650 rear wheel and shorter fork for 29" MTB->gravel bike conversion?

Geometry gurus, Can I get your opinion?

Premise…I like wide tires for gravel, 2" or more
Problem…most gravel bikes don’t take tires that wide with good mud clearance
Problem…the MTB BB height is a pain in the neck on gravel and road
And…I have a 29" hardtail bike sitting around (have switched to a 130/130 bike for trail riding)

So…can you fit a shorter fork…like 20-30mm shorter…and put a 27,5" rear wheel on and just go to town? Or will the change in trail make it completely unrideable?


Can you list your exact bike model, year and size?

I can use that to find the geometry chart and try a CAD sketch with your concept applied.

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Highly bike dependent (depends what you’re starting with).

I’d probably say don’t bother changing wheel size. Look at fork length - if it’s a 120mm you might run a 100mm or rigid equivalent. That’s drop the BB a hair and steepen the HTA. Then run a 50mm or 2” tire.

I just did a quick hack using my “Sagged” suspension sketch. This was made for another user, and likely has dimensions that skew the results a bit, but this is a decent swag to start. If make one change and just drop the fork length by about 25mm:

  • Head Tube Angle & Seat Tube Angle steepen by about 1*.
  • BB Height drops around 7-10mm.
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Basically yes… for a hardtail. The difference 27.5 vs 29 would be (622-584 / 2 = 18mm) of standover height recovered.

There are a lot of 47mm to 2.1in gravel bike options. By the time you need more than 47mm, you probably need a MTB unless you riding on sand.

Otherwise Bikepacking.com has a list of 29er drop at bikes.

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It’s a 2018 specialized epic ht pro in XL. I’m not finding any 450-ish mm rigid fork options with boost and a tapered steer tube so now I’m thinking….just swap to 27.5” wheels front and rear. Drop the whole bike by 3/4”. But what will that do to the trail and handling? It should reduce the trail and make it ride like a low trail rando bike right? Which wouldn’t be a bad thing.


I won’t be able to check on CAD until Monday, but I think your Trail will drop a tad as you expect. Maybe 2-5mm as a pure guess.

That should make it a tad more snappy to turn but not sure without a real delta.

Thanks in advance!


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Hey if it’s not a trouble…can you run the numbers on 26” wheels or show me how?


OK, some super simple numbers: 2018 Men’s Epic Hardtail Pro

BB Height = 309mm with 29" x 2.3" / 2.1" tire combo


  • 29" x 2.2" = 734mm Diameter = 367mm Radius (picked the middle between the stock sizes)

  • 650b/27.5" x 2.2" = 696mm Diameter = 348mm Radius (matched tire size above for simplicity)

  • 26" x 2.2" = 671mm Diameter = 336mm Radius (matched tire size above for simplicity)

Switching from 29" x 2.2" to 650b/27.5" x 2.2":

  • 367mm Radius - 348mm Radius = 19mm
  • 309mm BB Height - 19mm Wheel Drop = 290mm BBH

Switching from 29" x 2.2" to 26" x 2.2":

  • 367mm Radius - 336mm Radius = 31mm
  • 309mm BB Height - 31mm Wheel Drop = 278mm BBH

Consider those for a primer and see if one sounds like something you want to do. If so, I can take the extra steps into CAD, but you may know a bit from the above.

So incredibly helpful, thanks! I want to measure the bbh on my road bike trek domane 2021 60cm and then go from there. It says 80mm drop in the specs but I want bbh, would like my redneck gravel setup to be about the same.


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Cool, let me know if you zero in on a specific wheel/tire setup of interest, and I can do a CAD sketch to see the other values (like trail).

But you may be able to get some decent idea from the basic info and make a call from there.