Sanity check please - front spindle

[v8kid]

Could I get a sanity check from the members here?

I've been working out the geometry for a revised front spindle on a hillclimb/sprint car and have decided to keep the camber recovery lowish using medium length swing arms around the same length as the track and use caster to recover the camber.There was some compromise packing it all inside the wheel and I ended up with 3 degrees kpi, 9 degrees caster and to keep the trail down to 15mm placed the wheel spindle 27mm in front of the steering axis. The scrub, due to the available wheels ended up at 52mm. The geometric roll center - for what it is worth ended up at 35mm above ground and does not move relative to the c of g.

Does this sound sensible for a mid engined 60/40 car that is used on tight circuits/hillclimbs?

Cheers

David

David Child

 

[GregLocock]

I can't see that much castor really being a benefit, in itself, but other than that it seems ok. I assume it is short spindle, ie arms inside the wheel? Do you really need the camber-in-turn?

52 scrub is fine, might fidget a bit in straight line braking but at least that tells you which wheel is locking.

Cheers

Greg Locock


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[v8kid]

Thanks Greg, yes it has the spindle inside the wheel. I added caster to increase camber when the wheel is turned. As I only have 50% camber recovery through the suspension I need a bit extra. Also I have 9 inch front tyres and want the static camber to be close to upright to get good braking.

Is it worth reducing the scrub by increasing the kpi do you think?

David Child

 

[Yawing]

I would say it is worthwhile to reduce that scrub by increasing the KPI. negative being it takes off camber with turn (good for inside wheel, bad for outside) but with the caster you have should counteract it. positives of reducing the scrub is reduce the steering wheel pulling to one side when braking over uneven surfaces, sure its ok to have the feedback of which wheel is locking, but if you are braking over surfaces which vary from side to side you could find it too darty and you cant hold the line.
Also on trail braking you dont want the torque due to the scrub overpowering the feedback from the pneumatic trail/self aligning torque of the tyre.

 

[patprimmer]

9 deg of castor and 2" of scrub might make the steering very heavy unless the front end weight is very light.

Regards
Pat
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[GregLocock]

I'd be inclined not to increase kpi, the scrub radius is well inside the normal range (-25 to +100 mm) and the compound steering axis inclination is at the upper limit of what I'd like to see.

What apex speed are you designing for?



Cheers

Greg Locock


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[Yawing]

compound steering axis is at the upper limits? There are successful race cars getting around with close to 20 degrees castor and 7 degrees kingpin inclination.

also the steering weight is more a factor of the castor trail not the castor angle. 15mm castor trail is very little.

 

[patprimmer]

Sorry

I forgot about the spindle being forward of the steering axis.

Regards
Pat
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[GregLocock]

Hey just maybe what I'd /like/ to see isn't the extreme seen in practice.

Cheers

Greg Locock


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