Date: Sun, 31 Dec 1995 08:59:54 -0500
From: read_at_engr05.comsys.rockwell.com (Pete Read)
To: jrallen_at_halcyon.com, kkwollen_at_aol.com
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'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
>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
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
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,
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