Bottom Bracket Height and Crank Arm Length
New Bikes, First Rides, Pedal Strikes, Bottom Bracket Height and Crank Arm Length. Well it wouldn’t be BikeCo.com without courting a bit of controversy so might as well start it out on a Monday!
Let’s begin with the disclaimers – this is not a medical post. I do not have a kinesiology background so can’t speak to the physical mechanics of what you’ve got going on. I do feel I can speak to my riding experience over the years on topics like back pain, etc. If you find parallels or concepts that help I’m always stoked.
I do feel I can speak to riding dispositions as well as mechanical dimensions.
Next – there is such a thing as too low of a bottom bracket. There’s also too tall for modern geometry as well. The industry leaders are just that, industry leaders… Ibis, Yeti, Alchemy and Evil all do a lot of testing to ensure that they balance cornering, pedaling and smashing through terrain when they put their geo spec’s together… Bikes also sit into their travel slightly different – but again comparing the best of the best we can assume none of them are out in the weeds.
Don’t think of this write-up as gospel but rather as some concepts to think of when you test ride – or read about other people’s test rides… or how to review a review.
Bottom Bracket Height and Crank Arm Length
We get a fair number of crank arm queries here at BikeCo. We’re not here to tell you “yes” or “no” to what you want. It’s our job to help you navigate product benefits and decide what’s best for you. As avid riders we’re familiar with symptoms that might appear to be one thing, when actually are something else. Which brings me back to the bottom bracket height and crank arm length topic.
I’ve recently been spending a chunk of time on our demo Yeti SB130. (It’s so good… another topic for another day). As with a decent amount of bikes I test, on the first ride it seemed to have a couple pedal strikes early in the ride. But I didn’t notice them after the first little bit. I really didn’t notice them the next handful of rides either.
Then I had a bit of an eye opener late last week.
I came across a trail runner in a corner. It wasn’t a situation where I was going to blast over him (no one was diving out of the way nor was I putting the bikes into the weeds to miss him!), but I didn’t want to tear by him at full lean either.
So I grabbed a bit of brake and stood the bike up a bit. And thumped my outside pedal pretty good exiting the corner. The next turn was in quick succession and I really didn’t dive into it either and felt the outside pedal drag just a touch.
A light came on in my head. Usually when I notice pedal strikes is the first couple features on a bike I’m trying to get accustomed to. Typically during direction changes with a bit of chunk in them. It makes sense if I’m not leaning in as far, trying to grasp a bike’s balance point that the outside pedal is much closer to the terrain.
Having been testing bikes for so long I generally can get the grasp of a chassis within a couple turns and ride more aggressively. This would account for the strikes becoming less frequent as well.
(Nearly all of my test rides are in SoCal desert conditions – so I’m not speaking of rock climbing up baby heads and basically “walking up” on your pedals… that’s a bit different story if you’re having that type of issue in your terrain. We can help you solve it – reach out!)
Some things to take into consideration on bottom bracket height and crank arm length:
Are you riding faster / pedaling through more terrain?
Are you currently riding what would be considered modern or relativity modern geometry? Bikes went longer, slacker and lower a few years ago. If you’ve got a large technology jump the lower bottom brackets may seem more pronounced.
The center of gravity benefits of the lower bottom bracket allow modern bikes to corner more confidently, track better and ride faster. When I first went to the more modern geometry I noted pedal strikes as I was able to now pedal through sections that previously I tip-toed through! After a couple rides I got used to being able to sprint into and out of corners differently.
Is it a demo bike?
Couple things could be going on here. One, modern suspension requires the rear shock’s negative air spring to be correctly charged. If you’ve increased the shock’s PSI notably and didn’t allow the negative air spring to charge when it does the main chamber will lower.
This actually happened to me the other day. I just plain forgot to cycle the shock. Number on the pump was fine. Got to the trail head and was sitting at 50% sag instead of 33%. Didn’t have a pump in my truck. Crap. Ran it in trail mode to use the compression for support. Not ideal but it gets you through the quick after work rip.
If it’s a demo ride how comfortable are you on the bike? Are you flowing like you would on your own rig? It’s not the easiest thing to do for the first little bit – especially if something about the performance gets in your head…
So we’ve established I’ve spent a lot of time on the Yeti SB130 and noted a handful of strikes the first bit. The published BB height comes in at 337.7mm. My personal bike is an Ibis HD4 (which isn’t quite as long or low as say the SB130, SB150 or Ripmo) and the bottom bracket lists at 343mm.
So we’ve got a difference of 5.3mm. Or .209″. As a reference a quarter is 1.75mm thick. So 5.3 is just about 3 quarters stacked up…
OK. HD4 I don’t notice pedal strikes. BB drops 5.3mm (.209″ or 75 cents…) and I’m noticing it? I’m not sure I ride precise enough that could make a difference – which leads me back to it being a rider position scenario for me.
At the end of the weekend I had a chance to go for a quick spin on the HD4. I kept it tall through the corners where I noticed pedal contact with the SB130. Almost identically the same thing.
So to the point – it’s important to look at the “hows” and “whys”. Work with a quality resource who can help you analyze your riding style, terrain and fitment requirements.
Ok so we’ve said 5.3mm is 75 cents. And 10mm is just about $1.50? So why not just go to shorter cranks to make up for it automatically? Well it comes back to the kinesiology. How your body works, what stresses it and over what period of time.
Since cycling is a fairly repetitive motion generally over several hours a week issues may manifest. There are some really interesting articles out there on crank length compared to femur length, foot size, cadence, core strength etc. One that I find is both informative and appropriately vague is on Bicycling.com Here.
A problem you may come across researching crank length is it is often written from a road or tri point of view. Here’s the position, get in it, stay. In the mountain world bike fit is much more dynamic incorporating steeper ups and downs with varied body positions.
At BikeCo we work with our clients to ensure usable fit based on their body requirements and riding style. Don’t buy into the laser light show fit in MTB – there’s just too many variables.
For me, given that I’m on the taller end of the median rider I would be very, very hard pressed to risk knee or back issues going to smaller cranks… But if you’re thinking about it let’s discuss it! We can help you define what’s going to work best for you.
Questions? That’s what we’re here for. Chat, email or call our expert staff to help them define the best setup for your needs!