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Timing belt speed

prove_it

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This got brought up. I never thought about it, but having a gentleman's argument over the speed that the timing belt spins.

I would think he belt turns at the same speed as the crank sprocket which is the engines rpm. Kinda of a thinking question though. Cam sprockets spin at 1/2 speed of rpm, but that is due to the "gear" reduction in the cam gears. So in theory wouldn't the belt be moving the same speed as the crankshaft sprocket?
 

Coltsfan

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The belt would move at the same speed as any/all the pulleys/sprokets, but not the same RPM as any of them.
 

EHmotorsports

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The crankshaft sprocket moves the belt. But one full revolution of the crank does not equate too one full revolution of the belt. Being this is a 4 stroke engine. The belt should move 1/4th of the engine rpm.
 

prove_it

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Ah I see where your going. Ok, makes sense now.
 

iceman69510

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I don't agree it is necessarily moving 1/4 crank speed. What is your measurement of belt speed? RPM? Distance/second? Actual relationship of speed between like gears is based on the diameters (as mentioned with the cam/crank relationship). With a belt, you don't necessarily have a diameter, but you have a length, which made into a circle could be then calculated for the diameter. The speed would be the relationship of the length of the belt vs the circumference of the pulley driving it. Picture the belt as a big circle with the crank as a smaller gear driving it from inside the circle.

Example: The belt is 30" long, and the circumference of the crank pulley is 6". The belt would be moving 1/5 crank pulley speed.
 

iceman69510

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Quoting donniekak:
Crank gear teeth/belt teeth = ratio of belt rpm to crank rpm.



Donnie said it simpler. /ubbthreads/images/graemlins/worthy.gif /ubbthreads/images/graemlins/rofl.gif
 

EHmotorsports

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appologies it's been a rough morning. I will find out the exact speed when I get to the shop.

Update: if you are using a belt with 153 teeth and the crank sprocket is 24 tooth. the crank will rotate 6.375 times per 1 belt rotation. 6.375 to 1
 
Last edited:

prove_it

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So in reality, the belt is moving much slower than the crank.
 

EHmotorsports

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6.375 times slower

so lets say you are at 8000 rpm's. the belt would be at 1,255 rpms
 
Last edited:

donniekak

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Same surface speed, different rpm.

Let's say you have a bicycle with a 20" rear wheel, and 5" front wheel, you are traveling at 20 mph. The front tire is spinning at 4x the rpm of the rear, but the road speed is the same for both tires.
 

belize1334

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Exactly. At 3000rpms With a ~3" diameter crank sprocket, the linear speed of the gear teeth is about 60mph (3000rpm*6"*pi)*(60min/hour)/(12in/ft * 5280ft/mi). The belt runs on those same teeth so the belt also runs at about 60mph, as do all the pulleys and gears that it touches. But each pulley has a different diameter so they will SPIN at different rpms (ratio of rpm = 1/ ratio of diameter). The belt has the biggest circumference so it "spins" the slowest. But the rpm of the belt is sort of irrelevant. It's like talking about the rpm of water going down a water slide. It technically circulates through the system but is it really spinning? What's more important is the flexion rate, i.e. total curvature of the belt divided by time to traverse curvature. This is *roughly* equal to 1/diameter of each pulley, times it's rpm, all added together. That will tell you how much the belt has to flex and how often and will give some indication of how fatigued it is by the job it does.
 

prove_it

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We need a mind blown graemlin.
 

tektic

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Quoting belize1334:
Exactly. At 3000rpms With a ~3" diameter crank sprocket, the linear speed of the gear teeth is about 60mph (3000rpm*6"*pi)*(60min/hour)/(12in/ft * 5280ft/mi). The belt runs on those same teeth so the belt also runs at about 60mph, as do all the pulleys and gears that it touches. But each pulley has a different diameter so they will SPIN at different rpms (ratio of rpm = 1/ ratio of diameter). The belt has the biggest circumference so it "spins" the slowest. But the rpm of the belt is sort of irrelevant. It's like talking about the rpm of water going down a water slide. It technically circulates through the system but is it really spinning? What's more important is the flexion rate, i.e. total curvature of the belt divided by time to traverse curvature. This is *roughly* equal to 1/diameter of each pulley, times it's rpm, all added together. That will tell you how much the belt has to flex and how often and will give some indication of how fatigued it is by the job it does.



This answer is most correct.
 

prove_it

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Leave it to 1334 to make me reread the thread multiple times to get it down right. /ubbthreads/images/graemlins/rofl.gif
 

steve

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Quoting prove_it:
We need a mind blown graemlin.



/ubbthreads/images/graemlins/runt.gif
 

EHmotorsports

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/ubbthreads/images/graemlins/rofl.gif /ubbthreads/images/graemlins/rofl.gif
 
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