Converting 26er to 650b rigid gravel bike

commuterlegs

Hi,

The first in no doubt a long list of questions about this project I'm taking on.

The 80mm suspension fork on my 26er has corroded and has a lot of play in it so needs replacing so i figured its a good reason to have a go at this. I've watched some videos and done some research and think a 27.5 rigid fork will be approximately right to replace the 80mm suspension at c430m a-c. I will initially just replace the fork and will wait to do the wheels/groupset etc (small bit every month) so i need it to still work. The existing fork has IS mounts and new rigid forks have flat mount. I know i can get an adapter but the question is how the adapter affects the fit if it's on a 27.5in fork with a 26in wheel or am i overthinking it? It's a 180mm disc at the front.

Any guidance is appreciated.

whyamihere

A disc mount isn't affected by wheel size, they're the same regardless. Check if it is flat mount or post mount though. I'm not aware of any flat mount to IS adapters, so you may end up with some kind of bodge with multiple adapters which could be avoided by selecting a different fork.

commuterlegs

Thanks. I think the sheer volume of information is confusing in itself.

Any useful guidance on figuring out wheel clearances? I have 26/2.00 on the frame currently and about 1in of clearance to the frame. Is there an easy way of working out if a 27.5in wheel with a 1.5/1.75 tyre would fit? I feel like it should be capable of working out but failed to find any website that does it.

whyamihere

It's easiest to convert into ISO sizes. 26" rims have an ISO bead seat measurement of 559mm (this is the diameter of the bit of the rim where the bead sits). 27.5 have an equivalent measurement of 584mm. In other words, the tyre beads are going to have a diameter 25mm larger. For clearances we care about the radius, so it's a 12.5mm difference.

You can then find measurements for the height of the tyre you want to fit and add those in. As tyres are generally roundish in profile, if we assume your current tyres have a height of approx 50mm, you could fit a 37mm height tyre to a 27.5" wheel and have the same clearance. That's just under 1.5", so with the clearance you have that should be fine, but you'll need to check clearances at all points of the frame.

I wouldn't expect width to be a problem, but again you can measure. Work out where the fattest part of the tyre will be using the method above and just check the frame is wide enough at that distance from the rear axle.

In short, I'd be amazed if 27.5" wheels with 1.5" tyres wouldn't fit. 1.75" should probably be fine too, but you'd need to measure.

steve_sordy

Don't cut the clearances too fine; allow for wheel and tyre deflection that may start wearing the paint off the forks or worse, the fork itself!
I once fitted a wide tyre to the rear and although deflection wasn't a problem, the wider tyre was also a bit taller and the stones that the tread carried round were striking the seat stay brace. I had to put three layers of helicopter tape on the brace and keep replacing the tape as it wore away.

commuterlegs

Sacked it off. Once I planned it up there just seemed to be too many compromises when i could get a second-hand gravel bike for similar money. So that's what i will be doing and will see if i can donate the mtb frame.

commuterlegs

I've bought a 2nd hand frame and will transfer over the old 7spd groupset from my mtb while i work out what to do.

I've been watching some shifters / groupsets on the bay, principally Claris and Tiagra but for the money they don't seem great value given you don't really know what condition they're going to be in. Also, being a cynical person I question why certain sellers have so many random shifters etc for sale, not sure how you get away from the feeling of supporting bike thieves.

The other option therefore is buying direct from China. Seems to be some good deals on gravel groupsets and seen some decent reviews of them but again, never sure what you're going to get.

Any views from people who have built up bikes from either sources?