered up building the push rod bell crank zero roll systems for the rear of FVs. I produced close to 00 of those kits.

My bet is that you could come up with a droop limiting system that uses bump stops in droop and no shocks. The front suspension system under loads has enough friction to do all the necessary damping, the bump stops in droop would stop the droop limiting from causing a loss of grip when the limit was bumped into.

Just as a confession, my last FV was built in 1997. The car has been to a test track once. The design was a blending of the Z12 and the 84 Citation FV. There were also some new ideas thrown in for fun. the roll bar structure is based on what I have done on the FF and other cars since 1994. For engine cooling, you can look over the from beams and actually see the cylinders all the way to the case, not just the heads. At the rear, I have incorporated some ideas that have worked very well on the push rod bell crank cars I have done for decades. It would be fun to see this car on the track being developed, as my other FV projects had been.

[problemchild]

I believe the front limiters help combat rear squat, and resultant low-speed power-on push, as much as they impact the lateral dynamics of corner entry and mid-corner grip. Considering all the negative aspects of FV suspension (bump-steer, roll steer, link pin slop, steering box slop, lack of front camber, component deflection, etc) I never got excited about the fore/aft movement of the front wheels during ride height change.

[Daryl DeArman]

Daryl, I was just reading this thread now and saw your posts on the torsion arm angle and harshness of a curb hit. You mentioned that you lower the front end as much as you can. When you do that two things happen. The effective spring rate is increased as Steve Lathrop mentioned, but also the tire wants to move forward much more for a given vertical deflection than compared to when the turkey leg is parallel to the ground. (I'm assuming that you're not comparing to angling the turkey legs down.) Am I picturing/interpreting what you're describing properly?

I had different beam mounts fabricated and put a ride height adjuster in my beam. I ran the front end as low as I could without bottoming and still had the turkey legs angled so that the wheel is traveling rearwards during the first inch or two of bump travel.

Your point about the tire wanting to move forward into the face of a bump(curb) was exactly the point I made to Steve Lathrop about running the leg parallel at ride height.

[Hardingfv32]

....Bill Mitchel's WinGeo program. I use Mitchel a lot and I have found it to be very enlightening. I had always heard that the front roll center of a FV was at ground level. Mitchel shows the front roll center to be 13 inches above the ground. That is close to the rear. This does explain why the front of the car wants to jack up in corners, very much like the rear, and why droop limiting works so well on the front of a FV.

1) What measurements to you have to indicate the FV front suspension jacks while in a turn? My data has never shown that.

2) A front roll center at 13" contradict ALL suspension design conventions that I have ever seen for the FV front suspension. I think your statement needs an engineering explanation, otherwise this is just another example of 'garbage in garbage out'.

Brian

[Daryl DeArman]

Considering all the negative aspects of FV suspension (bump-steer, roll steer, link pin slop, steering box slop, lack of front camber, component deflection, etc) I never got excited about the fore/aft movement of the front wheels during ride height change.

Maybe you still wouldn't have got excited about it, but I bet that if you raced at a track that had a series of esses at the beginning of one straight and a chicane in the middle of another your ability to use the curbs without tossing you all willy nilly would have received your attention.

[Hardingfv32]

I agree with Steve Lathrop about the overall effect will be to raise the front end during cornering when you start with the torsions arms pointed up, since it is a rising rate when run like that. I'm adding nothing to this argument - just noting a good observation by Steve.

The other issue as Steven Saslow brought up many years ago on the FV Interchange, when the legs are pointed up there is a anti-dive effect under braking. We did a calculation on it back then to show how significant the effect was. Another good observation on this topic by Stephen. John

This is not directed just to John...

1) Rising rate: What is the generally accepted wheel rate for the FV front end? Not very great. Is a few pounds of rate change going to make that much deference?

The fact is we have no idea what spring rate is good for the current FV tire? There is no data available. We have no idea if a small change in rate actually makes a difference.

