zoldboy

Hello everyone!

so im looking at buying a new frame, a bassi bloomfield. This thing looks pretty sweet overall, but the bb drop is only 50mm, and i feel like thats not really low enough for what im trying to do with it.

My current bike is a surly cross-check and it has a bb drop of 66mm. The wheel size is 700x38c.

I would probably run 42’s on the bloomfield, and so its wheel size would be 650x42b.

This means i would be about 1.5cm lower to the ground overall, on the bloomfield.

But since the bb drop is 1.6cm higher than the crosscheck, i am kinda worried that its not going to be as comfortable on the road.

Am i crazy? Does this matter? Will i even feel a difference, in this regard?

I remember that when i tried a 650b conversion on an old ritchey mtb frame, everything felt very “on the bike” and not “in the bike” (i also used 650x42b there). For this frame, i believe the bb drop was around 40mm. considering it was originally designed for 26” wheels, though, the effect makes sense. Regardless, that was the main reason i parted that bike out.

The bloomfield will undoubtedly be zippier than my crosscheck, but im only going to have one bike (after i get a new frame), and i want something that will be fun but also pleasant to ride on road, for mostly mid-distance rides.

any comments are appreciated!

terrymorse 

I can't visualize how a bottom bracket height is going to affect comfort. Handling, certainly. Comfort, I doubt it.

As it seems the bassi bloomfield frame is intended for fixed gear riding, a higher bottom bracket is a very good idea. A low bottom bracket on a fixed gear runs the risk of grounding a pedal.

zoldboy

i don't plan on running it fixed gear. rather, single speed. anyway i do not doubt that the designers made a good choice, what i do doubt is if i'm going to feel planted on the road or not. which i guess i kind of neglected to say in the original post.

maybe i already have my answer to the original question, but i believe bb height/drop is directly related to stability. which is a factor of comfort, on a bicycle. at least, in regards to going in a straight line. for an example, you could have a bike with 20 inch wheels, and a 0mm bb drop, but the bb height could be the same as lets say a 700c wheeled frame with 70-80mm bb drop, and therefore you would have the same stability, going in a straight line, or even cornering, due to the fact that you wouldn't be any higher off of the ground(bb height), on either bike.

the two bikes would obviously have to have the same trail figure as well... but that's a different factor of stability that doesn't concern me, here.

TiHabanero

This is my favorite subject concerning frame design. I designed and built 5 frames thus far, all with differing BB drop. I can say with absolute certainty that 1-2cm in BB drop does not affect the ride quality of the bike, nor does it affect the handling of the bike. There is too much else going on that makes a big difference in those arenas to make 10 or 20mm drop stand out.
Assuming a straight top tube, BB drop does make a difference because it affects fit. If the top tube is sloped, BB drop does not make a difference because the head tube can be any length desired which in turn does not affect fit as much because it can be dialed in with head tube length. With a straight top tube, when the BB drop is deeper, it affectively brings the saddle height down bringing it more on par with handlebar height, which affect comfort on the bike.
I can go on, but the above sums it up fairly well albeit some other info is missing that will affect fit by moving the BB drop.

veganbikes

No fixed gear? That is so much fun. I would still want to avoid pedal strikes though on a single speed. It isn't fun.

In terms of having one bike if you are doing that then get a custom built bike and you can have whatever geometry you desire with all the attachment points you might want and whatever steerer tube you want and really dial it in to you. If I were only to have one bike which I loathe to think about I would probably stick with the Cross Check as it has a lot of versatility in it. If I was going to have one fixed gear only but other bikes than I have loads of options but the Bloomfield isn't bad it isn't my cup of tea as I am just not a threaded fork lover personally but you can get the Innicycle headset and keep the fork and still have modern 1 1/8 stems without needing a new fork.

Iride01 

You should really be more concerned with BB height. Not quite the same as BB drop.

zoldboy

Would to care to elaborate further on why i wont feel a difference between the two bb drops?

shelbyfv

I like to be able to put a toe down w/o having to move my butt completely off the saddle. Half butt on saddle is OK. I think a lower BB facilitates this but I could be wrong since numbers are involved.

ofajen

I think you would notice the difference but it’s up to you what is preferable. I have a nicer vintage road frame with a BB drop of about 90 mm and a cheap vintage road frame with a BB drop of about 63mm. I got the nice frame first and used it for many years on road rides. Even with quill style pedals on roads, I had to watch out for the occasional pedal strike.

When I switched to riding mostly our local trail system with more complex transitions and rougher surfaces, the pedal strike became obnoxious, especially when I started using larger, platform style pedals.

So for some years now, I’ve built up the road parts on the cheap frame with low BB drop. I get a BB height over 11 inches with 700x32 tires and pedal strike is never an issue. As always, YMMV.

Otto

Steel Charlie

Probably not a concern for most cyclists.

Lombard

This. And the only real advantage to a low BB is if you are planning on doing loaded touring, the lower BB will make the bike more stable.

icemilkcoffee 

OK. Let's do some quick math. The 650B wheel (584mm) has a radius that is 19mm less than the 700c (622mm) wheel. But you are putting on 4mm larger tires. So overall the 650x42b wheelset will have a radius that is 15mm less than the 700x38c wheelset.
Now you said the 650b bike has a BB drop of 50mm, vs 66mm for the 700c bike. In other words 16mm less BB drop. So the BB is 16mm higher relative to the axles, but the axles are 15mm closer to the ground. In the end you would be 1mm higher off the ground on the 650b bike, not 15mm lower.

