Whitbread
Vendor , w/Business number
I've never seen this on the forums and in lieu of recent events with my car, I felt it was time. Here's all the math needed to mathematically select a clutch (provided you can get the specs from the manufacturer).
T=(PNrCf)/12
T=torque capacity
P=pressure plate clamp force in pounds
N= number of friction facings on disk (single disk has two facings so therefore 2)
r=mean effective radius
Cf=coefficient of friction
The mean effective radius is calculated by r=Sq. Root(((ro^2-ri^2)/2)+ri^2)
ro=outside radius of disk (and friction surface)
ri=inside radius of disk to friction surface
20% is the general accepted average for width of friction surface compared to disk diameter, A sachs vr6 happens to be 1.5" wide (under 20% rule it would be 1.8")
So with our disks, they have an outside radius (ro) of 4.5"and an inside radius (ri) of 3".
Cf ratings for different friction materials are as follows using averages:
Composite (organic): .30
Kevlar: .32
Copper ceramic: .35
Sintered Iron:.47
and if your pockets are deep enough, Carbon fiber: .48 and twice the life of just about everything else
Lost yet? Ok, so for our cars we have a 9" diameter single disk clutch.
90% of us are running organic clutches so we'll assume a Cf of .30. We'll use an 1600lb pressure plate for our calculations as it's a very common clamp load among smaller stuff (unless someone knows an exact one).
First we must find r. Normally the the 20% rule is used, but we'll use 1.5" since thats what the vr6 disk in my garage measures.
r=sq. root(((4.5^2-3^2)/2)+3^2) = s.r(20.25-9)/2)+9) = s.r (14.625) = 3.82
Now that we have r we plug it in to the formula:
T=(PNrCf)/12 = (1600x2x3.82x.3)/12= 305.6 ft/lbs of torque capacity. Sounds just about right for what is claimed for a sachs OE vr6 clutch.
Equally useful to us is to turn the formula around to solve for needed clamp load:
P=(12xT)/NrCf
So for a tdi generating 400 ft/lbs of torque at the crank with an organic disk: P=(12x400)/(2x3.82x.3)= 4800/2.292= 2094lb minimum clamp pressure plate needed. A 2200lb plate would be a safe choice for this application.
Hope this sheds some light onto the subject and helps people to not over or under buy a clutch.
T=(PNrCf)/12
T=torque capacity
P=pressure plate clamp force in pounds
N= number of friction facings on disk (single disk has two facings so therefore 2)
r=mean effective radius
Cf=coefficient of friction
The mean effective radius is calculated by r=Sq. Root(((ro^2-ri^2)/2)+ri^2)
ro=outside radius of disk (and friction surface)
ri=inside radius of disk to friction surface
20% is the general accepted average for width of friction surface compared to disk diameter, A sachs vr6 happens to be 1.5" wide (under 20% rule it would be 1.8")
So with our disks, they have an outside radius (ro) of 4.5"and an inside radius (ri) of 3".
Cf ratings for different friction materials are as follows using averages:
Composite (organic): .30
Kevlar: .32
Copper ceramic: .35
Sintered Iron:.47
and if your pockets are deep enough, Carbon fiber: .48 and twice the life of just about everything else
Lost yet? Ok, so for our cars we have a 9" diameter single disk clutch.
90% of us are running organic clutches so we'll assume a Cf of .30. We'll use an 1600lb pressure plate for our calculations as it's a very common clamp load among smaller stuff (unless someone knows an exact one).
First we must find r. Normally the the 20% rule is used, but we'll use 1.5" since thats what the vr6 disk in my garage measures.
r=sq. root(((4.5^2-3^2)/2)+3^2) = s.r(20.25-9)/2)+9) = s.r (14.625) = 3.82
Now that we have r we plug it in to the formula:
T=(PNrCf)/12 = (1600x2x3.82x.3)/12= 305.6 ft/lbs of torque capacity. Sounds just about right for what is claimed for a sachs OE vr6 clutch.
Equally useful to us is to turn the formula around to solve for needed clamp load:
P=(12xT)/NrCf
So for a tdi generating 400 ft/lbs of torque at the crank with an organic disk: P=(12x400)/(2x3.82x.3)= 4800/2.292= 2094lb minimum clamp pressure plate needed. A 2200lb plate would be a safe choice for this application.
Hope this sheds some light onto the subject and helps people to not over or under buy a clutch.
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