Bottom bracket height?

lemonenema

On a MTB whats considered a "normal" range for BB height?
Ive had a look online at different manufacturers and its amazing how much it varies, and doesnt seem to correlate - some short travel HT have higher BBs than longer FS enduro bikes.
I understand the difference the height has on handling but there doesnt seem to be a definitive average.
Ive checked both my bikes - 27" HT trail BB 12.75"
26" FS AM BB 13.25"
Your thoughts and comments please good folks.

steve_sordy

The selection of BB height is partly proper bike geometry design (HT or FS, type of suspension geometry), some is partly fashion (what are the bike mags saying?), some will be influenced by wheel diameter (is the centre of the BB on the wheel axis, above or below).....there are probably many others.

When I first started in mtb, the bike mags banged on about BB heights being nice and high so that the bike could be leaned right over without catching the pedals on the ground, and with the ability to pedal through rock gardens.... Now it's all about low centre of gravity to give that agile feeling while you fling it around tight corners and weave through the trees.

But in one bike forum I'm a member of there is a long thread on where can they get short cranks to avoid pedal strikes! The old codgers all say that pedal strikes are a consequence of the rider not keeping his eyes open and a lack of bikes skills in moving the pedals around to avoid said strikes. The beginners just want shorter cranks so that they can keep on pedalling no matter what.

It's anal I know, but I keep records of the bikes I've owned and once the geometry became available I kept that as well. Not all the bikes I've had, but here they are: (all in mm)

Kona Kula (100mm 26er HT) 310
Kona Dawg (150m 26er FS) 356
Norco Sight (140mm 26er FS) 341
YT Capra (160mm 27.5 FS) 343
Whyte T130 (130mm 27.5 FS) 331
Focus Jam2 (150mm 29er FS) 330

They are roughly in the order of purchase.

At 356mm, the Kona Dawg was considered to have a high BB even ten years ago, but I could pedal through pretty much anything and keep pedalling whilst cornering. But it felt a bit unstable; the BB height was an inch above the wheel axis.
I bought the Norco after the Dawg and it felt so stable that my confidence improved immediately! I bought the YT Capra while I still had the Norco Sight and despite the YT Capra being a bigger wheeled Enduro bike 160mm vs the trail bike Norco, I felt completely at home on the YT. Could that have been partly down to the almost identical BB height? Who knows.
I sold the YT Capra and was left with just the Whyte T130. The Whyte is styled as an aggressive trail bike and I felt that I could ride it anywhere that I would have ridden the YT Capra as it felt even more stable,; was that down to the lower BB?
When I subsequently chose the Focus Jam2 29er, I chose it because it was the first eBIke I had ridden that just felt like fun. Could that be partly down to the almost identical BB heights, despite the bigger wheel diameter? Again, who knows.

Bikes are not chosen or rejected just because of the BB height, but it is an important dimension. Hitherto I have mostly ignored the BB height - just noted it, until you asked the question. Maybe I shouldn't ignore it anymore. :?:

lemonenema

superlative response once again mr sordy, im impressed you had that list up so quick
thats put my mind at ease thanks!

robertpb

What the BB is sagged on a bike is more to the point, a bike with 160/170 may have a BB of 342 but when sagged it's near 290, a 130/140 with a 330 will be around 290, a 160 hardtail with 310 will be down to 290 sagged.

These are just general figures but it does bring sense to some of the daft bike tests we see, "the low bottom bracket caused pedal strikes". In fact some of the longer travel higher static BBs on bikes have a lower BB when they are fully hucked.

Take a DH bike with over 350 BBs they run shorter cranks because of pedal strikes.

steve_sordy

From the figures robertpb provided, he is assuming a 30% sag at both ends of the bike dropping the bike evenly under rider weight. Evenly because the front and rear suspension both compress. With a hardtail, only the front compresses, so the effect will be about half that on the BB, as robertpb outlined. Excellent stuff.

The Rookie

steve_sordy wrote:

the effect will be about half that on the BB, as robertpb outlined. Excellent stuff.

About a 1/3 (BB being about 1/3 the way from rear axle to the front, but otherwise yes.

An FS will tend to have a higher unsagged BB height for that reason, so that sagged its similar to that on an HT.

steve_sordy

I have found this topic intriguing. Its not something I age considered before. If we assume sag at 30%, and the BB one third of the way from the rear axle, then very approximately, the BB will sit at 290mm above the ground. (All as noted previously by several members).

BB height: 290mm above ground with 30% sag. I will be looking out for that to see if it is a commonly occurring figure.

PS:
If you have a 145mm crank length and if you allow for the width of the bike, the thickness of the crank and the width of the pedal, again approximately, the outer edge of the pedal will be 145mm away from the bike centre line. That means you can lean the bike over 45 degrees on a level surface before the pedal touches the ground. But who leaves their pedal down on that side?