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I understand doing this "effects" the geo but what does that really mean? (im talking 2.8" plus so they are a bit shorter)
Surely the bike is just lower to the ground and thats it? But if that was the case then why do some bikes have chips to correct the geo from a wheel change?
I think that's it really, lowers the BB. It does so by quite a bit, as a 27.5x2.8 has a diameter about 30mm or so less than a 29x2.4 (i.e. they are nowhere near as similar as the industry originally claimed)
If you have a low bb already, that could be an issue, if not probably all good
assuming tyres are the same front to rear on both wheels, it just lowers it.
I think it changes the trail. The distance between contact patch and a straight line down through the head tube. This changes the steering.
How it changes it I couldn't tell you. But it's why 29ers have in general steeper bread angles than smaller wheeled bikes
https://en.m.wikipedia.org/wiki/Bicycle_and_motorcycle_geometry
I think it changes the trail.
Yes, it reduces the trail making the steering a bit more sensitive. i.e. If you reduced the wheelsize to a 1 inch wheel the trail would almost be non existent, if you ran a wheel that was 100 inches the trail would be huge (all other things staying constant)
The bigger the wheel the steeper the headtube angle to maintain the same trail.
Fork manufacturers sometimes have different offsets for 27.5+ or 29 normal. X-fusion have 46mm offset (McQueen 27.5+) and 51mm offset (Trace 29). I run both 29 and 27.5+ in my McQueens and it feels fine either way. That is about 4mm in terms of trail, not sure you would notice it.
Just BB height... Santa Cruz use a flip chip as they want you to run a longer fork for Plus wheels. Assuming you do, the hi position on the flip chip raises the rear of the bike an equivalent amount to the longer fork and maintains a similar BB height whichever wheelset you run
I have done it the other way around. My Scott Spark 710 came with 2.8 650+ tyres. However, my first ride I reckon I had 75 pedal strikes. Probably wouldn't have been an issue at a trail centre, but the rocky single track round here was lethal. So fitted a set of 29" wheels and it is now much better. Raised the BB by 12 mm or so. Still lower than my Liteville on 650 wheels though!
So if you put smaller wheels on it ideally you’d want to slacken it? But we are probably only talking 0.5 degree so wouldn’t notice?
I run 27.5x2.35 Schwalbe's on my Commencal Meta HT and it's great
You're meant to run 29x2.35 or 27.5x2.8 but I bought it as a frame only, originally planning to get the correct wheels/tyres later, but I can't really see the point now I've been riding it like this for the past 6 months and reckon it's just fine.
I probably will try some different wheels/tyres eventually, but they'd have to be a massive bargain.
It doesn't seem particularly low (although I haven't measured it), but it is a hardtail so may be less affected by lowering the BB by 15mm or so.
170mm cranks if you're interested too
welshfarmer: in reality you probably just needed to set rear shock sag to get rid of strikes?
my marin b-17 came with 27.5+ 2.8s and I have a 29er wheelset (27 internal) with 2.5 dhf front and 2.4 ardent rear. swap between both sets regularly.
the lower bb in plus mode and grippy tyres make it a winner, and faster when i'm hammering down trails. (i need to measure how much lower, and how much the 29er set weighs)
however plus sucks for for climbing in certain places (off road ok, but trail centres/roads it has so much drag!) so running it as a 29er is preferreed. I have a feeling I'm going to settle on that side more often than not. but if i'm doing a pure trail mashing with more tech stuff ride without much travelling between points etc i'll choose plus. becauase grip + low bb + have more aggro tyres on those rims. (also, according to strava I am faster in plus mode when descending most types of trail)