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Date: Sun, 31 Dec 1995 08:59:54 -0500
From: (Pete Read)
Subject: Re: M5 Suspension, Sway Bars

Keith Wollenberg writes:
>what you suggest is directly contrary to my seat of the
>pants experience. ... I spent many days at SIR (Seattle
>International Raceway), many years ago, playing with bars.
>Unless my memory is faulty, a large front bar and small
>rear bar is a recipe for oversteer, nor understeer. I am
>not surprised that increasing front pressure caused
>understeer. The later factory recomendations were for
>three pounds higher in the rear, which caused the car to
>oversteer more aggressively.
>As I understand it:
>When we increase the bar size, we reduce body roll. This
>causes *less* weight transfer to the outside tire,
>distributes the load between the two tires, and delays the
>incidence of slipping. This is why larger sway bars
>increase cornering force. If we increase one end more than
>the other, that end is less likely to slip.


Thanks for the response. Questions like this certainly make me think things through again and learn some fine points I've missed.

I'm sure your memory hasn't failed. Changing sway bars over and over has a way of sticking with you. After pondering this a while, and rereading a number of suspension articles, I found something that may explain your experience.

This is from the sway bar section of Puhn's book. "A stiffer front bar prevents excessive positive camber from occuring on the front tires during a turn. This effect is sometimes stronger than the extra weight transfer, and the result is less understeer."

So, after double checking, my M5 Suspension post agrees with every article I found. However, I see there are some exceptions to these "rules". In the case of your 1600, a bigger front bar increases the weight transfer that loads the front outside tire causing more understeer. But reduced positive camber change from the bigger bar (less understeer by keeping the outside tire flat on the surface) more than compensates for the understeering tendency from the bigger bar. The net result is less understeer that you experienced.

>From my (limited) experience, M5s react to suspension
changes as the books say. I've seen this basic table several places. It summarizes the effects of suspension adjustments and hopefully agrees with my M5 Suspension post.

Suspension Adjustment Table

   Adjustment           More Understeer     More Oversteer
   ----------           ---------------     --------------
1. Front sway bar       Thicker/stiffer     Thinner/softer
2. Rear sway bar        Thinner/softer      Thicker/stiffer
3. Front springs        Stiffer             Softer
4. Rear springs         Softer              Stiffer
5. Front wheel camber   More positive       More negative
6. Rear wheel camber    More negative       More positive
7. Front tire pressure  Lower               Higher
8. Rear tire pressure   Higher              Lower

Let me try to go through this and see if it makes any sense.

The centrifugal force (CF) from cornering is determined by car weight, cornering speed, and turn radius.

CF = (Weight lbs x speed^2 mph)/ (14.97 x Radius ft)

Total weight transfer is always the same, no matter what size sway bars or springs, as long as excessive roll isn't combined with a high Center of Gravity (CG). Lots of roll and a high CG cause a significant change in weight transfer due to the CG moving outboard. Performance cars, with limited roll and low CG, have negligible weight transfer from the CG moving outboard in a turn. Roll reduction is more important in limiting wheel camber changes than CG movement.

So total weight transfer is due to the cornering centrifugal force (CF), CG height, and track width.

Total Wt transfer (lateral) = CF x CG Height/Track Width

Total cornering weight transfer can be reduced only by lowering the CG height, increasing track width, or reducing the car weight (to reduce CF). It's hard to make a car wider (without flaring the fenders) or lighter, so most people just lower cars a little to reduce weight transfer to the outside tires. Then weight is reduced automatically when the exhaust system is torn-off from grounding!

While the total weight transfer can't be easily controlled, the front to rear distribution can be adjusted through sway bars and springs. The end with the most stiffness receives the largest portion of weight transfer. As you can see from the Suspension Adjustment table above, stiffer bars and springs transfer more weight and create more slip (e.g. stiffer front springs and bars cause more understeer). Think about replacing the front springs with solid blocks. If the front end doesn't roll at all, the total weight transfer is handled by the outside front tire.

Notice wheel camber though. More positive camber increases understeer. Sway bars reduce positive camber during cornering. In your case #5 (reduction of positive camber) is more important than #1 (weight transfer from stiffer bar).

Up to a point, increasing tire pressure reduces slip. Low tire pressure makes a tire more flexible and results in higher slip angles (understeer on front, oversteer on rear). Higher pressure resists distortion more and has smaller slip angles until the upper limit where the tread bulges from too much pressure (tire is no longer flat on surface).

That's why I needed six psi difference to reduce understeer at the track (40 front, 34 rear). With the sway bar changes (stiffer rear), I now run equal pressures front and rear. I think BMW's recommended M5 tire pressures of 36 front and 40 rear is intended to add more (safe) understeer.

Hope this makes sense,

Pete Read

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