Caster Setting

[Jean-Sebastien Stoezel]

Hi,

I have a Lynx B and it comes with some positive caster. This is built in, it's set with the way the H beam is bolted onto the frame.
When looking at modern frames, like the Lazer for example, it seems there is no caster, the H beam is perpendicular to the frame.

When I was running go-karts caster was a key setting, it allowed the gokart to switch to 3 wheels in corners, making it easier for the rear axle to go through turns (since there's no differential) with minimum drag. I'm guessing this is not an issue on a FV. I've read the only (?) effect of caster in a car with a differential is to make the car more responsive to steering input (the more positive caster, the more the car will be responsive).

Is this the only effect of caster? Why do modern FV cars seem to have no caster? What's a typical caster setting? Do people adjust caster, with shims for example between the front axle and the frame?

Thanks,
Jean

[samiam520]

Caster along with king pin inclination will control the amount of camber the wheel generates as it turns, just like in the go kart. In a open differential and zero roll car you don't need the jacking effect to transfer weight like you will in a kart with a solid axle.

I have recently purchased a FV that had 9 deg of caster built into the frame and I found it to be way to much for my liking. I have installed aluminum shims/blocks to reduce it to around 3 deg. which I found to be much better.

[Matt King]

I've never bothered to measure the actual static caster on my car (Citation XTC) but I do know that the beam is mounted with a 5-degree rearward inclination.

[Hardingfv32]

You are looking at a design compromise when viewing the caster on a Lazer. The goal was to lower the beam mounting location for what is a misguided attempt to improve the body shape. The CG and possibly ground clearance are also reduced with a lower mounted beam. The negative is very unfavorable control arm inclination or anti-drive. With this much anti-dive, the front end goes very stiff under braking. You also get a reduced spring and shock rate because of the big angle on the lower control arm. To alleviate these issues a compromise is made by removing the castor and there by reducing some of the anti-dive/control arm inclination. Driving on the straights now requires the driver's attention, but it is felt that the pluses out weigh the negatives with this design compromise.

Brian

[Campbell Motorsport]

Apparently someone changed the Lazer you are measuring because it normally would have 4 degrees built into the chassis. As Scott did, many people use shims or aluminum blocks to change to their liking.

Larry Campbell

[Bruce Livermore]

Larry, is the caster the same on the XTC as on a "normal" Citation? I ask only because by casual observation I always thought that a Citation looked close to zero (as "measured" by the beam mounting points.

My chassis (original Mysterian) had about 7 degrees built in as best I could measure it and since I "liked" it better shimmed to a degree or two more, I built in 8 degrees when I redid the chassis.

According to theory, more caster gives more on-center "stability" (the steering wants to return to straight ahead exiting corners and is stable there on straights). It also give some negative camber to the outside front wheel on turn-in. I consider all of these positives. The negative is heavier steering overall (since it always tries to return to center) which some people don't like..

The most caster I heard anyone use was 11 degrees, also on a Mysterian. That seems a bunch, but both the original driver (who set it up that way) and the subsequent driver (who I believe has kept it that way) liked it.

It appears that how much caster one likes is very dependent on the driver.

[Jean-Sebastien Stoezel]

You are looking at a design compromise when viewing the caster on a Lazer. The goal was to lower the beam mounting location for what is a misguided attempt to improve the body shape. The CG and possibly ground clearance are also reduced with a lower mounted beam. The negative is very unfavorable control arm inclination or anti-drive. With this much anti-dive, the front end goes very stiff under braking. You also get a reduced spring and shock rate because of the big angle on the lower control arm. To alleviate these issues a compromise is made by removing the castor and there by reducing some of the anti-dive/control arm inclination. Driving on the straights now requires the driver's attention, but it is felt that the pluses out weigh the negatives with this design compromise.

Brian

From the replies, I understand people are adding shims to set a caster angle between 3 to 7 degrees. By doing so you are also modifying the angle of the turkey legs, the ride height, the COG, the rates of the shocks/springs.