2) Tire for/aft movement from control arm movement: Exactly what is the concern with this? Is this some kind of wheelbase or steering concern? What is the negative effect?

Brian

[Daryl DeArman]

Tire for/aft movement from control arm movement: Exactly what is the concern with this?

As you know there are compromises with every design. Trading one benefit to the detriment of another. More than one way to skin a cat.

The best way I can visualize it is when hitting a curb with the trailing arm level as the wheel deflects upwards it is also being pushed into the face of the bump making the bump seem steeper/larger. The effective wheel spring rate is additionally rising.

If the trailing arm is slanted downwards towards the rear when encountering the same bump the wheel travels rearwards as it climbs the face of the bump making the bump seem shallower/small. The effective wheel spring rate is also decreasing as the lever arm length is increasing.

[lbacon]

Ok, need to clear something up. When the phrase, " the turkey legs up", are you saying the end of the turkey leg where it attaches to the knuckle?

[problemchild]

Maybe you still wouldn't have got excited about it, but I bet that if you raced at a track that had a series of esses at the beginning of one straight and a chicane in the middle of another your ability to use the curbs without tossing you all willy nilly would have received your attention.

Actually, no, but if someone else wants to worry about it, and put their time into dealing with it, that would be fine with me!

I have been concerned over the bump steer characteristics of putting steering racks on cars with VW trailing arm front suspensions for over a decade now, and been told by hundreds of people that I don't know what I'm talking about. That's OK. Prioritizing how we spend our time and resources, is generally the determining factor in racing success ..... after driver and HP of course.

[problemchild]

Ok, need to clear something up. When the phrase, " the turkey legs up", are you saying the end of the turkey leg where it attaches to the knuckle?

"turkey legs up" and "turkey legs down" is like "trailing arms" and "leading arms"

In the context of wheelbase changing with suspension travel, there is no difference.

[Daryl DeArman]

if someone else wants to worry about it, and put their time into dealing with it, that would be fine with me!

I have been concerned over the bump steer characteristics of putting steering racks on cars with VW trailing arm front suspensions for over a decade now, and been told by hundreds of people that I don't know what I'm talking about. That's OK. Prioritizing how we spend our time and resources, is generally the determining factor in racing success ..... after driver and HP of course.

Couldn't agree with you more. I loved when my competitors were concentrating their time, effort and money on things I didn't feel were as important as whatever I was focusing on.

[Daryl DeArman]

In the context of wheelbase changing with suspension travel, there is no difference.

Yes there is.

If the leg is horizontal there's no difference.

If the leg is angled downwards (in my context I'm referring to the beam being higher than link pin) the wheelbase decreases in bump until parallel.
If the leg is angled upwards (in my context I'm referring to the beam being lower than the ling pin) the wheelbase increases in bump.

[problemchild]

Yes there is.

If the leg is horizontal there's no difference.

Oh come on Daryl. I know you like to argue but .....
If its horizontal, then the leg is not up or down.
And if ride height is changing, then it is no longer horizontal and the wheel is moving forward with any ride height change..

No need to reply. I get your point.

[Daryl DeArman]

Oh come on Daryl. I know you like to argue but .....
If its horizontal, then the leg is not up or down.
And if ride height is changing, then it is no longer horizontal and the wheel is moving forward with any ride height change..

No need to reply. I get your point.

I don't doubt your abilities to successfully tune/prepare a race car. I'm not arguing to change your mind, or disputing your knowledge.

I'm arguing your explanation for the benefit of those that are attempting to get a better understanding. Relative to where that wheel was when the leg was horizontal you are correct.

But to say with "any ride height change" and that it matters not whether it was leg up/leg down is false.

[S Lathrop]

1) What measurements to you have to indicate the FV front suspension jacks while in a turn? My data has never shown that.

