Calculating RPM

[colinr]

My bike computer doesn't tell me this and I can't work out the maths but am curious as to how fast my legs are going round. Is there a formula?

For example, my top speed today was 31.4 mph.
Riding fixed at 46/17 on 700x25 tyres (1 revolution measured as 213cm)

Is that enough numbers to work out how many RPM I'd be pedalling at that speed?

 

[colinr]

Thanks! Now I have to make the numbers higher :?:

 

[scots_lass]

Presumably your bike computer has a clock? Would it not be simpler to just count the number of times your leg goes round in say 15 s or 30 s and multiply by 4 or 2? The maths above seem very complicated!

 

[Mike Rudkin]

Er147 rpm ??? I find 90 very fast and difficult to maintain-could the Maths be a tads flawed ??

 

[jimboalee]

There comes a point where your own reflexes can't get the muscles contracting fast enough to put power on the pedal.

I read a report once where the professional cyclist under test got to 150 rpm and then couldn't pedal any faster. ( on an ergonometer ).

I believe the terminology is 'spinning out'.

In the motor car engine business it's called the 'flame front velocity' where the expanding gases can't get fast enough to push the piston.

On a track bike, the next step would be to add 1 tooth on the chainring.

Then there comes a point where the riders capability 'tops out'.

I hope you have a good selection of chainrings?

 

[jimboalee]

Er147 rpm ??? I find 90 very fast and difficult to maintain-could the Maths be a tads flawed ??

No. The maths is good. User3143 did a good job.

If anyone has an idea to try this kind of cadence on a fixed, please, please warm up properly and stretch properly.
If you cramp at these speeds, its a bit like a motor car engine 'throwing a rod'.

 

[MajorMantra]

Presumably your bike computer has a clock? Would it not be simpler to just count the number of times your leg goes round in say 15 s or 30 s and multiply by 4 or 2? The maths above seem very complicated!

Counting whilst spinning is harder than simply running the numbers from the speed afterwards. This calculator makes things quicker though:

http://software.bareknucklebrigade.com/rabbit.applet.html

There comes a point where your own reflexes can't get the muscles contracting fast enough to put power on the pedal.

I read a report once where the professional cyclist under test got to 150 rpm and then couldn't pedal any faster. ( on an ergonometer ).

What kind of pro couldn't top 150? Even I can do 180+ (a little under 40mph on my fixed) and track riders sometimes do over 200.

Matthew

 

[Sittingduck]

+1 for the old skool counting method. I find the addition of a bar mounted abacus looks stylish and is a great ice breaker when stopped at the lights and sexy female cyclists arrive!

 

[colinr]

If you cramp at these speeds, its a bit like a motor car engine 'throwing a rod'.

I'd had a 20 mile warmup and this was a sprint to the finish top speed. I'd be very impressed with myself if I could hold a 30mph+ speed for more than a few seconds downhill!

Would it not be simpler to just count the number of times your leg goes round in say 15 s or 30 s and multiply by 4 or 2?

In theory, yes. But I can't multitask at that speed

 

[jimboalee]

Counting whilst spinning is harder than simply running the numbers from the speed afterwards. This calculator makes things quicker though:

http://software.bareknucklebrigade.com/rabbit.applet.html



What kind of pro couldn't top 150? Even I can do 180+ (a little under 40mph on my fixed) and track riders sometimes do over 200.

Matthew

I read a report once where the professional cyclist under test 'spun out' at 150 cadence.

He must have been a roadie.
I don't disagree track sprint specialists could get 200 rpm, for only a few seconds.

On a 95" gear, 154 rpm is 43 mph. That's about 1.2 kWatts if the Cd is 0.75.

How fast do they want to go?

 

[fossyant]

I can't spin as fast on the road bikes. I can just about do 160 rpm (computer that has cadence) on the fixed though. I top out at that at the minute.

 

[ColinJ]

Er147 rpm ??? I find 90 very fast and difficult to maintain-could the Maths be a tads flawed ??

I typically pedal at about 90 rpm and I don't consider myself to have a particularly high cadence.

On my MTB yesterday, I was doing well over 100 rpm up a 25% climb but I was using a very low gear (22/28).

I think that I spin out at about 130 rpm (I'll check on my next road ride).

I have an old video which shows Eddy Merckx training on rollers in his garage. He ends up pedalling so fast that his legs become a blur and the sweat is bucketing off him. I reckon that he must have been doing close to 200 rpm.

 

[yello]

On another forum once upon a time, a similar discussion resulted in someone claiming a cadence of 220 on fixed. I must admit, I found that difficult to believe, not saying they were lying just that it's pretty darn quick.

Personally, I have hit 150 but it wasn't comfortable. Anything more and I think I would have been off.

 

[MajorMantra]

On another forum once upon a time, a similar discussion resulted in someone claiming a cadence of 220 on fixed. I must admit, I found that difficult to believe, not saying they were lying just that it's pretty darn quick.

Personally, I have hit 150 but it wasn't comfortable. Anything more and I think I would have been off.

220 is possible I suppose but I would be sceptical too. You do get some outlandish claims on the forums.

I find 150 fine and I often come very close to it when riding around Edinburgh. With my current setup I find it gets tricky around 160rpm as I start to bounce slightly but it's possible that my saddle is slightly too high or something. I'd lower it but my seatpost is currently seized.

Matthew