Derailleur on a fixed gear?

Boredman

Active Member
I was wondering if anyone knew why a fixie has to be single speed? Also why you can only have gears on a freewheel bike?
 

biggs682

Smile a mile bike provider
Photo Winner
Location
Northamptonshire
You can have a fixed gear bike with a 3 speed sturmey archer set up

ie Claud Butler 531 fixie - but rather different!

Which i think is a great idea
 

Ian H

I am an ancient randonneur, & I stop often for tea
Location
East Devon
Sturmey made or make 2 and 3 speed fixed hubs. I've never ridden one but I understand that the considerable backlash made them less pleasant to ride than a fixed single cog. Also, you could find a 'false neutral' between gears which, if used on a fast descent, could possibly break either the hub or an ankle if the lever moved.
 

Edwardoka

Shambling ruin of a man
Mechanical reason: the spring in the derailleur keeps the chain under tension regardless of the length between the front of the chainring and the back of the cassette (which varies for different gears) but it only does so in one direction, whereas in a fixed gear, the chain is of fixed length and is always under tension and therefore power can be transmitted in both directions.
Once rode a hire bike when its freewheel seized, as soon as I stopped pedalling, the chain would instantly go slack and was at risk of getting jammed in the spokes. The backpressure that would in a fixie exert a force on the wheel causing it to slow instead caused the spring to be pulled forwards, slackening the chain and causing swearwords. An exciting few miles!
 

robgul

Guru
The fixie listed for sale is mine - with 3 speed hub.

Have to say I haven't experienced anything that put me off the thing (I just have too many machines, hence the sale) - everyone asks how you change gear! The technique involves anticipating the need to change before a hill (up or down) - as you pedal just release the pressure your legs are outputting and move the lever. There's very minimal backlash - especially if you keep the chain fairly taut (which in my view is a good idea with a fixie anyway). I would add that other than when I was about 15 I've always ridden fixies with two brakes, rather than just the front that the law requires.

Rob
 
I've had an S3X and an ASC. The ASC was better but I never trusted either. They both have neutrals between gears and both need quite high cable tension in low gear - a slight lack of tension and it will pop out, probably painfully if you're standing on the pedals.

The ASC was such a slow seller when new that NOS hubs were still available ten years after production ended. Then Sheldon Brown raved about it and everyone wanted one. The unique ASC trigger is far harder to find than the hub itself bur, with a grinder, you can modify the ratchet plate of a 4-speed trigger so it works.
 

Tigerbiten

Veteran
Mechanical reasons ..........

An easy way to see what would happen if you put a derailleur on a fixed wheel is to just jam the chain in the rear derailleur on a normal bike and back pedal.

The bottom chain run then pulls tight , so will the rear derailleur survive the chain pulling tight or will something bend/break in it ??
The top chain run develops a big loop in it and the higher the gear then the bigger the loop.
With the chain going so slack will it even stay on the chainrings and what happens if/when it falls off because it won't stop moving even if it does ??
Plus the risk of that loop of chain getting jammed between the rear wheel and the chainstay or just in the rear wheel inself.

So not a good idea .......... ^_^
 
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