Help on ratios for kids bmx please

[Rake722]

Ok, so my 7 yr old is flying out and dropping in but hits the CW on the cing as his bike has 33/16.
It's a GT el centro 16" bmx.

So I want to change the driver to a 9 tooth but not sure what to put on the front to make the wheel smaller and keep the gearing realistic for his age.

Any ideas would be great.

Thanks

Ron

[Rake722]

Think I got it. 33/16 = 2.01
25/9= 2.77

Closest to 2.01 would be 18/9 or 20/9 = 2.1

[nosepickben]

Or go 20/10 for a 2.00 ratio
Depends what sprocket you can get hold of really. Do you have the 9t already?

[Spen69]

Can you get smaller than 25T on the front? Thinking of the drive pin in the crank having nowhere to bear on if it's smaller?

I'd go 25/13 or 28/13 if you want to use the metric flip flop thread side of the hub (assumingyou have a flip-flop hub?). An Oddysey 13T freewheel is easily available for change out of £20 and either a 28 or 25 front are also easy to find.

[nosepickben]

I've seen 23t sprockets, 23/10 wouldn't be unreasonable in terms of ratio for a 16" wheel.
If he's still catching the sprockets at that, he needs some time to learn riding in a different way!! 

[Spen69]

Oh, also watch for chain clearance on a 9T as you will hit an AD caliper if it's on the lower chainstay. You might also hit the chainstay itself.

I'd stick with a 13T freewheel and easy to find stuff

[Dark Diggler]

Teach him to ride using what has

[HEYWOOD BMX]

http://bmxultra.com/downloads/flash/gearchart.htm#20x1.75

[fugazi]

Anything down to 23t should fit a standard crank pin/bolt. Anything smaller You need cranks with the 2 bolt setup to run micro gearing. Bigger risk of chain wear/snapping at those sizes & as Someone else said might run into problems with it all hitting brakes/chainstays/dropouts.  Probably not worth doing as He'll be on an 18" setup soon anyway?

[Rake722]

Cheers All that have replied.

I found a 23T front and gave the wheel in to loot bmx yesterday for the strip so I do hope it works out now.

Its looking like 23/9 if it works ! If it binds then a waste of cash. The brakes are on the lower tube too.

[Rake722]

Managed to catch em before the wheel got stripped.
Those who warned about fouling the chainstay - 10/10 ! I made a mock
tube for the 9T and the chain fouled the brakes badly so thanks for helping us avoid a double wheel rebuild.

I showed him a vid of approaching the coping at right angles and turning into the pipe from right angles and he said he'd try that instead.

Will check clearance and go from there.

13T may be the better option afterall.

Thanks.

[Spen69]

My work here is done

But Si was right when he said teach him to ride with what he has - deffo approach at least 45 degrees to the face of the pipe and turn in at the last moment

Managed to catch em before the wheel got stripped.
Those who warned about fouling the chainstay - 10/10 ! I made a mock
tube for the 9T and the chain fouled the brakes badly so thanks for helping us avoid a double wheel rebuild.

I showed him a vid of approaching the coping at right angles and turning into the pipe from right angles and he said he'd try that instead.

Will check clearance and go from there.

13T may be the better option afterall.

Thanks.