Fred W B
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| posted on 25/2/15 at 05:32 PM |
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Design clearance between chassis and road at full bump?
Anyone care to comment on what would be a suitable design clearance dimension between chassis and road at full suspension bump travel for a car to be
used on road and track.
For example, if a car had 100 mm of ground clearance, and a suspension travel of 60 mm in bump from ride height, the chassis would be 40 mm from the
ground at full bump. I am talking here purely of the damper travel, spring rate does not come into it at this stage.
I understand that this clearance will vary depending on the actual profile of the road being driven on. I see some cars do "bottom" in
certain circumstances, and you see people winding up static ride height or spring rate to try to prevent this, but in the case of a clean sheet design
it would seem incorrect to design so that the clearance would be 0 mm.
Cheers
Fred W B
[Edited on 25/2/15 by Fred W B]
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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Angel Acevedo
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| posted on 25/2/15 at 06:54 PM |
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Hi Fred.
At some point in time I commented on this subject.
I´m looking for 4.5" (114.3 mm) front 5" Rear Ride Height.
This to the lowermost part of the chassis.
Bump travel will be 3" approx maybe a little more on the front due to Wheel Travel being larger than shock travel.
This will put me in the 1.5" to 2" clearance at full bump. (38 mmm to 51 mm)
My sump will end maybe 12 to 18 mm below my rails, but the only time clearance would be compromised is crossing Sleeping Policemen...
Roads here in Mexico are bad, but I think I can get away with it.
I think 40 mm is more than enough for a track regardless of condition.
On my previous post on the subject I wrote that Sacrificial Rubbing Strips may be installed to avoid damage to the Paint/Chassis, and reply was that
even though you may do so, ride would suffer as spring rate would rise ad infinitum when the strips contact the ground.
Maybe racers may have a more definitive answer on this.
Regards.
AA
Beware of what you wish.. for it may come true....
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Sam_68
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| posted on 25/2/15 at 09:30 PM |
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quote:
Originally posted by Fred W B...in the case of a clean sheet design it would seem incorrect to design so that the clearance would be 0
mm.
Two blindingly obvious comments:
* Suspension travel should never be limited by damper travel. You should be running bump rubbers to ensure that your dampers never go solid.So your
maximum wheel deflection will always be less than that permitted by the damper stroke.
* Conversely, you're not running on cast iron tyres. So if there's enough of a load on your suspension to be compressing the suspension
onto the bump stops, there's enough to be compressing the tyres quite a bit, too.
You'll need to take both these factors into account when assessing what you judge to be suitable clearance (though to some degree they cancel
each other out).
Tyre rates are complicated - they're rising rate and depend on pressure, apart from anything else - but you can make an initial assessment by
measuring the static deflection against a known corner weight.
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Angel Acevedo
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| posted on 25/2/15 at 10:57 PM |
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quote:
Originally posted by Sam_68
quote:
Originally posted by Fred W B...in the case of a clean sheet design it would seem incorrect to design so that the clearance would be 0
mm.
Two blindingly obvious comments:
* Suspension travel should never be limited by damper travel. You should be running bump rubbers to ensure that your dampers never go solid.So your
maximum wheel deflection will always be less than that permitted by the damper stroke.
* Conversely, you're not running on cast iron tyres. So if there's enough of a load on your suspension to be compressing the suspension
onto the bump stops, there's enough to be compressing the tyres quite a bit, too.
You'll need to take both these factors into account when assessing what you judge to be suitable clearance (though to some degree they cancel
each other out).
Tyre rates are complicated - they're rising rate and depend on pressure, apart from anything else - but you can make an initial assessment by
measuring the static deflection against a known corner weight.
Hi Sam_68.
I never put thought on tyres, but for my limited building experience the only time this would happen (Both Tyres in Bump) is whenever I hit Sleeping
policemen -or a curb head-on, if I am doing any speed beyond barely moving on those two scenarios, I have worse problems than just chassis-Ground
clearance.
By the way, I´ll try to upload some time what we have here in Mexico.
Best Regards
AA
Beware of what you wish.. for it may come true....
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Fred W B
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| posted on 26/2/15 at 07:16 PM |
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Thanks for the comments
quote:
Suspension travel should never be limited by damper travel. You should be running bump rubbers to ensure that your dampers never go solid.So your
maximum wheel deflection will always be less than that permitted by the damper stroke.
Yes, that is understood. The 60 mm travel mentioned as an example would be up to, and include compression of, the bump stop.
quote:
Conversely, you're not running on cast iron tyres. So if there's enough of a load on your suspension to be compressing the suspension onto
the bump stops, there's enough to be compressing the tyres quite a bit, too.
Fair point, that had not occured to me yet.
Cheers
Fred W B
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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