mstateglfr 

My gravel bike has 70mm. A frame I am considering is 77mm. If I build a frame, I had figured I would just repeat the geometry of my current gravel bike so it would be 70mm.

More BB drop means you sit more 'in' the bike than 'on top' of the bike. And the stack height is effectively higher the lower the bottom bracket goes, so less spacers are needed to get the bars where you want. And there is more control with a lower BB drop which is also a good thing for gravel riding.

So all that seems like a plus. The only potential downside I can think of is pedal strike and I simply dont turn hard enough on gravel for that to be an issue.

Am I missing something and there actually is a big downside to 75+mm of drop?

redlude97

67mm BB drop on a size 49cm Superx and a CAADX frame. Occasionally I'll bang my chainring when I do a ****ty bunnyhop over a log during races or clip a pedal through a turn. I don't run the same tires, but compared to when on my road bike through the same descents I feel super top heavy when leaning the bike on pavement which is pretty unnerving

79pmooney

A consideration perhaps: If you change what you do with this bike, you are stuck with your BB height choice. Anything you can do with a low BB bike you can do with a higher BB, The reverse is not true.

An example - my Peter Mooney. I ordered it as an all-arounder though that term was not yet in use. Big enough clearances for big tires and fenders. Ability to carry racks. Horizontal dropouts. Highish BB (driven by the fact that I was coming off a very high BB racing bike I loved and that I pedaled hard corners all the time). Almost 40 years later, Cycle Oregon announces the week ride to Crater Lake with ~30 miles of gravel. Crater Lake! I'm going fix gear! (Already done it once. Now I get to go up the North Entrance. But 30 miles of gravel with 1000;' climbs and descents? Not on my regular fix gear and 25c tires!) So I set up the Mooney as a fix gear that could handle huge tires. Proved the concept on the Trask River Trail. And fell in love with the bike as simple a classic English fix gear roadster from any time last century.

So this old bike in its new life as a fix gear? That high BB? What a good decision!

Ben

trailangel

77mm drop for an offroad bike.....I don't think so.

fietsbob

More drop lowers the pedal BDC and so you cannot pedal through a corner, Or you will dig a pedal into the road surface..

dsaul

Bottom bracket drop is not the only consideration here. Tire size and crank length will also affect the height of the pedals above the ground. The first gravel frame I built had 80mm of drop with 700 x 35mm tires and 170 cranks and I had occasional pedal strikes on roots and rocks in singletrack. It was never a problem on gravel roads. I built my current frame with 60mm of drop with 650b x 47 tires and 170 cranks.

chas58

I’m not sure BB drop makes any difference if you are not mountain biking.

My track bike has 50mm drop, and it rides just fine (although I don’t do gravel on it, lol). If you are doing lots of hairy down hills or something similar to mountain biking and are worried about going over the handle bars – they yeah maybe you have a case for more drop.

Drop is often adjusted to change the chainstay length. Higher BB means shorter chain stays and more responsive acceleration. Personally I find bikes in the 75mm drop range to be a bit sluggish. Again, if you aren’t hammering on acceleration, you may not care.

Lower BB means the chain stays are going to be longer.

I do ride hard, and power through turns. So, I need a high BB – mine is 65mm.

Pedal strike lifts your rear wheel off the ground and can have catastrophic results. I’ve done it on tight turns, at speed on the velodrome, powering out of downhill (30mph) so I have momentum going up the hill. It always sux when my BB is too low and I strike. No real negatives to a higher BB in my riding.

Your mileage may vary.

TimothyH

I haven't the slightest idea what my bottom bracket drop is or why.

BluesDawg

My custom road bike which I sometimes ride on smoother dirt roads has a BB drop of 80mm. It has never been a problem by any stretch of the imagination for me. The guy who built the frame for me (Nate Zukas) also has 80mm drop on his CX racing bike. If it was a problem for him, I'm sure he would build himself a frame with less drop. I don't pedal through turns on pavement or gravel. My gravel bike has 70mm drop. I can tell very slight difference if I concentrate, but both bikes handle very well.

I can see two situations where a drop that low would be problematic. If you were doing a lot of singletrack riding or riding on muddy roads with deep tire tracks, you might find the pedal strikes bothersome. If you decided to run 650B tires, the BB would be a bit lower, and any marginal situation would be that much worse.

Based on what you said about how you ride, I don't see any negatives.

Bryan C. 

I'm working through a similar situation here. I have my eye on a Specialized Diverge but the 85mm BB drop has me a bit concerned. My road bike has a 77mm drop. My current hybrid/gravel bike has around 50mm and the bike feels pretty tall, a large frame with 700x40 tires and long chain stays. While it isn't a major issue I do notice it when maneuvering at slow speeds. When test riding the Diverge I felt more stable but granted this was only on city streets near the shop so I can't compare it to off road.

Seems that most gravel bikes have around 70mm of BB drop. The 15mm lower drop in real terms is less than 5/8". That doesn't seem like a meaningful number to me. The 35mm more than my hybrid is pretty significant, but thinking about how I would use the Diverge mostly on flat gravel I probably won't miss it.

But I do agree that if you ride single track with rock gardens then you need every bit clearance you can get. Open gravel roads? Probably not an issue.

Kapusta

Caliper

If your gravel biking doesn't include trying to compete with mountain bikes, I'd go for the additional drop. One of my favorite bikes is an 80s Schwinn with sports-tourer geometry, it has 85mm of BB drop and the "in the bike" feel is amazing. Very different from my other bikes even with similar HT/ST angles. In terms of pedal strike, I've not had problems on dirt. Lets face it, you aren't leaning the bike as far over as a paved road bike because the traction isn't there.

Stack wise, it depends. The head tube can be shortened to keep stack low in larger frames (the frame above is a 58cm with a 155mm HT) but I can see on the smaller side of things reaching a point where the HT can't get any shorter.

Koyote

That statement about stack height is only true if everything else about the frame's geometry is unchanged, which will probably not be the case when BB drop is different.

And for what it's worth, my gravel bike has 75mm BB drop with 175mm crank arms and 700cx40mm tires. I haven't had any problems, even when on singletrack. (Though now I will probably have a wicked pedal strike on my very next ride!)