pandabear

what is the lowest acceptable bottom bracket height? contemplating a custom build. 170mm cranks. 32c tires. do not need to pedal through turns. do not need clearance for obstacles.

Thanks!

cjenrick

170 mm plus clearance for pedals.

we used to make what was called an "upside down bike."

anybody old enuff to remember those?

you would flip the frame, take the forks out and put them in the other way, and ride it with no seat.

this put the bottom bracket about 3 feet in the air.

you need butterfly handlebars for this.

shelbyfv

"Custom build" seems to mean a couple of different things. If you are having a custom frame built, trust your builder. If you are buying a frame and putting on parts, any frame from a legit company will have adequate height. BB drop will vary depending on intended use of the frame.

ofajen

It will depend on the pedals you are going to use, as well. I benched my nice 70s road frame when I started using big platform pedals, because of pedal strike. I moved the parts over to a cheap 80s Schwinn frame with low BB drop, like only 64 mm, IIRC. The 70s frame has a lot more BB drop and the top tube would be about an inch lower, even though both are 25” frames.

With 700x32 tires, my current BB height on that bike is 11.25”, which is more than enough. I ride SS and could probably ride fixed on that bike, if I wanted to.

Otto

TiHabanero

Fortunately I build my own frames and have had the pleasure of experimenting with BB drop over the years. For obvious reasons concerning pedal strike a crit bike requires less BB drop than a touring bike. The daily rider I built almost 20 years ago has a BB drop of 75mm. It is 5mm lower than a typical race frame had at that time. That bike has 170mm crank arms and 28mm tires. Zero issues with pedal strike when pedaled through right handers.
The touring frame I made has an "extreme" drop of 90mm. I am running 38mm tires on it,175mm cranks and have zero issues with pedal strike. Of course I tend to ride at a very mellow pace on that bike, and there is no need to pedal through turns for any practical reason. The purpose of the 90mm drop is to provide more stand over height, which turned out to be very effective, but I suspect if pushed through some hard right handers pedal strike will be an issue.
The last frame I built has 80mm drop and runs 170 cranks with 28mm tires. I t feels just like the 75mm drop bike as far as handling is concerned, and I have yet to strike a pedal, but I do not race it.
Conclusion I have come to is that BB drop differences of .5-1 centimeter makes no difference in handling characteristics, but it could affect pedal strike clearance in the right circumstance such as crit racing or pedaling through tight right handers. Keep in mind crank arm length and pedal width will also have an affect on ground clearance when pedaling through tight turns.

63rickert

Acceptable to who? Acceptable for what?

Lowest BB I ever owned was a 1967 Falcon M90. When Falcon was still totally handbuilt. Depending on tires glued BB height, ground to center, was 9-3/4” to 9-7/8”. This was before road tubulars narrower than 25 existed. It was a 23-1/2” frame and arrived with 165mm cranks. Wide Campy pedals of course and they scraped constantly. Which is not a problem unless you are determined to make it a problem. If scraping a pedal is going to throw you into a complete panic don’t ride a bike like that.

By 1967 that frame was a throwback. Earlier British bikes it would have been frequent.

Lowest I ever saw was the frame Ron Boi built for himself and raced Category I in early 70s. Bottom bracket drop was 101mm, coming to a height of about 9.3”. His review of his own bike was that it was completely impossible to pedal into a corner but he still came out of the corner quicker than anyone else. He got better draft off other riders and gave less draft to his opponents. He rode with 170s. He never convinced anyone else to try it in his next 45years of frame building.

Most builders will decline to go that low. You won’t be able to argue with them about the point. You might find someone who would do it. Most are simply slaves to fashion. Fashion trumps function every time.

pandabear

What kind of pedals do you use on the bike with 90mm drop?

Thank you for the response! I am leaning toward 90mm now. I also will have shorter crank arms than yours, and I also ride mellow. Gone are my speedy days..

pandabear

I am not able to meet the builder for fitting and need to submit the geometry myself.

veganbikes

Surely you know someone who can use a measuring tape and submit that to the builder.

I would use section 3 and 4 of the Indy Fab Custom order guide if the builder does not have their own:
https://ifbikes.com/wp-content/upload...erGuide_17.pdf

If they aren't willing to accept that then find a different builder or ask them what info they want for your measurements. Guessing stuff on a custom bike is kind of silly. If you have to come up with the geometry entirely and you aren't sure of certain measurements as you are here, you could come up with something pretty funky that you don't want.

79pmooney

This is so user dependent. I'm an ex-racer. Learned as a racer that, for me, having a high BB and being able to pedal deeply into corner and sooner coming out offset my weakness - few fast twitch muscles and no sprint. I raced a Fuji Pro with an 11" BB on race tubulars. Very high. Pedal strike (with Campy copy road quill pedals) happened at big leans and was scary. Only struck twice and both were memorable.

My Mooney has 10-3/4" with small road tires. I struck Campy quills a few times in the early years (being a newly ex-racer and riding like it). Over the years I have moved to narrower pedals (LOOK style clipless, Shimano semi-platforms) and strike rarely happens. My workhorse fix gear is an early Trek with 10-5/8" and the Shimano semis. I can strike (175s - my knees insist) and do so early enough that it isn't too scary but with 28c tires, I don't very often.

Now I set up a sport ~1990 Peugeot as a fix gear. 10-3/8". Even with 170s, LOOK Delta copies and 25c tires, I would scrape just looking at a corner. Likewise my Raleigh Competition which might even have more drop. 32c tires, 175s and double sided Shimano MTB SPDs scrape regularly, both on corners and pedaling over speed bumps. Good thing is that they are bullet proof pedals but the city is probably wondering where those scraped in the pavement come from.

My two customs, 2008 road and 2011 road fix gear have (by my request) 10-3/4" with LOOK Delta copies and 25c, 10-7/8" with Shimano semis and 25c. I can and have hit both but not often. Both bikes are a joy and I wouldn't change anything. Now, take into account I always (except those few years on the Peugeot) ride 175s. If you ride 170s you can subtract roughly 3/16" from my BB heights for the same strike. I ride as narrow a Q-factor as I can get (side-to-side crank width governing where the pedals sit and how bowlegged we ride). If you ride more standard Q-factors, you will strike sooner on corners though speed bump clearance doesn't change. Wide and deep pedals obviously make for quicker strikes.

And comments on the strikes themselves. Not all strikes are equal. First, strikes at big leans on hard, fast corners are scary, even crash causers. Strikes at shallow leans from any combo of deep/wide pedals, low BB, tiny tires are not scary unless you really hit hard. (Irregular road surface, you very aggressively dive into a turn ...) And different pedals take kindly or otherwise very differently. If the pedals have dustcaps that can hit, well they unscrew and roll off into the gutter. (Right-hand threaded left pedal caps. I used to regularly shed my left Leotard platform ones and kept robbing the caps off my old, retired rights.) Hitting those press-fit construction pedals accelerated loosening those fits and speeding their demise. On some pedals the cages hit first and are replaceable but now, they are probably not easy to find. The good Shimano MTB pedals can probably take years of hard strikes because that isn't so different from aggressive MTB'ing.

If striking pedals was like striking oil, I'd be a rich man.

rydabent

One of the biggest factors here is the size of a persons foot, and the angle they keep their foot at while pedaling. It will vary too with the geometry of the bike. It will be different from and up right frame to a crank forward frame.

On a recumbent trike I can attest to the fact that 1 foot off the ground is about as low as you can go to avoid heel strikes.