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      09-03-2010, 01:54 AM   #1
rzm3's Avatar

Drives: E92 M3
Join Date: Aug 2007
Location: USA

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Arrow Technical White Paper: Lowering Springs and Bump Stops, Take Three

After playing with various suspension set ups over the past two years (H&R, Dinan, Moton Street Sport, and soon Moton CS), I've learned quite a bit about this M3's suspension, and I thought it's time to write another article regarding the highly contentious topic of lowering springs and bump stops.

I will attempt to do this with no equations and pure common sense.

Bump Stops

Bump stops, as most of you know, are engineered elastomers. The main advantage of bump stops vs. a traditional spring is that the former can be engineered to provide variable stiffness at a very light weight.

In the old days bump stops are simply installed to prevent the suspension from topping or bottoming out; they do not affect the suspension except at the limits of travel. However, modern day bump stops have started to play an increasingly more important role on street cars, especially on a every day sports car like the M3.

Comfortable on the Street and Handles Well on the Track

The E9x M3 has been praised by almost all car magazines in the world as the best all-around sports car on the market today... because you can comfortably take your kids to school and then pass everyone on the race track.

The M3 is comfortable on the street because its springs are relatively soft, both the front and rear have wheel rates (lb/in) in the mid-hundreds. Having soft springs means that the wheels have greater travel with less abrupt transitions over bumps, which eliminates the rough ride experienced dedicated track cars.

But softer springs are not usually well suited for the race track, because they allow too much suspension travel when the car is under high G forces, i.e. in a corner, or under braking, and this prolongs transitions and unsettle the platform.

M3's Bump Stops

The extensive use of bump stops is one reason why the M3, even with a very comfortable ride, can handle the race track so well.

As Steve Dinan has pointed out, there is only about 0.5" of suspension travel before the front bump stops come into play; a similar situation applies for the rear.

This is exactly the intention of the BMW engineers.

These bump stops are engineered so that they are initially very soft (compressible by hands), but as compression increases, they get progressively harder. The bump stops gradually increase the effective wheel rate to control suspension travel and maintain the platform of the car.

This is how the M3 gets away with soft springs.

The "Cardinal Sin" of Riding on Bump Stops

Bump stops have become a catch phrase for those who are considering lowering springs. Dinan has of course extensively used "modified bump stops" as the chief marketing slogan for why their kit is superior.

But as I have explained above, even in stock form, the M3 is designed to ride on its bump stops more often than you think.

Consider this, when there are four people in a car, your M3 will be already on its bump stops... and that is perfectly OK, as the bump stop is initially very soft.

What is NOT OK
  • Excessive lowering: M3's bump stops are initially very soft, but when compressed to a certain length, they will become very stiff.

    This is why it is not a good idea to excessively lower your M3 with spring kits. The M3 has roughly 2.25 inches of compression travel. If you lower too much you would be constantly bottoming out on the bump stop.

  • Uneven front/back lowering: the M3 is designed to be pretty neutral (with slight understeer) at the limit. The point here is that the front and rear suspension are balanced to produce this effect. If one end of the car is lowered significantly more than the other, that axle will likely bottom out, loose grip sooner, and change the balance of the car.
My Recommendations
  • Do your research!
  • Be moderate when lowering!
  • Have a qualified mechanic install your kit.

Remember, it is OK to ride on bump stops, but ride with good judgement.
Appreciate 0