This is getting interesting as it looks like there is a compromise, between several factors.

- The turkey legs should be horizontal to offer the best aerodynamic penetration
- The ride height should be minimum to lower the COG, but high enough to accommodate diving. What is a typical "diving" distance by the way?
- The angle the turkey legs make should provide "appropriate" spring/shocks rate. What is the best angle (with reference to the H beam) for the turkey legs to provide adhoc spring rates?

It looks like all these conditions could be met by a combination of proper beam mounting height on the frame, and proper caster.

[Campbell Motorsport]

Standard Citation and XTC were both the same at approximately 5 degrees.

[samiam520]

- The turkey legs should be horizontal to offer the best aerodynamic penetration

And the best position from a geometry aspect also.

The ride height should be minimum to lower the COG, but high enough to accommodate diving. What is a typical "diving" distance by the way?

Depends on your spring rate, spring preload, weight distribution, brake bias, etc. My car is just over 2" static ride height in the front and it will hit under hard braking.

- The angle the turkey legs make should provide "appropriate" spring/shocks rate. What is the best angle (with reference to the H beam) for the turkey legs to provide adhoc spring rates?

The angle of the turkey leg has no effect on the spring rate. Only changing the amount of leafs in the spring pack can change the spring rate, however, the angle does change the motion ratio which will effect the shock rate and wheel rate.

I recently raised the beam in my FV which will lower the frame. I wanted the turkey legs level with the ground and the frame about 2" above the ground.

[Bruce Livermore]

And the best position from a geometry aspect also.

Quite possibly, but in my experience maybe not.

The angle of the turkey leg has no effect on the spring rate.

True if you define spring rate for a Vee as the force per degree of rotation of the turkey arm.

But what matters as far as handling of the car goes is the effective wheel rate not the angular spring rate. As the angle gets steeper (away from horizontal in either direction) the effective spring rate the wheel sees relative to VERTICAL forces goes up because the effective lever arm acting to rotate the arm goes down. To visualize, imagine the extreme case where the turkey leg is vertical, the effective spring rate is infinite because there is no length to the effective lever arm at all.

Only changing the amount of leafs in the spring pack can change the spring rate, however, the angle does change the motion ratio which will effect the shock rate and wheel rate.

What the tire and the chassis sees and what affects the handling of the car is the wheel rate, not the rotational spring rate.

Running more "rake" in the front turkey legs does (at least) three things (make it 4).
1. For a given torsion spring set-up, it increases the effective wheel rate.
2. It provides a more "rising rate" effective wheel rate since every degree of rotation as the car goes into jounce decreases the leverage the vertical force in the tire contact patch has to rotate the turkey leg. The further you go from horizontal the more quickly this happens for a given vertical displacement.
3. It provides more anti-dive under braking than horizontal legs (which also provide some anti-dive if they rotate far enough).
4. It increases the frontal area of the turkey legs which might hurt aero some.

Some of these are good some of the time and bad at others. My personal experience in raising the beam on my Mysterian (along with a bunch of other stuff so the issue is admittedly very confounded) was that I liked driving the car better when it was stiffer in front and had gobs of anti-dive. I never had trouble with insensitivity of the front brakes over bumps because there was still compliance to vertical forces. To me it was a more stable platform and had better balance overall.

I'm sure you can go too far, and I'm sure driver preference has a lot to do with what works for any one person. There are also other aero things to consider when selecting your beam height. Compromises, compromises...

Your mileage may vary as always.

Bruce

[samiam520]

Running more "rake" in the front turkey legs does (at least) three things (make it 4).
1. For a given torsion spring set-up, it increases the spring rate.

It does not change the spring rate, it will however increase the spring loading or wheel rate. The spring "rate" never changes unless leafs are added or subtracted.

