# Weed-eater bike conversion - Safe rpm?

I've recently started working on retrofitting an old bike with an even older weed-eater engine. The engine will sit atop a bicycle rack and directly drive the rear wheel tire (engine has a built-in clutch). I've already constructed the engine mount and obtained other hardware. http://imgur.com/a/we3bn

My concern is that once I get moving (maybe 15-20mph) the chain will be moving so fast around that tiny sprocket that it will come off.

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Are you driving the chain or driving against the tire? – WTHarper Jul 1 '13 at 18:58
First, do the arithmetic to see what RPM corresponds to what speed. – Daniel R Hicks Jul 1 '13 at 21:02
The chain will be directly driving the rear tire. @DanielRHicks With a 1 1/2" sprocket at 3000rpm (an estimate, I can't find specs on this engine) it will be about 1200 ft/min. – Evidlo Jul 2 '13 at 16:47
How will the chain drive the rear tire? Aren't you going to need another sprocket on the wheel? Your 1200 ft/min number is chain speed. Wheel speed will be higher, depending on sprocket diameter. – Daniel R Hicks Jul 2 '13 at 18:27
@DanielRHicks, yes, I'm adding a 54 tooth gear to the rear wheel to get about 1:8 reduction in rpm. – Evidlo Jul 3 '13 at 18:58

My concern is that once I get moving (maybe 15-20mph) the chain will be moving so fast around that tiny sprocket that it will come off.

You can use a `chain tensioner` to keep tension on the chain, so some slack on the chain won't cause the chain to skip off the sprocket.

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A quick check around the interwebs (http://www.lmgtfy.com/?q=chain+drive+rpm+limit) took me to this site: http://www.practicalmachinist.com/vb/general/roller-chain-sprocket-rpm-210682/

It looks very practical to use some sort of chain for what you want to do. Bike chain might work, might not, but the cog you're using doesn't look suitable for bike chain anyway.

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The sprocket is actually a #40, but I found out once it arrived that it was much too thick for any bike chain I'd ever seen. I might grind/mill it to a thinner thickness. – Evidlo Jul 2 '13 at 16:38