BMW e30 steering geometry 3

[ke70gt]

Hi there
I’m trying to set up an BMW e30 for drifting and after a good look at the front suspension I noticed the front axle was placed around 50mm forward of the bottom ball joint so that the front wheel has move left to right slightly and forward and backwards due to the 14.4mm scrub radius it has from standard I haven’t seen any other steering arm with the axel so far forward and wanted to know what the theory or reason was behind this before I start modifying it all
cheers
Just a pic of the bmw e30 suspension setup looking forward

This pic shows the later model e36 setup ignore the one on the left it the best pic I could find to help explain it better

 

[Tmoose]

Hi cibachrome,

A while back you said "The way to tell an O.E.M. (BMW) tire is with the dime sized star symbol on the sidewall."

Well, this past weekend I was Cleaning up the garage nether regions and found a couple of tires given to me by a buddy about 10 years ago. One was a Brandy new 70 series Michelin MX something or other on a steel wheel. I still recall he said one of them was a spare off some departed BMW car. Lo and behold there is a star molded in the sidewall. I study tire sidewalls from time to time and can't recall ever seeing that symbol before. I guess I know why.

Still has a paint stripe around the tread, full of air, and a quick inspection revealed nary a sign of cracking in tread grooves or on sidwalls.


thanks

Dan T

 

[NickJ67]

So we seem to be saying that BMW base their steering geometry design to a large extent on a particular tyre construction (presumably developed with/for them) marked with a star, but made by several manufacturers. They then send these cars out in large numbers to parts of the world where those tyres are not available and no-one, not even the dealers, knows the significance of the star-marked tyres? How is that supposed to help with the "ultimate driving experience" (or even basic safety) once you've worn out the OE fitment tyres? Doesn't sound that smart to me.......

This notwithstanding I have learned alot from this thread and appreciate the knowledge shared.

Nick

 

[GregLocock]

That is the big ugly truth of steering and handling. The tire is the most important determinant of steering. The car is optimised around at most 3 tires, and you as the consumer have no way of deciding what replacement tire to use. Even worse than that, in 3 years time, when you replace your factory fit tires with identical named and sized ones from a tire retailer, you won't even be fitting the same construction and compound in some cases (this may not be as common as it was).

I can think of no other safety related item where this would be tolerated.





Cheers

Greg Locock


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[dynatune]

Some Background information on German cars:

Rear wheel driven cars like BMW, Mercedes and Porsche drive with very high castor angles. Typically BMW is around 7°, Porsche around 11° and Mercedes in between. In order to not arrive at immense mechanical trail values the steering axis is having at wheelcenter an elevated castor-offset (side view) of up to 15mm. Going for a pointed forward damper tube on the BMW is usually done for 2 reasons: a) to maximize the overlap between damper piston and strut housing increasing thus the camber stiffness and 2)the extra inclination of the damper increases the caster in jounce travel. Caster creates in a steering motion on the outside wheel more negative camber reducing understeer. It is a brilliant way to reduce understeer and works in effect only when needed (steering = cornering). This allows suspensions to have relatively low jounce motion camber gain values which is important for having a non-road-copying car at 250 kph on the german highway. To some extend the low speed non returnability that has been mentioned here so much is also due to a compromise versus high speed stability. Porsche has never had complaining customers on lacking returnability in a parking lot but boy they have had their part of angry customers on high speed handling .... and when BMW is referring to be the ultimate driving machine they could not care less about low speed returnability (even if it is an error state, they just do not care, just as porsche brakes do always squeal, the Brand DNA is strong enough to cope with "unimportant issues" like that ...).

Now to drifting, as far as I can see on the pictures the cars have been modified quite heavily. I could be wrong but the nissan skyline seems to have a steering gear in front of the wheel center line with a trackrod fixing at the upright behind the wheel center line. This would create all the clearance necessary for the full lock inner and outer wheel positions and most likely also enough design area for more or less parallel steering. In case of a BMW you would probably get yourself a LHD or RHD (if your car is LHD) steering gear, custom made left and right uprights and some specially formed track rods and off you go ....

Cheers

Dynatune,

http://www.dynatune-xl.com