2. It provides a more "rising rate" effective spring rate since every degree of rotation as the car goes into jounce decreases the leverage the vertical force in the tire contact patch has to rotate the turkey leg. The further you go from horizontal the more quickly this happens for a given vertical displacement.

I agree but shouldn't it be described as a falling rate, not rising rate? As the turkey leg compresses past horizontal the motion ratio gets worse, wouldn't this be a falling rate?

3. It provides more anti-dive under braking than horizontal legs (which also provide some anti-dive if they rotate far enough).

Don't have any real world experience as you do with this one.

4. It increases the frontal area of the turkey legs which might hurt aero some.

Maybe, but it isn't much.

[Bruce Livermore]

Sorry Scott, I got sloppy and started subbing spring rate for wheel rate. And of course you are correct: the angle of the turkey arm does not change the torque /degree property of the spring itself. I've edited my post above.

My point is that changing the turkey leg angle DOES change the wheel rate and I should have been perfectly clear on that. I'd argue that wheel rate is what matters regarding handling, thus my focus on how the turkey leg angle affects it.

For a given spring, if the turkey leg starts pointed up at ride height (link pin center higher than its beam bushing center) the wheel rate will be higher than if the turkey leg is horizontal. The wheel rate will also go up as the car goes down (in jounce). I'd say that is "rising rate" since the force per inch (of wheel displacement) rises as the tire goes up (chassis down). In terms of motion ratio, in this case, the further the wheel goes up, the more it rotates the turkey leg (in degrees) per inch of vertical travel of the wheel, thus increasing the spring deflection (twist) per inch of vertical wheel travel. Is the "worse" or better as far as motion ratio? Depends on what worse means. The spring is getting more mechanical advantage (or the wheel less) which is what increases the wheel rate.

If on the other hand, if you mount the beam so high on the chassis that you start with the trailing arm pointing down (at your chosen ride height), the wheel rate will start higher than if the turkey leg is horizontal and the wheel rate will go down as the tire goes up (until the trailing arm becomes horizontal). This would be falling rate (until horizontal).

The most constant wheel rate (least wheel rate change per inch of wheel movement) will occur in the zone where the arm stays closest to horizontal. So if that's the goal one would design the mounting position and ride height to keep the trailing arm closest to level throughout normal suspension travel. And for a given spring, this would also give you the lowest average wheel rate (for better or worse).

I agree on the shock. The motion ratio goes down (less shock length change per inch of vertical wheel travel) as the turkey leg points up and this effect is even worse when using the offset shock adapters.

I hope I got it right this time!

[samiam520]

Yea, this stuff is confusing mainly because of some of the nomenclature.

I agree with you on everything that you have described other then your description of rising/falling rate.
I agree that any time the spring is compressed, or in our case twisted, it adds wheel rate.
The upward motion of the turkey leg, which could be described as going over center when it is above it's center of rotation, causes the motion ratio to continually decrease as it continues to move upward, thus, in my mind it would be a decreasing or falling rate. Even though the spring is still applying more force as the turkey leg rotates up, the falling motion ratio causes the wheel rate to fall also.

Does that make any sense?

[Daryl DeArman]

Rising rate means the rate gets higher as the spring is compressed/twisted.
Falling rate means the rate goes lower as the spring is compressed/twisted.

For those of you with lots of FV experience---has anybody effectively measured their front wheel rates? When having some front shocks custom valved, some spring rate data was requested. I attempted to measure it and on a FV it is quite apparent that the relatively super stiff sway bar is a huge factor when encountering a bump one wheel at a time.

Thoughts?

Further, is the spring rate per degree of twist really linear? I'd think at some point of twisting it would bind up, like a coil spring that has bottomed.

In regards to the lots of positive caster and the heavy steering---that can be tuned out by playing with steering arm lengths/geometry.

