Suspension mods

[Vegas_M]

Actually, I thought it was a combined linear rate, not a progressive rate, Marko. The formula I have for dual rate springs is (sp1*sp2)/(sp1+sp2)=(150*550)/(150+550)=118lb/in.

You can get a nifty triple rate calculator here:
http://www.quebec4x4.com/outils/swayaway14inr3c.xls
Use the 1st rate is the soft spring, 2nd the stiff, 3rd can be used for the bump stop.

I was just presenting a simplified version and being a bit flippant...[^]. Much of what I wrote is a simplification of some very complex calculations. But, as I had a hard time finding it explained in one place, in a straight forward fashion, I thought it might be good enough to give the basics of what most people are interested in. I gave two url's if anyone wanted to look further, and even those sites simplify things.

M.

[Marko]

those 2 formula's are the same , if you toss the numbers around a bit , is a formula for spring stiffness for springs connected in series,

i expressed myself in a wrong way, overall graph is progressive , since you have a linear spring rate of combined springs ,then the smaller spring bottoms out , then you have another linear spring rate but as a much steeper line . thats what i ment, it goes from lower combined value to higher value of the bigger spring

[Hasbeen]

Martin, I may be old & a bit past it, however, I believe that the
angle of the strut, from vertical, & therefor the spring, increases
as the suspension is compressed. The angle between the lower ball
joint, & the strut mount must increase, as they become closser
together.

This is why we get an increase in negative camber, as the suspension
is compressed. This would give a slight decrease in spring rate, as
this occured, but probably not enough to alter the balance of the
car, after the corner was set up.

I have found that it requires a pretty heavy rear anti roll bar to
get the understeer of our cars down to reasonable.

It also requires the thing to be driven quite close to the limit,
particularly in slow corners. Otherwise, the understeer quickly
reasserts, as the rear grip is increased with moderate applications
of power. The famed slow in, fast out, simply promotes understeer
in these cars.

This is quite different to the rear end breakaway, induces by wheel
spin, when excessive power is applied. This can be fun. &
moderately quick, but the rapid tyre wear makes it unsuitable for
competition.

At higher speeds, the balance established at turn in, can be
maintained through quite long corners, if the spring rates, & shock
bump settings are high enough, in my experience. Admittedly this is
only at the lower limits, achievable, with my road going 7, & 8.

Hasbeen



Hasbeen

[Troy ODoherty]

Dont forget that the rear springs are mounted on the bottom control arm and not directly over the rear axle. This means that the wheel rate is approx half of the spring rate in the poundage range that are likely to be used in a road car.
John Wolfe quotes the formula for calculating the wheel rates in his book on racing TR7s.The wheel rate on Macpherson struts is close enough to the spring rate so as not to be to concerned about the change in a road car application.[:)]

Cheers Troy

[Vegas_M]

Beans,

I don't know about "old", but I'm sure you are not past it..... [:o)]

Re: "I believe that the angle of the strut, from vertical, & therefor the spring, increases as the suspension is compressed. The angle between the lower ball joint, & the strut mount must increase, as they become closer together. "

Correct, as long as the radius arm is below horizontal. As the suspension compresses the radius arm pivots up and the angle of the strut, from vertical, increases slightly. However, once past horizontal, the angle again begins to decrease slightly. I believe that this really applies more for MGB and N.Am. style A-arm suspensions than our struts. Many A-arms are set up to have lots of above horizontal movement range.

On the hoist, the radius arm of my TR7 drops about 4" below horizontal at the shock. I believe that it will reduce by about 1.5 - 2" once the weight of the car is on the wheel, (with my old springs anyhow...)leaving about 3 - 3.5" of travel before it's level. That's basically at the bump stop and leaves us about a half inch of travel (crushing the bump stop)before the suspension totally bottoms out. Not enough distance to have any impact on the angle of the strut.

M.