THSCC Forums

I'm definitely interested to see what goes on in this thread as it affects me quite a bit.

I still haven't figured out what spring rates I want to run...much less anything else so all information is helpful at this point.

Tom,

The wheel rate is defined by squaring the motion ratio.

The motion ratio is the ratio of wheel movement to spring movement. For example, if you move the wheel "up" one inch and the spring compresses 1/2 inch the motion ratio is 0.5. The wheel rate would then be .25. With a .25 motion ratio squared a 400 lb spring rate results in a 100 lb wheel rate.

Unless you have really accurate drawings or models, the best way to determine motion ratios is to measure the relative movement of the spring seat for a given amount of wheel movement.

Note that modern race cars which utilize wings and downforce and therefore must run really stiff wheel rates to keep the aero platform constant will actually be designed with motion ratio's greater than 1 so that the shocks travel further than the wheel. This allows the shocks to move far enough/fast enough to actually work better.

Note that I have not studied the link you posted but unsprung weight has nothing to do with calculating wheel rate. It is relevant to calculating suspension frequency and it is certainly relevant to handling.

Edited to add: I've looked at the link and it looks totally screwed up. Commonly used terminolgy DOES NOT use sprung or unsprung weight to determine wheel rate!!!!!

Dick (who has spent far too much time learning about suspensions. )

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Dick Rasmussen

FS 50 2018 Mustang GT

With my car being a Mcpherson strut suspension the Motion Ratio is REALLY close to 1 if not exactly 1. In that case with 550lb springs, I'm just a little under the weight of the corner which is at ~650 or so. What this means is that I should run ~900lb front and 650lb rear springs on the car? That would be insanely stiff.

-Tom

A nice thing about strut suspensions that I learned:

For our purposes the motion ratio is 1 - sin(theta) where theta is the angle the strut makes with respect to vertical. The other movements of the suspension that affect motion ratio are too small to matter in this case.

--Kevin H.
(Hope I got that calculation right -- it's been years since I did any trig...)

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2003 WRX (again!)

Regarding roll stiffness and picking up tires: There have been a number of Pro Solo and Tour photos posted showing inside front tires well off the ground for a FAST BMW in BSP! Talk about weight transfer! Even F1 cars lift inside front tires despite really low cg's. I honestly don't know how "decambering" the rear tires to improve balance compares to, for example, simply running smaller tires on the front of a mid engine two wheel drive rear engine car. What do the all wheel drive road racing cars like the Audi's do assuming they have a lot of room within the rules and budgets? For street based designs, especially awd where the rear tires also need "forward bite", the compromises are more complex than I care to deal with.

Tom, what kinds of front, center, and rear diffs do you have and do you have any legal/practical options for increasing the "drive" to the rear to help kill understeer and/or increase oversteer?

Also, is there any legal way to get either the front or the rear of the car lower to lower the cg at that end?

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Dick Rasmussen

FS 50 2018 Mustang GT

Maybe. See my comment about what do the racing AWD Audi's run?

Seriously, with appropriate shock valving and some way to keep the springs seated, it might work a lot better than you think, especially if it allows significant lower ride heights. I assume you have some sort of adjustable spring seats but are your struts short enough to lower the car more without running on bumpstops?

Also, when did insanity bother you?

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Dick Rasmussen

FS 50 2018 Mustang GT

Gary Sheenan on his USTCC WRX was running 12K rates (650lb) springs all around. That is in the rear equal to the amount of weight per corner, but way off on the front.



open front, LSD center, LSD rear. I have NO way of increasing rear drive in any way shape or form.

Any lower and the control arms start pointing up and we run into REALLY REALLY bad camber curves.



Good point, but my car with 550lb rear springs is already scary on a bumpy surface (and shocks are valved correctly for the spring rate), I cannot imagine it with 650-700lb springs.

-Tom

Tom,

What size front and rear bars is Sheehan running and does he have the same diff constraints that you have? Of course, he is also dealing with totally different corner radii and generally is in a lot higher gear (less wheelspin front or rear and less "power push" tendency).

Just brainstorming but is there any legal/practical way to raise the inner pivot of the lower control arms? A friend and I did that with offset bushings in a Datsun Z (probably before you were born since it was around 1975). Rather than set the bushing offset for more camber we moved the control arm pivot UP. Since you may already have enough negative camber, this may be an option. Also, keep in mind that if the suspension is stiffer that the camber curve becomes less significant.