[samiam520]

Daryl,

For those of you with lots of FV experience---has anybody effectively measured their front wheel rates? When having some front shocks custom valved, some spring rate data was requested. I attempted to measure it and on a FV it is quite apparent that the relatively super stiff sway bar is a huge factor when encountering a bump one wheel at a time.

Yes, on my last FV I measured the wheel rate and the motion ratio of the shock, If I remember correctly the wheel rate was about 25lbs and the motion ratio was around .350"/1" of wheel travel. Of course because of the angle of the turkey leg those numbers will vary car to car. And, IMO the front sway bars on most zero roll cars are to stiff.

Further, is the spring rate per degree of twist really linear? I'd think at some point of twisting it would bind up, like a coil spring that has bottomed.

I believe it is linear, and you are absolutely correct on the binding. In fact my latest FV was doing exactly that, so I raised the beam in the chassis to keep the chassis low to the ground and get the turkey legs parallel to the ground at normal ride height.

[Hardingfv32]

How in the world can you say the front bars are too stiff? Assuming a non-flexing sway-bar, you would only have a one wheel rate of 50 lb. in a one wheel bump situation.

1) FF front wheel rates are 150-200 lb. at minimum, probably much higher.

2) The std rear FV wheel rate is in the range of 100 lb. Do you find the rear tires tramping off coarse on rough surfaces?

Brian

[Daryl DeArman]

Daryl,

Yes, on my last FV I measured the wheel rate and the motion ratio of the shock, If I remember correctly the wheel rate was about 25lbs and the motion ratio was around .350"/1" of wheel travel. Of course because of the angle of the turkey leg those numbers will vary car to car. And, IMO the front sway bars on most zero roll cars are to stiff.

Thanks for the info. How did you manage to measure the wheel rate? PM if you wish.

[Daryl DeArman]

How in the world can you say the front bars are too stiff? Assuming a non-flexing sway-bar, you would only have a one wheel rate of 50 lb. in a one wheel bump situation.

1) FF front wheel rates are 150-200 lb. at minimum, probably much higher.

2) The std rear FV wheel rate is in the range of 100 lb. Do you find the rear tires tramping off coarse on rough surfaces?

Brian

Brian,

I guess that depends on what theory you subscribe to; are you a soft spring/big bar or big spring/soft bar type guy.

For me, in a FV, the wheel travel doesn't cause the camber to go all goofy so I'm not too worried about travel other than the ground clearance required. While camber goes all nuts with roll---and you don't have much roll resistance in the rear of a zero-roll car. So in theory, I wanted soft spring, big bar. Had good success with the set up. Drawbacks were the transitions off of the infield to the banking were a little nervous, and the car didn't really like to encounter tigers' teeth with the front wheel.

[Hardingfv32]

25-30 front wheel rate is not soft... it is nonexistent! How soft do you need to be? The rear works fine at 4X the rate.

Brian

[samiam520]

How in the world can you say the front bars are too stiff? Assuming a non-flexing sway-bar, you would only have a one wheel rate of 50 lb. in a one wheel bump situation.

1) FF front wheel rates are 150-200 lb. at minimum, probably much higher.

2) The std rear FV wheel rate is in the range of 100 lb. Do you find the rear tires tramping off coarse on rough surfaces?

Brian

Brian,

I don't understand what you are trying to say or ask ? What is a non-flexing sway bar? The wheel rate on my car is 25lbs for each front wheel, that would be 50lbs total for the front. Not 50lbs per side. I am not sure what the spring rate of my sway bar is so I can not add it into the "one wheel bump" wheel rate calculation. I will have to look through my notes to find that number.

Also, please note that I am not working with a conventional zero roll suspension. I have a sway bar in the back for rear roll resistance.

Thanks for the info. How did you manage to measure the wheel rate? PM if you wish.

Daryl,

Removed the shocks, then staked 5lb weights on the front of the car until the beam went down 1" It took 50lbs to achieve this. The front tires were also on scales to confirm the results.

