assumed roll rate

[abhishek502]

This is my first year, so its a rather stupid question but its been bothering me for sometime now..
i am right now doing roll and ride calculations from milliken & milliken,but i am not able to get my head around how they've assumed the roll rates to be 70000 lb.ft/deg and 50000 lb.ft/deg for front and rear respectively and how do i assume the roll rates for my car?..
i have all the values- front weight, rear weight, front track width, rear track width, Cg height, CG to roll axis height, roll axis height above ground for rear and front, wheelbase and cornering conditions.

if someone can please help me with this,it would be really appericiated.

THANK YOU!!..

[acedeuce802]

There are formulas that relate wheel rate, and ARB rate into roll rate. They're pretty simple formulas, and can be figured out with an FBD... or do some searching around. Figure out your roll moment, decide a roll gradient (deg/G), then calculate required roll rate.

[MCoach]

Those values are just assumed from experience in the book (remember, these guys have been doing this for a while).

It really doesn't matter a whole lot of what you originally assume in your calculations, whether it be 5ft-lb/deg or 5,000,000 ft-lb/deg because you will have an overriding target that you will be working towards (roll stiffness).

Your spring selection will make up for part of this based on your ride choice (frequency), the rest is made up by your ARB.
The key to it not mattering is that you will iterate to get your final solution.

So, even your second iteration may be close to what you are looking for, even if the first guess is factors away from being correct.
By deciding a roll gradient and putting in an initial value, you can see how far off you are from where you want to be. When you perform the ride iteration and then circle back to the roll iteration, and eventually come up with an answer of what you are looking for though a couple iterations.


One thing I might suggest is to make a plot alongside a spreadsheet of your roll axis compared to your CG points as slices. It will give you something to think about in terms of chassis balance.

Working through the solution in the book with their values will give you something to check against if you are building a spreadsheet.



Hope that helps.

[Edward M. Kasprzak]

Note that those values in RCVD are given in lb-ft/rad, not lb-ft/deg as you stated.