Uhhh - weird. I'm building the same thing for my car. Haha

 

Yes, I'd think wheel offset and camber would matter to whatever extent that it changes the effective center of mass of the wheel/hub assembly. That's the point that determines the effective length of the suspension lever arm.

 

Not really - the motion ratio basically describes the relationship between vertical displacement of the spindle's lower ball joint to the spring displacement. Generally this calculation is done assuming the LCA is horizontal. Wheels don't need to be on the car to figure it out, and thus are non-factors in the calculation.

http://www.eibach.com/ERS_Worksheet.htm

 

Yeah, that's the definition, but as a practical matter, isn't motion ratio mostly used to find the wheel rate (spring rate at the wheel)? And wouldn't the wheel rate change given sufficiently large change in the wheel size/offset/camber? I don't see how the "true" lever arm of the wheel/hub assembly could remain constant.

 

Wheel rate is the amount of force it takes to move the suspension up at the hub. Here again, wheel size and offset are not factors in determining wheel rate, which is defined as:
(spring rate) * (motion ratio) * (Cosine(a))
where (a) is the angle of the spring from vertical. Obviously this value changes a bit as the suspension moves through its travel.

You might be thinking of how to find the Instant Center and FVSA (front view swing arm) length, which are dependent upon the placement of the tire's contact patch.

 

I've seen (Milliken&Milliken) wheel rate was defined as
(spring rate)*(installation ratio)^2
where installation ratio seemed to be defined in the same way as motion ratio, i.e. equivalent to cosine(a). That's how I got my numbers above. (The data I have quotes spring rate and wheel rate directly.)

Either way, still a little confused at how changing the wheel/hub geometry wouldn't change the lever arm of the suspension. As Conan might say, "must contemplate this on tree of woe."

 

I agree. Moving the wheel changes the level arm just as much as moving where the shock attaches to the lower a-arm.

John, I take it since you based your numbers on the actual wheel rate vs. spring rate relationship, your equation factors in for the angle the shock sits at?

 

[QUOTE=The Reverend,Dec 2 2004, 04:47 PM]I agree.

 

Jason, why would it be unusual for the spring to deflect more even if the weight of the car doesn't change. I may be seeing things too simply, but that's basic leverage. Imagine you had wheel spacers ten feet wide (but of negligble mass) so the wheel is sticking out 10 feet from the car, it would take relatively little force but a lot of movement of the wheel to deflect the spring an inch. So the motion ratio would be very high and the car would ride very soft compared to without the spacer. The ball joints would still see the same movement and force as you explained, but seems to me the wheel movement is what matters in choosing spring rate.

 

OK!!! I think I see where the two schools of thought are coming from now and I believe the truth is squarely in the middle...

Richard and I were thinking of the wheel motion almost like if it were around a single axis - as if there were just one arm to the suspension. We were partially neglecting the second arm keeping the wheel mostly perpendicular as the suspension travels up and down.

JZR is assuming too far in the other direction, that the two suspension arms are identical length and parallel to each other. If that were true, he would be correct that the wheel rate wouldn't be effected by offset. It WOULD put all kinds of stress on everything is you had a +TONS offest, but in the end, the wheel rate would still be unaffected.

However, the truth is in the middle. The arms are unequal length and are not parallel, so while they accomplish some of what JZR is thinking, the motion still is somewhat like if there were just a single arm swinging up and down.

I don't know if that made sense, but my end thought is that the offset WILL effect it, but the suspension geometry will minimize the effect somewhat.