[bugcollections]

Did a search and this thread was the most informative I found on caster. My question is: What is an ideal caster angle to shoot for on a vintage vee? Relative new to vee racing (13 race weekends). Have a vintage Zink C-4, Z bar rear. Weight with driver and fuel is almost exactly 1,000lbs. Running Hossier vintage vee tires. Am running 4 degrees negative camber (loaded) on rear each side with -0- droop. Running 3 degrees negative camber on front. 1/16" toe-in front and rear. Turkey legs set horizontal parallel to ground. Current angle of front beam is about 4 degrees positive caster. Have not measured caster at king pin. If I understand the VW link pin front end; as the car raises (turkey legs going down as measured at link pins) the caster increases positively, and vice versa; as the car goes down (turkey legs go up as measured at link pin) the positive caster decreases. Since almost all vees I have seen have the turkey legs somewhere around horizontal parallel to the ground which is obviously lowered considerably relative to a stock vw suspension, then a setting of 2 degrees positive caster setting of the torsion bars (that is the factory setting on a stock vw) then the actual caster on the king pin would be even less positive than on a stock bug.

So where is a somewhat ideal setting? It would appear to me with limited suspension knowledge that you would want considerably more positive caster, or as much positive caster as you would feel comfortable driving. Increasing positive caster has a lot of benefits, but you can always have too much of a good thing.

I am not trying to reinvent the wheel: there are just a lot of vee guys out there with more knowledge than myself. The vintage vee drivers I have asked around the track usually come back with a reply on how far to shim the torsion tubes and the consensus seems to be about 4 degrees. But this is not a true measure of caster as measured at the king pin.

[Diamond Level Motorsports]

Wow, that's a loaded question.

Depends on so many factors. Is the car a zero roll? Many vintage cars are not zero roll. The more caster the more cross weight that is jacked into the chassis. Of course this doesn't happen if the car is a zero roll. One could probably write a whole book on just this subject.

The best answer I can give is start with around 4 deg and experiment from there. Everyone has a different driving style and a different goal for the dynamic balance of the chassis.

[BLS]

BugC,

Perhaps one of the vintage racers will chime in with specifics on your car.

I can tell you that back in the 70's, on a Zink like you have, with the Z bar rear suspension, but running slicks, most of us were increasing the caster by a few degree's. On my car I ran around 6-7. This just required a 1/8 or 3/16 shim at the bottom torsion tube as I recall.

What I found was the greater caster reduced understeer that was present. This was I believe a result of the camber that occurred under load. Increasing the caster decreased the positive camber that would occur. As I recall, it was a fairly significant improvement over the stock Zink value.

[Ed Womer]

I have always built my cars with the beam straight up and down with no angle. If you like the castor in the car you then shim the bottom tube to get what you want. I actually like it with no castor since it makes the steering very light and you don't need gorilla arms to drive the car.

I did rent my car a few times through Ski Motorsports and would put a 1/4" shim in the bottom tube which makes it around 3 degrees of castor and left it there when I drove it the next time and just keep it that way. I also usually ran a 5/8" sway bar or a 11/16" one.

But you need to realize that I liked a lose car feel so these set ups gave me what I liked and not everyone likes a lose car but as the old saying goes, lose is fast you just need to keep on the road!

Ed

[budawe]

The factory caster settings for a beetle were set by the mounting points on the tub and designed to deliver 2 degrees, +- 1/2 of caster. If during an alignment this number was off, we'd check ride height. Unlike many cars, the VW front end changes caster during vertical travel. As the car goes down, caster is reduced. Bounce on your car and watch the king pin, it will tend to move forward as the car goes down. This forward movement reduces kingpin inclination and therefore, caster.

Caster does two things. It provides straight line stability and drives the steering wheel to center position. The more caster you have, the more stable the car will feel at speed but at a cost of greater steering effort. If you look at the early Porsche 356's with H beam suspensions, you'll see their caster is much closer to 6 to 7 degrees since they were a bunch faster than a 40hp VW. Mercedes Benz ran as much as 11 degrees on some big sedans.

