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BMX Conversion?


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I thought it was because Stocks were 26'', so you're modding a mountain bike, not a BMX.

20″ – 'Mod Bikes'

The first purpose made and commercially available trials bikes were manufactured by Montesa[3] a mototrials company and were based on the modified bmx bikes that riders had been using. As a result of being based on BMX bikes, the rear dropout spacing is 116mm. These 20″ wheeled trials bikes have become known colloquially as Mod bikes. It should be noted that some mod bikes have a 19" rear wheel to make room for a bigger tyre, but the overall tyre diameter is similar regardless of the rim size, so this is mostly important as a compatibility issue for spare parts.

26″ – 'Stock Bikes'

Early mountain bikes were well suited to the trials riding and so a separate class was introduced for them. 'Stock Bikes' used to refer to a 26" wheeled mountain bike kept in original or 'stock' condition, i.e. not modified like a mod bike. Stock classed bikes were required to have at least six working gears and a seat. Now however, this designation is used to describe any trials bike with 26″ wheels, as current 26" trials bikes look about as similar to mountain bikes as 20" trials bikes look like BMXs.

Historically stock bikes all had 135 mm spaced dropouts to take a standard mountain bike rear hub. There is a subcategory of Stock trials bikes, sometimes called 'Modstock Bikes', which have 26" wheels but have horizontal dropouts spaced at 116 mm, similar to mod bikes. Horizontal dropouts at both spacings are becoming more and more common in stock bikes as they eliminate the need for a chain tensioner, making the bike lighter (The chain is tensioned by moving the wheel backwards in the dropouts; vertical dropouts are designed to hold the wheel in one position only and therefore require a separate means of tensioning the chain to accommodate wear).

Edited by Ash-Kennard
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because BMX is rad, and so is trials, bmx's have good geo (short chainstays, highish bb, steep head angle), an easier gear would allow more trials moves and so would brakes. A longer frame would be needed, maybe like a 21.3 or something. Most streety 20" bikes are still a bit too trialsy for my liking, long stem....why?

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  • 2 weeks later...

I just ran a 18t freewheel on the back with a 25t front cog.

but you can get smaller front cogs now

That's probably the best way. Run a decent trials freewheel on the back or run a Profile with a big driver.

As for turning an existing bmx into a trials rig, not worth it, in the sense of changing stuff. You'll need a long ass frame, 22" minimum if you want to sidehop anywhere near a reasonable height comfortably. And running a trials gear isn't going to benefit on a short bike.

You can happily hop up walls, do some back hops on a normal un-modified bmx. Just tweak them brakes, and if you get somewhere with gaping on the bmx then lighten up that gear slightly.

Though a 24" trials or bmx would probably be a nice inbetweener.

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