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  #1  
Old 02-11-2006, 01:58 PM
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Join Date: Feb 2006
Posts: 10
brake bias front to rear...

I have driven several BMW's over the years and recently picked up a 300e.

After some interesting minor episodes in the snow and ice - I decided to try a panic stop on dry pavement and was able to lock up the front left (possibly both) caliper as the antilock engaged. I felt like the car nose dived a bit - again compared to the lighter cars I have been driving.

I do not remember being able to do that as easily with my lighter (2800lb) bmw.

we have also had trouble with the car sliding down the driveway in the AM with any snow on the ground - seems with the revs up as the engine races that it is much harder to control the car on the snow in reverse. Car has a set of well used Blizzaks on for winter.

I think the rear brakes are working - tried the brakes with the car jacked up - the seemed to work as I was expecting - very hard to turn the rear wheels with the brake on.

So,

is the fact the 300e is heavier ( what is the weight anyway? ) causing this or is there a brake bias valve needing adjusted or other issue?

Thanks for any input..

Don
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  #2  
Old 02-11-2006, 04:25 PM
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Location: Carson City, NV
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Quote:
Originally Posted by old300e
I decided to try a panic stop on dry pavement and was able to lock up the front left (possibly both) caliper as the antilock engaged. I felt like the car nose dived a bit - again compared to the lighter cars I have been driving.

I do not remember being able to do that as easily with my lighter (2800lb) bmw.
Heavier car+softer suspension=more nose dive on braking. You should have seen my '73 Chrysler.

Quote:
we have also had trouble with the car sliding down the driveway in the AM with any snow on the ground - seems with the revs up as the engine races that it is much harder to control the car on the snow in reverse.
Have you tried letting the engine warm up until the idles comes down? If the engine is running right this shouldn't take long.

Quote:
Car has a set of well used Blizzaks on for winter.
How well used? If your tires are bald or nearly so, that would explain a lot.

Quote:
I think the rear brakes are working - tried the brakes with the car jacked up - the seemed to work as I was expecting - very hard to turn the rear wheels with the brake on.
This one concerns me. If you have someone step on the pedal hard, it should be next to impossible to turn a jacked-up wheel by hand.

Quote:
So,

is the fact the 300e is heavier ( what is the weight anyway? ) causing this or is there a brake bias valve needing adjusted or other issue?

Thanks for any input..

Don
Most likely, you are experiencing normal operation. Brake systems are normally designed so that the front wheels will lock just before the rears. Some nose dive on hard braking is typical for most cars. However, it is possible you are having an issue with your rear brakes. Generally speaking, how well does the car stop?
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  #3  
Old 02-12-2006, 11:22 AM
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thanks for the reply...

I forgot that this car could use rear shocks - I bet that will make a difference in the nose diving effect...

I try things one at a time and see how things improve...
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Old 02-12-2006, 02:01 PM
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If in proper working order, any modern brake system will lock or engage ABS with enough pedal effort. It comes down to available tire grip, and dedicated winter tires have poor grip in warm weather. It's ultimately the adhesion coefficient of the tires that limits deceleration.

Weight transfer under braking is a function of wheelbase and CG height. The amount of dive in response to weight transfer is a function of spring rates and suspension geometry, in particular, the amount (if any) of "anti-dive" built into the suspension geometry. Same with the rear under acceleration, but the engineeering term is "anti-squat".

Shocks have nothing to do with it. Shocks only provide a resisting force proportional to stroke velocity. They will affect the RATE of dive when the brakes are applied, but will not effect the amount of dive once steady state deceleration has been achieved. Ditto for body roll in cornering.

If you have a worn out set of winter tires, THAT is likely the source of your braking problems, not shocks.

Control arm supensions allow quite a variety of anti-dive geometry by angling the axis of the pivot points. The vertical distance from the CG at the point where the axis lines intersect is the key, and this is why the upper front control arm is often angled down at the rear. There is a formula to compute "percent anti-dive". If the axes intersect at the CG height, the anti-dive would be 100 percent, but the car would be unstable under braking, so 50 percent anti-dive is usually the maximum that is designed in. Same for the rear except the term is "anti-squat". Multilink suspensions offer the same anti-dive design flexibility, but it's a little harder to calculate because of the additional reaction force paths.

On strut type suspensions, anti-dive is achieved by tilting the top of the strut to the rear. The brake reaction force line of action originates at the top of the strut and is normal to the strut axis. The lower line of reaction is along the axis of the control arm pivots.

You could actually design a front suspension that LIFTS the front under braking by angling the control arms so the intersection of the pivot axes is ABOVE the CG, but this would require some severe angles, which would cause a wide variation in caster with suspension movement, which is bad. Also, "banking" suspensions have been designed where the car rolls toward the center of the turn radius, like a V-hulled speedboat, but such designs result in so many other poor compromises in suspension geometry, they have never been successful.

Duke

Last edited by Duke2.6; 02-12-2006 at 02:15 PM.
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  #5  
Old 02-12-2006, 03:00 PM
t walgamuth's Avatar
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Join Date: Apr 2005
Location: Lafayette Indiana
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i dont very often

disagree with mr duke, but i disagree about the effect of bad shocks on braking. the dive will be more severe, coming on faster and the control of the wheel specifically from hopping will be compromised. if you have bad shocks the rear wheels will hop out of control under hard braking. it can't be good for the front either.

having the rear wheels hop under hard braking is a little too exciting for me.

now with abs ;you may keep going straight but if the rear wheels are in the air half the time they wont be stopping the car and your distance will increase. all systems are made to work best if maintained properly. this includes the whole package, suspension bushings, springs, shocks, brakes and of course the most important, tires.

tom w
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