Bike Geo Question - BB Height/Wheel Diameter/Travel

[GoSlowGoFar]

Hey all, hoping there's some bike geometry nerds out there. I'm toying with the idea of putting some 29" wheels on my Pyga Hyrax. I know this will lead to about a 19mm increase in BB height due to the increase in wheel diameter. However, I also plan to decrease fork travel by 10mm and flip the switch on the bike to the "low" setting which reduces the BB height by about 5mm.

Can anyone explain how much the net increase in BB height would be? Also, how does that relate to drop? For reference, I'm measuring my current BB height at 345mm with 27.5 wheels/160mm fork travel unsagged.

[mntlion]

lowering fork will lower BB by 2/3ish that value. (6mm)

re-measure B drop with chip in low position.

BB in low possition + 19mm increase with wheels - 6mm from dropping fork = new number?

but doesn't that bike come with 29 already? or did they re-use name for other sizes bike in the past?

[GoSlowGoFar]

Awesome, thanks. So it looks like it’ll bump my bb height up 8mm to 349ish. Seems like most trail bikes nowadays are in the 330-340 range so I’m not sure what it’s going to do handling wise if I get closer to 350, or if I’ll even notice.

The US distributed Hyrax from last season was a 27.5. They just moved to an alloy 29er this season. I’ve found myself more on the Trail/XC end of the spectrum lately as opposed to Trail/Enduro so was thinking of seeing if putting some 29s changes some of the climbing characteristics/pedal efficiency of the bike.

[HAB]

Keep in mind that a 29er fork will be taller at a given travel, so even if you go to 10mm less, the net fork length won't be any shorter.

Does the rear end even clear a 29er? Check at bottom out too, not just static.

[GoSlowGoFar]

Keep in mind that a 29er fork will be taller at a given travel, so even if you go to 10mm less, the net fork length won't be any shorter.

Does the rear end even clear a 29er? Check at bottom out too, not just static.

Got ya, thanks for that tip. The rear end clears according to the factory. I believe the South American version was built up as a 29er but the US folks decided to make it a 27.5.

Maybe it’s not a great idea, love the bike overall. Just sometimes I wish it was a little more efficient on the climbs.

[NuMexJoe]

If you love the bike overall and are looking for better climbing, then you'll probably be pleased with the mod. FWIW, I deliberately sought out a V1 (aluminum) GG Smash because of the 348mm BB height (that's with a 150mm fork). I love the capability of the bike on the ups and downs, and now I'm no longer smacking pedals on techy climbs as I was doing on my Canfield Balance with 165mm cranks.

[GoSlowGoFar]

Yeah, I guess I'm just hung up on not having ever ridden a bike with a bb height more than 345ish. But that wasn't necessarily a conscious thing. I might ask around here locally to see if someone has an extra fork and wheels laying around that I could toss on and just get a feel for it.

[toast2266]

If a 29er fits in that rear end, I'd bet it'll only fit with a fairly skinny tire on there.

I also think your BB will be much, much higher than the current 345 height. I'd expect it to end up somewhere in the neighborhood of 15mm higher than current, so 360ish mm. And that's accounting for putting the frame in a lower position and lowering the fork travel (keep in mind that a 150mm travel 29er fork is still ~10mm longer than a 160mm travel 27.5 fork).

Personally, I think all of that would completely ruin the bike and make it handle like utter shit, but maybe that's just me. If all you want is better pedaling clearance, maybe just get shorter cranks? Or see if you can put a spacer under the headtube to raise the front end a bit? Or get better at ratcheting your pedals?

[GoSlowGoFar]

I also think your BB will be much, much higher than the current 345 height. I'd expect it to end up somewhere in the neighborhood of 15mm higher than current, so 360ish mm. And that's accounting for putting the frame in a lower position and lowering the fork travel (keep in mind that a 150mm travel 29er fork is still ~10mm longer than a 160mm travel 27.5 fork).

Is that based of your assumed increase in crown to axle length? Mainly just asking because the Mattoc Pro I have on there is 555mm crown to axle and the 29” Pike is 551mm so it’s actually shorter no? Or is there another measurement I’m missing?

[HAB]

Is that based of your assumed increase in crown to axle length? Mainly just asking because the Mattoc Pro I have on there is 555mm crown to axle and the 29” Pike is 551mm so it’s actually shorter no? Or is there another measurement I’m missing?

That's comparing a 160mm Mattoc 27.5 to a 140mm Pike 29. He's right if you only drop the travel by 10mm instead of 20mm.

[GoSlowGoFar]

Ah, got ya. Ya I didn’t catch the difference in travel apologies. But ya been contemplating going down to 150 or even 140 just to make the geometry work. I’m not sure how much I’ll miss the 20mm since most I’ve ridden most of them on an old Trance with 140mm fork anyway. It’s more the geometry, slacker head angle and slightly steeper seat angle and reach that I’m worried about saving.

[Andeh]

Raising up the rear end while keeping the front end static height will increase the head angle. A 360mm BB sounds awful too.

[GoSlowGoFar]

Raising up the rear end while keeping the front end static height will increase the head angle. A 360mm BB sounds awful too.

Is there any way to tell by how much? Right now I'm at around 65.6 deg. I can live with a degree increase or so.

[toast2266]

Is there any way to tell by how much? Right now I'm at around 65.6 deg. I can live with a degree increase or so.

I'd bet the headtube angle will get slacker, not steeper. Lowering rear end will slacken it. Decreasing fork travel would steepen it, but you're actually getting a longer axle to crown, which will slacken it. I'd guess net result will be 65.0-ish, or even a bit less.

[GoSlowGoFar]

I was under the impression that it wouldn't change since the crown to fork of a 140mm Pike is the same as the crown to fork of the 160mm Mattoc. If anything I thought it'd maybe steepen it by a half degree or so due to the larger wheel in the rear and the front staying the "same".

Anyway, I think I talked someone into letting me toss some parts on it to see how the geo changes. They seem to think that the BB would come in right around 350. Which doesn't really seem too crazy of an increase considering I'm measuring 345 right now.

[toast2266]

So if you're decreasing fork travel by 20mm, and thus keeping axle to crown same-ish, then head angle will slacken, but not by too much. There'd be no net change in head angle due to the wheel size swap since it's the same at both ends (it just raises the whole bike up ~19mm). The only effective change to head angle would be due to lowering the rear end, which I'm guessing would slacken the head angle by .4 degrees or so (and also drop the bb, as you said earlier in this thread).

[GoSlowGoFar]

Great, thanks for the clarification toast.

[Eluder]

Got ya, thanks for that tip. The rear end clears according to the factory. I believe the South American version was built up as a 29er but the US folks decided to make it a 27.5.

Maybe it’s not a great idea, love the bike overall. Just sometimes I wish it was a little more efficient on the climbs.

Don't go butchering your bike man, just get yourself some better 27.5 wheels if you want a more efficient climber. A high BB sucks at a lot trail things, getting them to transition from turn to turn is slow and floppy. IMO there is nothing worse than feeling like you are on top of your bike instead of in your bike.

[GoSlowGoFar]

Rest easy, didn't do it. I did demo the Stage Max which is a blast on the up and can party well enough on the down. Just need to come up with good reasoning for the lady...any ideas?