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?

[g-roadster]

Norm: Interesting discussion on the Satchell Link. Thanks for your input. A couple of points about the Satchell Link: The roll center is determined by the intersection of the two lower links, If you want to minimize roll center migration relative to the frame bring these inner pivots as close together as possible. Making a large A frame with a ball joint would be a solution for minimum roll center migration but would increase stress on the single ball joint rather than two and reduce your alignment adjustment possibilities. Roll steer is controlled by the upper links, adjusting the length and inclination affects roll steer. You hinted at this in your discussion.

I am designing the g-roadster with a Satchell link rear suspension. So far I have only laid this out graphically. I feel comfortable doing this as I have designed everything from large space craft to small optical systems from the drafting board. But I do realize the limitations of graphical only design. What program are you using for your suspension analysis? I will need to get a program, or access to one, to refine my front and rear suspension designs. Recommendations appreciated.

Norm, if it wouldn't be too much trouble could you do computer run for my Satchell link design?

Not sure what: RideHeightChange, LCAchassisZchange, LCAaxleZChange are in your inputs.

[Norm Peterson]

RideHeightChange allows you to input a ride height change due to a change in vehicle loading or a spring height change for a quick ,if slightly dirty, set of results.

LCAchassisZchange is simply a lowering (or raising) of the chassis side LCA pivots relative to some baseline value, and perhaps useful if you have multi-hole or otherwise adjustable chassis side pivot pickups. It's just another way of modifying the LCAchassisZ data that could provide a hint about what you were thinking when you look at the sheet in a year or whenever. Also slightly approximate.

Ditto for LCAaxleZChange, which is where one would enter changes in LCA attachment elevation at the axle brackets. AKA relocation or anti-squat brackets.

P.S. One can perform moderately sophisticated geometry and engineering analyses using Excel or similar spreadsheet software once the necessary equations have been worked out.

P.P.S It's not much trouble at all to run a few of these every so often. After all, there are less than 20 separate inputs (symmetry has been assumed). But I'd rather handle this sort of consulting via e-mail, as the amount of allotted attachment space is limited.


Norm