As Ed mentioned, vee builders use the front end mounting points to establish initial caster. The more the mounting points on the frame are tilted back, the more caster you get. In the early days, builders followed Porsche and MB ideas and ran fairly high caster. I say initial caster since the final number will be dependent on ride height and turkey leg inclination. If your turkey legs are pointed upwards at ride height, the final caster will be reduced.

For the most part, vee caster is a function of feel. I run just enough to get positive wheel return and try to minimize arm energy. If your car is skiddish and the toe is set properly, ( toe in, never toe out) then a bit more caster may be called for.

I use 2" tubing and slice it in half and use the halves for shims between the h-beam and frame. It is the same technique we used in the front end alignment business. Caster is difficult to measure since it requires degree turntables and camber measurements.

If the steering is really hard, try adding shims to the top until it feels better without getting skiddish. If it is kinda skiddish and has little steering effort, do like Ed said and add a shim to the bottom.

One big caution about shimming and caster. As you brake, the nose goes down taking caster with it. If your car passes through zero caster and possibly into negative caster during braking, you will get tire shake that will knock out your fillings. Remember those terrible grocery carts? same deal.

On cars that have the steering box mounted somewhere else besides the h-beam, it will require rechecking toe setting after adding or sub-tracting shims.

One final thought: If the caster is different from side to side, it can cause the car to pull to one side. While more common to Macpherson strut designs, a caster split can be caused by a twisted h-beam or bent frame. If you suspect you have a caster split, add a shim to the side to which the car pulls and see if it helps.

[Dean Curtis]

That was a fantastic explanation. Hope everyone reads and gets it. Check your caster. Thanks

[bugcollections]

Thanks guys. I'm learning.

[Drake]

"Depends on so many factors. Is the car a zero roll? Many vintage cars are not zero roll. The more caster the more cross weight that is jacked into the chassis. Of course this doesn't happen if the car is a zero roll."

Please explain

[BLS]

It is a Zink with a "Z" bar.

Have a vintage Zink C-4, Z bar rear.

[budawe]

Because caster becomes camber in turns, it tends to lift the car up as camber increases and the axis moves backwards and towards the ground. For caster to cross jack or change the weight of one side of the car, the caster change must be different side to side. Generally with any kind of roll resistance found in most race cars, that's not likely to happen. In vees where the caster changes with ride height, it's possible to have a zero roll car to roll over hard to the outside tire, carrying weight and causing ride height to change side to side and generate a caster difference. This is not the case for Z bar cars and most vintage vees.

I'm going to have to set up a test to measure that caster change, it may contribute to our inherent push. I know that a stiffer front roll bar does tend to reduce push and this may likely minimize the caster split.

[problemchild]

I agree but shouldn't it be described as a falling rate, not rising rate? As the turkey leg compresses past horizontal the motion ratio gets worse, wouldn't this be a falling rate?
Don't have any real world experience as you do with this one.
Maybe, but it isn't much.

I would say falling rate, once the arm goes over-center. The force at the wheel is still increasing because the spring is being twisted, but the rate of that change is decreasing.

I raised the beam on my later cars so I was getting rising-rate on the front.

On a side note: My last 4-shock version of the Shirley MacLane had decreasing-rate rear suspension that I designed to cancel out the increase of the the wheel rate on compression. I was attempting to create the benefits of zero-roll with the advantages of diagonal weight transfer of a conventional car. As with most of the stuff I tried back then, I was so underfunded, that I could not develop its potential, but it was incredible on some medium speed corners, like corner 1 at the current Pocono configuration, or end of the back straight at Mid-Ohio.