2) A front roll center at 13" contradict ALL suspension design conventions that I have ever seen for the FV front suspension. I think your statement needs an engineering explanation, otherwise this is just another example of 'garbage in garbage out'.

Brian

I think that I said that I modeled the FV front suspension using the Michel software. I have not modeled the FV suspension using the techniques that I use for all my other cars. The Mitchel software assumes that the control arms, for the FV front end trailing arms. What may be an issue in this analysis is the fact that the 2 arms for a FV front end have the axis of rotation perpendicular to the center line of the car.

The jacking effect is a function of the roll center height either above the ground or below the ground. At first I did not trust the the results but from my experience with the FV suspension, the roll center location is consistent with the handling and the effects of other changes I have made to FV suspension over many years. Also there were no calculation error in the Mitchel software.

I have dealt with jacking effects on many of the cars I have built or engineered and I have actually been able to measure them with data logging systems.

I will agree that other than the Mitchel software, all the literature I had read either said that the front roll center was at ground level. But the Mitchel software is more consistent with what I found in testing with a FV.

[nrider42]

I have nothing technical to add to this great discussion...yet. Someday I hope to. For right now I am thankful that y'all are willing to share your knowledge...and entertain me Mid-Ohio, end of April...my turkey legs are going to be somewhere...

--Nate

[jpetillo]

I had different beam mounts fabricated and put a ride height adjuster in my beam. I ran the front end as low as I could without bottoming and still had the turkey legs angled so that the wheel is traveling rearwards during the first inch or two of bump travel.

Your point about the tire wanting to move forward into the face of a bump(curb) was exactly the point I made to Steve Lathrop about running the leg parallel at ride height.

Thanks - that clears it up. I understand what you did. John

[jpetillo]

This is not directed just to John...

1) Rising rate: What is the generally accepted wheel rate for the FV front end? Not very great. Is a few pounds of rate change going to make that much deference?

The fact is we have no idea what spring rate is good for the current FV tire? There is no data available. We have no idea if a small change in rate actually makes a difference.

2) Tire for/aft movement from control arm movement: Exactly what is the concern with this? Is this some kind of wheelbase or steering concern? What is the negative effect?

Brian

Brian, those are excellent questions. I'll try to respond to some of them. I enjoy the dialogue.

1) In your question about what is the generally acceptable wheel rate, are you asking what wheel rate we have or what is best? I know we've calculated what is it (both you and I) when using a single spring pack, but I don't have it off the top of my head. (~100 lb/in? - I forget)

About the rising rate, it may not be that significant, but the more the rear of the torsion arm is up in the front the higher fraction the rate rises with a given vertical deflection. It's very nonlinear. I'll try and dig that graph up so we can rub our chins over it and pass judgement.

Getting back to what's best for a wheel rate. Good question on its own (with regard to suspension and handling). But then you asked what's best for our tires, which is really the ultimate question in my mind. I expect they'd play against each other at some point - especially when you look for the optimum balance between what's best for handling vs. what's best for the tires keeping their performance throughout a race. I assume this is what you were considering.

2) I'm not sure there is a significant concern with the fore/aft movement. Daryl feels that it would make a bump harsher if it moves forward with a vertical deflection. There's no doubt about that being the case, but again, how much would be the question. With the rear of the arm angled up, the more the angle it starts at, the more the wheel rate increases with deflection. The wheel rate increase may be more of the cause of it feeling harsher than from it moving forward. We can figure this out - put some numbers behind it - not too hard to do.


Front jacking: Unless we include the fact that parts bend, I don't see that the front has a mechanical jacking. Steve L., this is why the books say that the roll center is on the ground, right?

But, if you have a risen rate suspension added to it, then the front will rise up in a corner. If the torsion arms always stay tilted up, then it will always have this effect in a vee. Could the Michel software be reporting an "effective" roll center, where the rising rate causes jacking?

Nate - Thanks for starting the thread - we may have strayed from some of your questions!

Great discussion - John