Smaller front rim on mountain bike

Question

I have a Repco Warrior 29, I recently twisted the front rim doing some stuff I probably shouldn't have been doing at my advanced age.

There are no 29 rims available here, but there are 26's. Has anyone tried putting a 26 on the front of a mountain bike (I don't care about the brakes problem). What are the disadvantages of doing so? (I don't ride offroad)

I am aware that handling will change, that goes without saying. What I want to know is how will the handling change. Specifically will having a lower front end have a detrimental impact on hill climbing. But also in general, if the bike for instance would become unstable or unsafe.

Móż · Accepted Answer

The specific handling changes you will get come from two effects.

First, lowering the front puts your weight further forward, and tilts your torso down. You'll find the bike tends to "dig in" more then you go down into a dip or rut, and you're more likely to go over the handlebars as a result. You'll also have more weight on your arms, so you're likely to find the bike less comfortable on longer rides. You might also have more neck soreness since you're looking up more when riding.

Secondly, you've changed the head angle and trail. A steeper head angle normally makes the bike more twitchy (responsive to steering input), but since you also have more trail (if you extend the line through the head tube to the ground, and measure back from there to the centre of the contact patch, that's the trail). More trail means more castor effect - the steering tends to return to centre if the bike is upright, or turn in towards the direction of lean. The combination is likely to make the steering feel heavier, and you'll have to work more to keep the bike on track if you're not leaning perfectly on a  banked corner. On a flat track when cornering the bike will want to oversteer, twisting the handlebars into the turn until the wheel flips sideways and you crash.

The change from 29" to 26" is relatively small, so the above effects are unlikely to be dramatic, and you will probably be fine. I expect you'll ride for a bit, re-read this answer and go "yeah, it does that, a bit".

You could eliminate the first effect by buying a stem or bars with a bit more rise in them. You're going from ISO622 to ISO559 (bead seat diameter (BSD) in millimetres), or a difference in diameter of 63mm, so the bars will drop about 31mm (assuming tyre size stays the same). A stem 30mm higher should be fairly easy to find.

This assumes you're using disk brakes, since rim brakes won't work with the smaller wheel. If you don't have disk brakes, the missing front brake will be the main thing you notice. You'll lose 70-90% of your braking power, and that's bad.

Batman · Answer

700c and 29" and ISO 622 wheels are the same rim size (typically ones marketed as 29" are wider than ones marketed as 700c, so you can run fatter tires), so if you can go with that, thats what I'd do.

Most front wheels are 100 mm hubs, so the wheel will go in. With rim brakes, the brakes won't align (and you'll have to remove them, since you don't care about brakes), but with disc brakes they should work with some adjustment.

The geometry of the bike will change (the front end will become lower) and handling will be affected, but its been done before many times. These bikes are often called 69'ers or 96'ers (normally with the smaller wheel in the back; Trek used to sell bikes this way). and were designed to combine the rolling advantages of a 29er some better acceleration.

That being said, I'd strongly advise against doing this if you don't have disc brakes since you really should have 2 independent braking systems, or use an adapter for  brake posts to move them (Mavic used to make them for 700c->26", but you could fabricate your own).

Smaller front rim on mountain bike

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