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Old 7 Feb 2004, 14:56 (Ref:866378)   #1
Matt~Dobbs
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Designing Suspension

Hi all

I am a Student at Coventry University, and I am designing the suspension for a space-framed minivan with a mid-mounted Porsche engine.
I have chosen a bump rebound travel of 125mm (5”) and my roll centres are 35mm front and 45mm rear, I am designing the geometry so that the wheel will have no chamber change with roll.

How much roll angle should I design it to?

And

How much Ackermann angle should I have, and do you design it from the point that the KPI intersects with the ground, as this is the rotational axis.


I would be very grateful for any advice.
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Old 7 Feb 2004, 17:00 (Ref:866443)   #2
StephenRae
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Without knowing what use the vehicle is to be put to it's not possible to advise...5% roll is a lot! If the steering arms are in front of the front track you will be limited to as much Ackerman as you can get, for racing it doesn't really matter. Use the centre of the tyre footprint for all calculations.
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Old 8 Feb 2004, 11:21 (Ref:866964)   #3
Matt~Dobbs
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Stephen

The car will be used mainly on tracks but it will also be road going, so my main concern is for tracks, I am design most parts (within reason) to be adjustable so I can adapt if needed.
I have been told that today’s road sports cars have about 3 degrees roll, so I assume that it will be between 1 and 2 degrees,I wanted to be certain, as I am not sure on the difference it will make.

Designing the Ackermann angle from the centre of the wheels contact patch,
If I had centre point steering I would not have a problem with this, but I have a scrub radius of 47.4mm and a KPI of 6 degrees. As Ackermann is to make the inside wheel rotate more than the outside, would it not make sense to use the wheels rotational axis? And do I take into account castor as this has inclination on the KPI.

I can understand that the centre of the contact patch is used for load calculations (apart from steering torque), but I am struggling on this on as it doesn’t seem logical.

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Old 8 Feb 2004, 13:40 (Ref:867083)   #4
StephenRae
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Are we talking about a 'Morris' minivan shape or an American people carrier...if it is the latter I can understand your concerns about excess roll.
KPI is only there to ease the stress on the wheel bearings, but together with castor angle, it does create scrub if the wheel is turned from lock to lock when stationary. You have to base your calculations somewhere and the centre of the tyre is a fixed point, in effect the car is moving about this point It is of little consequence what the inside, lightly loaded front wheel is doing while cornering.
Ackerman discovered a 100 years ago that if you align the steering arms with the centre of the rear axle then the inside wheel will follow a tighter radius than the outer wheel, hence no tyre scrub. Racecar designers often use varying ammounts in anticipation of the outside wheel doing most of the work and having a greater angle of slip.
Current racing cars run varying degrees of negative camber to keep the tyre contact as big as possible, and that is with minimal suspension travel. With a dual purpose vehicle if you were to be able to design out camber change in roll then you would be left with extreme camber and toe in changes during bump and squat......it's all a compromise!
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