Camilo

On any given wheel/tire size, one is easily calculated from the other. For the common range of tires on a 700C road or gravel or touring bike, the axle to ground measurement is pretty constant within a few mm of 350mm depending on actual tire size. It =BB drop+BB height, so one can get calculated from the other. I think 350 for road and 355 for gravel/touring/etc. (because of wider tires) is what you'd find for the "constant" And from what I've seen, if a bike is designed for both 700C and 650B like a gravel bike, the tire size used for their general specs result in a similar overall diameter like 700x35 and 650x45 give or take. It all ends up within a couple of insignificant mm of 350 to 355. I don't know anything about mountain bikes.

I've been going through this finding a good frame for my wife who is not only short, but has short legs. I tried a Cross bike for her gravel bike and it was too tall by standover even though it fit every other way. Cross bikes have high MTB-like BB height/drop. Newer "gravel" bike designs are good in the respect you can easily find one with a lower BB and more sloping TT if that's what you're looking for. And the height can be easily calculated if they only list drop.

Camilo

It can impact standover. High BB increases the height of the top tube. That's why typical cross frames fit different than a typical road bike with similar geometry other than BB height. It doesn't impact me personally but has significantly impacted my wife's overall fit. Cross frames don't work, but similar sized road bikes or gravel bikes that tend toward road design do. The downside is possible pedal strike which is why cross bikes are designed that way, but bikes primarily ridden on pavement and moderate paths don't need it.

I think the new-ish gravel bike niche is great in that there are options that tend toward MTB or cross design with higher BB and options that tend towards road designs with lower BB I prefer the latter.

Camilo

For fun I just went out and measured some wheels I have. Smallest is 650b-35, and my 700c wheels have 23 and 35. I also have 650b-50

700c-23 and 650b-50 are both about 348 and 700c-35 is about 353 from axle to ground, as far as I can tell. Maybe 345-355. But they're all within a half a cm from "350".

The only wheel that is an exception to simply using 350 as close enough for BB drop and height calculations is the 650b-35 which is closer to 335 which is exactly why I put them on my wife's bike to squeeze a bit more straddling clearance for her and she only rides smooth and rough pavement and moderate dirt and gravel with this bike so typical large 650b tires aren't necessary.

TiHabanero

Zoldboy, I intentionally built 5 frames with the same seat tube and top tube angles, same top tube length, same fork offset. BB drop ranges from 7.5 to 9.0. As far as stability, handling, or ride quality I cannot feel any difference between the frames. Pedal strike with the 9.0 drop can be a concern with lower profile tires, however I went with the deeper BB drop to improve stand over height with the larger tires.
Chain stay length will affect handling because it affects wheelbase. 1cm of difference is not enough to dramatically affect handling, but 5cm is noticeable. Tire width has a noticeable impact on ride quality provided the difference is large enough to be noticed. For instance I cannot tell the difference between 23 and 25mm, but do notice a difference when I step up to 28mm. The difference between 28 and 38 is very noticeable even though there is only 10mm difference. Tire profile coupled with width add up to a lot of change.
Addressing stability, I have found that it is affected by chain stay length, trail, tire size, and handlebar width, not BB drop. Even my Miyata Ridge Runner from 1985 is very stable on the road and it has something like 2.5cm drop!
I'm sure I have overlooked some other factors, however these come to mind right away because it is what I have concentrated my efforts on.

Lombard

Not sure I understand why you can't just lower the top tube to compensate. Yes, saddle and bars will need to be higher to compensate for higher BB, but that doesn't mean TT needs to be higher, it can be more sloping.

Polaris OBark

On my bike, the outside diameter of 650b x 44mm is the same as 700c x 28mm. The difference should be closer to 2mm, not 15mm.

Camilo

You're absolutely right. A lot of gravel bikes I've been looking at do just that. There's two ways to lower the TT: lower the BB and/or make the TT more sloping with a shorter seat tube.

This is one of the reasons - at least in the past when I was using them for what now is called "gravel" riding - that cyclocross bikes had a relatively high stand over. They had a high bottom bracket for clearance on rough trails, but also had a fairly level top tube so the main triangle could be roomy so the bike could be hefted on the shoulder by actual cross bike racers, the target market.

Now that you can find a bike with clearance for wide tires, that isn't targeted at cross racers, but to people who simply want to ride a road-type bike on rougher surfaces, some more hard core than others. They can slope the top tube more and/or lower the bottom bracket. I've seen bikes that do both or one or the other - the bottom brackets tend toward either the lower, road bike design, or the higher, mountain or cross bike design. A lot of bikes in this niche, it seems to me, are also emphasizing low standover/very sloping top tubes more than in the past. Like I said in an earlier post, it's not something I look for because in the size of bike (mostly looking at effective top tube and head tube lengths, I don't usually have problems with standover. For people who are very short exacerbated by disproportionately short legs, it can be a big deal.

It's interesting to see the options that have appeared in just the last 5 years. I've read a lot of comments to the effect, why a "gravel" bike, but I think the designs are changing for this sort of riding and the riders who are using them. Some are pretty much road bikes with larger tire clearance, and some are pretty much rigid 27.5" mountain bikes with downturn handlebars.

zoldboy

yo thank you all for the extremely informative replies.

it seems like bb height will be basically unaffected, if i choose to run 650x42 on the bloomfield. if i could go custom, i would lol.

main reason i want to switch to this from my cross-check is that the bloomfield has less trail, and less (or non-existant) toe overlap. also, riding fatties on 650b just feels incredibly nimble compared to a fat 700c wheel, lol

also, a 650b, single speed frame with RIM BRAKES is just so cool in my opinion.

thanks again guys!