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My SC Hightower, like many, can take both 29 and 27.5+ wheels. It even has a flip chip in the shock linkage to allow you to adjust geo between the two. The stock 27.5+ wheels would be 2.8 Minion DHs on RF AR 40 rims.
So how narrow a tyre could this reasonably take before it just became a 27.5 bike?? And would the geo suffer?? And would this matter??
I don't know as I have 27.5+ tyres (2.8 front and 2.6 rear) on a 27.5 bike non boost and never worried about the name. My front tyre is taller than my rear so I assume the delta in geo is on the HA which has probably gone down a fraction of a degree, which doesn't really matter.
I am wondering what the "chip" does - if you change 2 wheels then the bike doesn't get steeper or slacker, just BB height and trail change.
Anyway it's not going to be binary, if you changed 1 wheel from 29 to 27.5 it's be 3/4" which is about 1 degree. Unless you are a riding God of course.
INTRODUCTION
This service applies to MY17 Tallboy and Hightower models. The wheel size adjust allows you to run either a 27.5+ or 29 inch wheel size using a “ ip chip” design at the upper link, while maintaining frame geometry measurements such as bottom bracket height and head angle.
NOTE: You must change your fork when changing wheel sizes.
Basically that, make the little correction as they are assuming you are running a different fork when using them (in the examples above)
Tyre height is fairly close to tyre width on a MTB tyre, assuming the rim width is appropriate.
A 29" rim is 622mm OD and a true 2.2" tyre is 56mm wide and high, so the full diameter is 734mm. A 27.5" rim is 584mm OD and a true 3" tyre is 76mm wide and high, so the full diameter is 736mm. Factor in the lower tyre pressure and thus more tyre sag and a true 3" 27.5+ tyre will give you pretty much the same BB height as a 2.2" 29 tyre.
Now the reality is that most tyre manufacturers lie about widths, especially with bigger tyres, so you need to figure that in.
If you drop from a true 3" 27.5+ tyre to a true 2.6" 27.5 borderline plus tyre that will drop your BB by about 10mm. That's quite a bit, towards the outer limit of adjustable geometry on most bikes but conversely it's little different to changing your fork travel by 30mm or changing your shock sag from 28% to 35% on a 150mm full-sus.
The flip chip raises the BB by extending the rear suspension a bit, so it also steepens the angles, slightly increases reach and slightly decreases stack.
Sorry, i might not have been clear. I'm OK with the flip chip stuff. The chip simply adjusts the geo. I'm wondering if there will be a noticeable height / geo difference between 2.8 and 2.5 tyres.
noticeable height / geo difference between 2.8 and 2.5
Height - probably around 0.3" or 8mm but if it's that close then you'd want to look at individual tyre heights.
Geo - changing wheels or tyres only affects hieght - ie BB height and trail (if you don't use the chip and change forks)
I don't expect you'd notice a difference.
BB height
One of my bikes is a 27.5+ bike.
Use 2.6 inch tyres and 35 mm inner rims. Yes - the BB is a bit on the "low side". Bit of an hassle on difficult trails but a blast on fast downhill runs...
And your questions: not sure - but 2.6 inch rubber is no longer plus...
"I’m wondering if there will be a noticeable height / geo difference between 2.8 and 2.5 tyres."
The only geometry difference will be a change in BB height. And that'll be about 0.3" (~8mm) because that's the difference in width and height of the tyres.
If a bike is designed to be low slung as a big tyred (2.5") 29er then it'll be even lower as a 27.5x2.8 and and on 27.5x2.5 probably unrideable except on groomed trail centre runs or steep non-pedalling trails.
but 2.6 inch rubber is no longer plus…
Showing my age a bit, but I remember when 2.7in was just DH gnar. Although they were Maxxis tyres that came up about the same as anyone else's 2.4 or 2.5.
3in Nokians from the late 90s on a similar note?
It is good to know your tyres will fit these days before you buy them though.