ChrisW
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| posted on 8/5/12 at 12:01 PM |
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Brake efficiency vs wheels size
Having a bit of a brain fart this morning and can't work this out!
For a fixed size of brake caliper, disc, clamping force, etc. Would the braking effect increase or decrease with wheel size?
Thanks, Chris
My gaff my rules
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A1
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| posted on 8/5/12 at 12:07 PM |
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wouldnt it be easier to stop a smaller wheel cause it carries less momentum? but then would that benefit stopping the car...
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nick205
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| posted on 8/5/12 at 12:14 PM |
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I don't think the wheel diameter would have a significant effect other than bigger diameter wheels tend to have wider tyres therefore a bigger
contact patch.
To change the efficiency you'd to increase the friction area or disc diameter (i.e. move the caliper further from the hub to increase mechanical
advantage.
(happy to be contradicted with physics BTW)
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Fred W B
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| posted on 8/5/12 at 12:18 PM |
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quote:
Would the braking effect increase or decrease with wheel size?
I would say decrease, because the bigger wheel has more leverage
Cheers
Fred W B
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liam.mccaffrey
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| posted on 8/5/12 at 12:25 PM |
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the deceleration of the vehicle would be less with bigger wheels. If it helps think of it at extremes say with 20' wheels and the leverages
involved there!
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ChrisW
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| posted on 8/5/12 at 12:26 PM |
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That may be true, but the relative speed of the disc surface to the pad would be less for the same speed with bigger wheels. Will that make a
difference?
Chris
My gaff my rules
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DavidM
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| posted on 8/5/12 at 12:36 PM |
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The leverage would be greater with a larger wheel, but for a given vehicle speed the larger wheel would also be rotating more slowly than a small
wheel. It would be the same regardless of the wheel size as it would be the ability of the brakes to overcome the forward momentum of the vehicle, not
the rotation of the wheel.
If the large and small wheels were just spinning at the same speed then the larger wheel would need more braking to stop.
David
Proportion is Everything
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coyoteboy
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| posted on 8/5/12 at 12:40 PM |
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Disc/pad combo can only create X amount of torque (ignoring RPM). Wheel can produce more torque with increasing diameter. Brake will rotate more
slowly but with any given PSI of line pressure will be able to produce less reaction torque to slow the car. Meaning either more pressure is required
or different compounds. Frictional heating effects, for same pressure, will be lower but to stop in the same time the same power must be dissipated so
no gain there, in fact some loss possibly due to lower centrifugal ventillation.
So all in all bigger overall wheel diameter = worse braking for any given system. That said, you can now fit a bigger rotor and larger calipers which
means you may reduce the likelyhood of overheat.
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ChrisW
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| posted on 8/5/12 at 01:13 PM |
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Thanks all. Seems the consensus is that a given braking system will be more efficient on a smaller wheel, which is what I need for my application!
Cheers, Chris
My gaff my rules
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phelpsa
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| posted on 8/5/12 at 01:15 PM |
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Take it back to the basics of what brakes do... kinetic energy into heat at a certain rate (power). If the kinetic energy (mv^2) stays the same and
your brake power stays the same them you will deccelerate at the same rate, completely independent of wheel size (other than its effect on mass 
).
Edit: Ahhh but your braking power doesn't stay the same because you've specified a constant clamping force.
The capability of the brakes to absorb power remains the same, but you need to apply a greater torque because your speed has decreased (power is
torque x speed). In other words, you'll need to press harder to achieve the same decceleration but you'll boil the fluid in the same
amount of time.
[Edited on 8-5-12 by phelpsa]
[Edited on 8-5-12 by phelpsa]
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britishtrident
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| posted on 8/5/12 at 03:13 PM |
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Tyre rolling radius rather than wheel diameter , with all other variables on hold the tyre rolling radius is smaller it will be easier to lock
the wheel but stopping energy that brakes can dissipate won't change much.
But Tyres with a smaller rolling radius lock earlier than the same width tyre with a bigger rolling radius.
Larger diameter wheel allow better brake cooling.
Wide wheels give poorer brake cooling than narrow wheels.
Aluminium alloy and magnesium alloy wheels sink more heat energy than steel wheels.
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coozer
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| posted on 8/5/12 at 03:40 PM |
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So, Ive gone from 15's with 195/50's to 13"s with 205/60 on the back, 185/60 on the front.. I see the advantage there is the 4kg
weight saving on each corner. However rolling diameter is just about the same.
Can I expect to see an improvement in braking performance??
1972 V8 Jago
1980 Z750
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matt_claydon
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| posted on 8/5/12 at 04:19 PM |
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quote:
Originally posted by coozer
So, Ive gone from 15's with 195/50's to 13"s with 205/60 on the back, 185/60 on the front.. I see the advantage there is the 4kg
weight saving on each corner. However rolling diameter is just about the same.
Can I expect to see an improvement in braking performance??
No, if rolling diameter is the same then braking will be the same.
Greater rolling diameter = less deceleration for a given pedal force
Smaller roller diameter = more deceleration for a given pedal force
Maximum possible decel will be the same (assuming same tyre width) as unless your brakes are really poor you will always lock up the wheels before you
run out of leg strength.
Fade performance will be unchanged as the amount of energy you are asking the discs/pads to absorb has not changed.
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