Bottom bracket height

[ccrdave]

Bearing in mind a lot of ebikes come with big tyres which I am not a fan of but putting more reasonable size tyres on will lower the bottom bracket so how low is too low and while we are at it how high is too high?
I guess too low is when you get peddle strikes but what other effects does a too low or two high BB have?

 

[knut7]

BB-drop and BB-height are two relevant measurements here. I'd say it's the BB-drop that affects the riding charecteristics of the bike the most. Draw a line between the center of the hubs. Draw a vertical line from the center of the BB until it hits the other line. The lengt of the latter line is BB-drop. Lot's of BB-drop is stable in turns, stable at speed (more OTB resistant and more wheelie resistant). Less BB-drop makes it less stable at speed, more eager to OTB/Nose-manual, more eager to wheelie/manual. Lowering the BB height with smaller tyres shouldn't have much effect on these things. I'm thinking it's just the pedal/crankarm, chainring, and the ability to pedal across a puddle or snow that will suffer.

 

[ccrdave]

Agree with most of what K says above. Except. Tyre outer diameter does actually have a noticable effect on handling. Using smaller tyres will lower the BB and add grip while cornering and add stability at speed. This is just one of the reasons I still prefer 26" wheels. I even have 26" wheels on my YT Capra (a 27.5" 170mm travel Enduro race bike)
From the E-bikes I've looked at none have what I'd call hugely low BBs. Afew are quite high though. I was surprised and very pleased to find a BB height of 333mm (or 13.1") on my Vitus when measured. right in the ball park I like. it's listed in vitus' geomtry chart as -10mm BB drop. It isn't. it's actually over 15mm.
Whether you get on with low BBs is going to depend hugely on your skill level and riding preferences (how and where you ride) Properly low BBs need a different technique to get the most from. You can't simply pedal blindly into things either up or down and even compressions need a more cautious approach if on the gas. For me the trade off is really worthwhile. Others would hate how low some of my bikes are.
I also hate big tyres BTW. I'm running a 2.3 semi slick Minion SS rear and a 2.5 DHF up front. The advice that seems rife in the Emtb community and media says you need big tyres for grip to make use of all the extra power. You don't. it's absolute rubbish. You do need a half decent idea of how to weight your bike when extra grip is required tho. Just like all you should riding all bikes.

What bikes are you looking at? and What riding are you wanting to do on them?

thank you
its a turbo levo, with 165mm cranks, i have tried
2.8 minions 27.5 = 340 bb height
2.5 shorty on 29ers = 353 bb height
2.35 magic marys 27.5 = 333 bb height
all plus/ minus a mm or two
riding is a mix of trail centre, off piste stuff in the FOD which is very rooty and rocky stuff in the peak district

 

[100 Cols]

How do you measure BB height (or drop) on a full suspension bike?

 

[ccrdave]

The manufacturer measures it static centre of bb to the ground so i do the same

 

[100 Cols]

Static meaning without any load? Does not tell much of anything?

Also, with fatter tires you probably use lower pressures, so the bike and BB stay lower when loaded, in real ride situations?

 

[ccrdave]

Yes no load but dont all manufacturers measure like that? If not the measurement will vary depending on sag set

 

[knut7]

Agree with most of what K says above. Except. Tyre outer diameter does actually have a noticable effect on handling.

Yeah, reducing the BB-height does lower the center of gravity. And I agree about not needing Plus tyres because the motor needs the extra grip. But when my local, rocky trails get wet, the 2.8" makes stuff much more rideable compared to my old 2.35" setup.

 

[Gary]

Yeah. no doubt. I'm more of a one tyre for all conditions and just deal with it kinda guy and favour fast rolling dry tyres over drag.
I kinda enjoy getting my bike sideways as much as possible TBH.