rear axle frequency

[vixens]

Are the coil spring perches directly on top of the axle, or are they on the arms? If they are on top of the axle, for jounce and rebound, just use the direct 1:1 motion. For body roll, use the ratio of the distance separating the spring perches to the distance separating the tire contact patches - that's how much the springs move relative to the tire contact patch (relative to the body).

Given the quote above from Brian Petersen does this spreadsheet use the correct formulas for the rear axle?

 

[BrianPetersen]

Context, please. None of us know what you are talking about. None of us know what problem you are trying to solve. None of us know what connection your spreadsheet has to the real world. I don't even know where you extracted that (apparent) quotation from. (If I said that, I don't know where I said it, or what the circumstances under discussion were.)

 

[vixens]

A 'rear axle frequency' google search showed these posts in eng-tips.com.

Brian,
"damir130 6Sepp10
Hi everyone, congratulations on the high level of the discussions that have been going on here for the past few years. I've been reading and learning as much as I can, but there is one problem where I hope someone can provide some insights..

I am preparing a classic toyota for classic car races here in Europe. I have read a lot about basic setup procedures regarding initial spring, arb and damping rates. However most of these procedures seem focussed on A-arm or McPherson suspensions and no mention is made of the old solid axle type setups. Question that I am trying to tackle are:

Is it correct that a solid axle's motion ratios vary between one wheel bump (vertical rotation around other sides contact patch), two wheel bump (motion ratio == 1) and roll?

I'm having special difficulties visualising what happens during body roll. Do I consider the rear axle fixed and move the chassis around the spring base (attachment of springs to body), do I do the usual and take the lower spring attachment point and reference it to the wheels?

The ARB is giving me similar issues.. its bolted to the rear axle and the ends are attached to the car. The opposite of the usual setup. The mounts on the axle are spaced really close together compared to the arm-mounts.

Any hints on how to model a solid axle rear end are more then welcome.

Thanks!"

"Brian Peterson answer 7Sep10
In 1972, that car originally came with leaf springs, so presumably you are dealing with some sort of retrofit. Corollas had leaf spring rear ends up until the square body style came out around 1980. I presume by "5 links" you are dealing with two trailing links on each side and a panhard rod. Your effective roll center is at the height of the panhard rod pivots, and I sure hope that at nominal ride height, the panhard rod is horizontal (and as long as possible).

Are the coil spring perches directly on top of the axle, or are they on the arms? If they are on top of the axle, for jounce and rebound, just use the direct 1:1 motion. For body roll, use the ratio of the distance separating the spring perches to the distance separating the tire contact patches - that's how much the springs move relative to the tire contact patch (relative to the body).

Draw a diagram with the bodyshell straight, and again with it leaned over (say) 5 degrees. It's easier to see what's going on that way."

The attached spreadsheet shows information about a tvr being built to run scca solo2 in emod. The rear axle uses a 4 link in a satchell arrangement with shocks/springs mounted on top of axle. See attached photo.
In that photo there is a a 250 lb weighted 10' lever arm attached to the frame to see what effect that has on corner weights. The spreadsheet should give most of the data relevant to this question.

This test was without sway bars so the sway bar data was reduced in the spreadsheet. Some error would be expected with this type of test since the lever arm scenario won't create the same effect as cornering g force. The tvr ballast test does produce the same 2+ degrees of roll as the spreadsheet. Moving the ballast to the end of the lever arm changes the front corner weights to 637 left and 192 right for a ratio of 3.3 versus the spreadsheet's 3.6. That ballast changes the rear corner weights to 556 left and 352 right for a ratio of 1.6 versus the spreadsheet's 3. Is it safe to assume both front and rear ratio should be close to 3?

Hopefully this better explains the original question, are the rear axle forces being interpreted correctly by the spreadsheet formulas?

corner weight and %, spring height cw and %, spring height

weight in center of car lf 450 lbs 24.9% 5.24" rf 442 lbs 24.4% 5.29"

weight at end of lever 637 lbs 36.6% 3.54" 192 lbs 11% 6.34"


ballast centered lr 467 lbs 25.8% 5.49" rr 451 lbs 24.9% 5.74"

ballast at lever end 556 lbs 32% 4.97" 352 lbs 20.3% 6.45"

Thanks, Kevin Wallace

 

[GregLocock]

I don't know, debugging spreadsheets is not something I do willingly, but there is no such thing as a sprung cg height at each axle of a vehicle. You may be thinking of the dreaded roll axis, which is a fake concept. Vehicles do not roll about the roll axis as it is usually defined, or the principal inertial axis, or any other easily defined axis. The only reliable way I have found to determine it is to measure the lateral motion of the sprung mass at three locations.

Cheers

Greg Locock


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