Crank Sizes

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rsvdaz

New Member
Location
Devon
currently undertaking a bmx re-build for my son,,planning to keep a 1 piece crank

the crank I took off is 140mm..but I notice replacement ones are 140mm or 170mm....any benefits of a longer crank?

He is 11 years old...and average height for his age..if this helps?
 

PpPete

Legendary Member
Location
Chandler's Ford
170mm is the standard length for road bikes & hybrids. Probably too long for a BMX - especially a kid's one. Could hit the ground on corners.
 
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rsvdaz

rsvdaz

New Member
Location
Devon
cheers all!

now....funny you mention gear ratio...he wants a small 25t front sprocket...in anodised red of couse! :becool:
do smaller sprockets mean he needs to pedal faster to match a larger sprocket?..its fairly hilly here..would that also help him up the hills?

when i took him to the local skate park last weekend there was a lad with a rear sprocket that had more of prenounced wirrrrrrrrrr to it than the rest of the BMX's
it sounded pretty trick..anyone know what they are called?
 

Zoiders

New Member
BMX's are a tricky subject.

If you want to go over to a smaller front ring you need a 3 piece crank which works out a bit pricey to say the least, the drilling that secures the ring is different as well. The real reason they run the smaller ring is to avoid damage to the ring on street courses and parks or on the coping of skate ramps, concrete and steel can be unkind if you get hung up on it.

If you go for the smaller ring then you have to make the switch over to a much smaller sprocket as well which again is a different system which ironically runs on an over sized axle that won't drop into a bog standard frame. The sprocket is similar to a cassette in design and has more engagement pawls, this is what creates the whirring sound you happened to hear.

The gear ratio isn't that silly at all, the smaller ring runs with a much smaller sprocket so you get what you lost in gear inches with the smaller front ring.

I would not go mad trying to upgrade the existing bike, ride it into the ground on the existing components and if he really wants to get into using it to rip around the park on then go for a new bike with the 3 piece cranks and the low profile drive train.

As for crank length, you only really need the 170's if you are on an actual BMX track with the run in ramp and start gates, it's a way of getting more welly into the first corner as the first person in to that corner tends to win the entire race.
 

migrantwing

Veteran
when i took him to the local skate park last weekend there was a lad with a rear sprocket that had more of prenounced wirrrrrrrrrr to it than the rest of the BMX's
it sounded pretty trick..anyone know what they are called?



If you take all the grease out of the rear hub, it will make the hub sound louder. Silly idea, if you ask me.
 

gaz

Cycle Camera TV
Location
South Croydon



If you take all the grease out of the rear hub, it will make the hub sound louder. Silly idea, if you ask me.

Taking the grease out will make it louder but for a freewheel to make a loud clicking sound it must first be made to make one. Fulcrum freewheels make a loud clicking sound even when the hubs are fresh from the factory

 
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