Designing the Satchell Link

[boodlefoof]

Thanks again for your insights Norm.

I've been playing with my design more and have found something interesting that seems to go against conventional wisdom... In fact, I think I might have to add these observations to the "Really Long Trailing Arms" thread over at CC.com.

Typically, you hear that the longer the links, the better. Many people seem to think this will help to keep RRCH from moving, thus making the car more predictable. So, I had been designing to get the longest links possible. On the bottom side, this comes out to the roughly 28'' links. On the upper side, the spaceframe design really will acommodate anything up to 60'' at the extreme. Currently, I had been playing with a roughly 30-35'' upper link (as this is a convenient mount point on the frame and a link this length won't need to be as beefy as a longer one).

The thought occurred to me though that perhaps there is an optimum upper link length relative to the lower length, much like in a double A-arm front suspension. I thought "Perhaps going with too long an upper would actually be detrimental to the design?"

So, as I tried to flatten the AS curve I found that I could do so by shortening the upper link as I mentioned earlier in this thread. I had been looking primarily at the AS curve and the understeer curve, trying to reduce AS loss while not increasing roll steer too much.

Then, I happened to look at what was happening to RCH depending on upper arm length. I found that there does appear to be an ideal upper length relative to the lowers for the purpose of stabilizing RCH migration in bump. It appears to be around 21.5'' on my setup.

Here are the numbers, which I think are pretty interesting... RCH change does the following in bump all else being equal:

Ride height / 30'' upper arm RCH / 21.5'' upper arm RCH

0'' bump / 11.30 / 11.31
1'' bump / 10.90 / 11.30
2'' bump / 10.50 / 11.30
3'' bump / 10.10 / 11.31
4'' bump / 9.71 / 11.35

The same thing happens in lift - the long arms gain roughly .4'' RCH per inch lift while the shorter arms don't change RCH by more than .04'' over the entire range of motion!



Additionally, with the shorter arms AS loses about 20% per inch of bump as opposed to 30% per inch with the longer arms.

Meanwhile, roll steer with the short arms moves from 0% at ride height to 7.9% at 4'' of bump, while figures for the long arms are a range of 0% to 5.2%.

It seems odd that the shorter arm actually appears to be a better setup on paper??? What do you think?