Regarding the shock valving, lots of shocks are valved fairly stiff in both high and low shaft speeds, especially for stiff springs. However, as the springs get stiffer, the shocks don't have to control the movement as much and can maybe be softer, especially at high shaft speeds which are the ones relevant for bumps. You might want to contact Guy Ankeny (Penske and others) or Mark Ball (Koni) to see what they suggest. FYI when I increased my spring rates I ended up with shock settings identical to the settings testing had shown to be best with springs with half the stiffness. I actually tried full soft in front but that was too soft and allowed wheel hop under braking over Petersburg bumps. I don't know how "sophisticated" your shocks are so I don't know what is possible.

Also, can you go stiffer with the springs and softer with the bars? Keep in mind that for "racing" the total cornering wheel rate (bars and springs and high shaft speed shock valving) are what count regarding effect of bumps. Bumps when you are going straight are only a concern if they hurt straight line traction. Autocrossing fast (like you) involves very little traction limited straight line accel or braking so bumps may not be an issue then and the spring/bar total may be able to be changed to more of the total being spring. If so, especially in the front, you may end up with more inside front tire grip.

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Dick Rasmussen

FS 50 2018 Mustang GT

Yeah,but what's the maximum negative camber that I can expect to get on the front of my wrx from an alignment shop?

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2003 Matrix XRS notaminivan
2004 Subaru WRX #38
2000 Rally Focus
2007 Honda NFR S2000

From an alignment shop, expect about -1.1 - -1.4 in the front; it is adjustable, but without moving the strut tops in, it will be kinda random. In the rear, about -1.3; it is not adjustable, unless you count "bolt slop", but an alignement shop will not go through all that trouble.

When I had mine done, I just told them, "Max equal negative camber in the front, zero's on everything else."

I would say it is worth the effort to do the prying/moving yourself first, then have them fine tune it. Thats what I need to do.

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-Bill Martin
02 WRX DS#44

Somehow this just doesn't jive with me.

On a strut, the wheel is attached directly to the bottom of the spring. Regardless of what angle the spring moves at, the wheel moves up one inch for every inch of spring movement. Yes, there does seem to be a small amount of gunk that makes this ratio only be very *near* 1, but it's certainly not "much less than one" that I can see.

From your example if the strut sits at a 20 degree angle you'd have a motion ratio of .66, which is way low.

Also, at what position would you measure theta? Full droop? Full compression? Weighted?

I'd definitely like an explanation of where you get that. According to Staniforth in _Competition Car Suspension_, "An inboard mounted coil with a high leverage on it will be crushed more (and thus appear softer, or weaker) than a similar coil under similar load and fitted to a McPherson strut which exerts direct weight but no leverage, when Coil Rate and Fitted Rate will be the same."

Also according to Staniforth, Wheel Rate (lbs/in) = Coil Rate (lbs/in) divided by Suspension Leverage Squared.

As near as I can tell, the "Suspension Leverage Squared" part is near one since the wheel is mounted so very close to the spring. It is *slightly* outside the spring (tire has to clear it somehow), but relative to the length of the lower control arm, this distance is fairly negligible.

Or am I missing something?


--Donnie

Hmmm...

Donnie,

It looks like Staniforth neglects spring inclination in all his leverage diagrams on page 184/5 of my 1988 edition. He covers the effects of spring inclination on page 77.

The angle is part of the "equation" since it will cause the spring to move less than the wheel. I found one book where the approach taken was to measure the control arm length and measure at 90 degrees from the strut to the inner control arm pivot point. The more "inclined" the strut, the shorter the distance from the strut to the pivot and, therefore, the "smaller" the motion ratio.

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Dick Rasmussen

FS 50 2018 Mustang GT

He is running a tad bit bigger bars then I am, but the same "ratio" of F/R (at least size wise). Good point though he does have a front diff and upgraded kaaz helical center and rear units.

If I had someone manufacture me a bushing I could perhaps do that. How much do you have to move it to be beneficial? I could perhaps move it a few mm as that bushing is pretty small.



Even though they are Tein shocks, Lee Grimes from Koni went over the "proposed" valving and OK'd it for the springrates I was going to be using. I don't know much other then that.



How would that work. With not enough shock, wouldn't the car be very bouncy over bumps? Once you hit a bump the shock wouldn't control the spring and the car would oscilate(sp). Right?

That was definetly a thought. With a bit stiffer springs but softening up the bars, I could get the "independency" back of the suspension and maybe make the car BETTER over bumps.

Can you explain THAT to me? I have no idea how changing to a smaller bar/stiffer spring would help me with inside tire grip (which is a HUGE problem right now)

-Tom