[Diamond Level Motorsports]

Because caster becomes camber in turns, it tends to lift the car up as camber increases and the axis moves backwards and towards the ground. For caster to cross jack or change the weight of one side of the car, the caster change must be different side to side. Generally with any kind of roll resistance found in most race cars, that's not likely to happen. In vees where the caster changes with ride height, it's possible to have a zero roll car to roll over hard to the outside tire, carrying weight and causing ride height to change side to side and generate a caster difference. This is not the case for Z bar cars and most vintage vees.

I'm going to have to set up a test to measure that caster change, it may contribute to our inherent push. I know that a stiffer front roll bar does tend to reduce push and this may likely minimize the caster split.

When the wheels are turned caster will cause the inside wheel to go downward, or lift the car, and the outside wheel will go upward. Caster has a huge cross weight jacking effect.

Now you could argue that cross jacking has no effect on a zero roll car, but the caster still has an effect on how the front end grips. I would deduct that more caster increases the work of the inside tire.

I see roll resistance having a very small effect on the caster split. Caster, KPI, Wheel angle, and Ackerman would have a greater effect the caster split.

[budawe]

I agree Scott, but given the small steering angles and reduced Ackerman most folks run, it's probably well right of the decimal. Same can be said for roll and other issues.The vees are pretty rustic but caster loss under braking is critical to maintaining stability.

[jpetillo]

Sorry Scott, I got sloppy and started subbing spring rate for wheel rate. And of course you are correct: the angle of the turkey arm does not change the torque /degree property of the spring itself. I've edited my post above.

My point is that changing the turkey leg angle DOES change the wheel rate and I should have been perfectly clear on that. I'd argue that wheel rate is what matters regarding handling, thus my focus on how the turkey leg angle affects it.

For a given spring, if the turkey leg starts pointed up at ride height (link pin center higher than its beam bushing center) the wheel rate will be higher than if the turkey leg is horizontal. The wheel rate will also go up as the car goes down (in jounce). I'd say that is "rising rate" since the force per inch (of wheel displacement) rises as the tire goes up (chassis down). In terms of motion ratio, in this case, the further the wheel goes up, the more it rotates the turkey leg (in degrees) per inch of vertical travel of the wheel, thus increasing the spring deflection (twist) per inch of vertical wheel travel. Is the "worse" or better as far as motion ratio? Depends on what worse means. The spring is getting more mechanical advantage (or the wheel less) which is what increases the wheel rate.

If on the other hand, if you mount the beam so high on the chassis that you start with the trailing arm pointing down (at your chosen ride height), the wheel rate will start higher than if the turkey leg is horizontal and the wheel rate will go down as the tire goes up (until the trailing arm becomes horizontal). This would be falling rate (until horizontal).

The most constant wheel rate (least wheel rate change per inch of wheel movement) will occur in the zone where the arm stays closest to horizontal. So if that's the goal one would design the mounting position and ride height to keep the trailing arm closest to level throughout normal suspension travel. And for a given spring, this would also give you the lowest average wheel rate (for better or worse).

I agree on the shock. The motion ratio goes down (less shock length change per inch of vertical wheel travel) as the turkey leg points up and this effect is even worse when using the offset shock adapters.

I hope I got it right this time!

Bruce, you did!

[bugcollections]

Since posing the initial question by re-upping this thread I have read all of the replies and talked to a lot more vintage vee drivers. So I decided to experiment. The initial caster setting on the C-4 vintage Zink was approximately 4 degrees as measured at the beam. I shimmed until I got 9 degrees at the beam which results in about 8 degrees at the spindle. First race weekend with the new setting was at MSR Houston. Car still handles just about neutral and seems to stick much better particularly on long sweeping turns. Only downside is that the steering is noticeably heavier on the long sweepers, but not so much that it is a problem. Conclusion: From seat-of-the pants feel the car seems to handle much better really sticking, particularly in long sweepers; I am happy with the results so far. Since MSR was a new track for me I can't compare the new setting to performance on the same track with the old caster setting. When we get back to some repeat tracks I will have some better analysis.