[Vitamin G]

I have a question about bottom bracket drop in regards to the feeling of bike stability, and how much difference in BB drop is needed before a rider can feel the difference (subjective I know, but any experience would be appreciated).

My old road bike had 7cm BB drop, and 417 mm chainstays. My newer bike has 6.75 BB drop and 407 chainstays. The new bike feels less stable descending, I can't describe the feeling other than it feels like the bike is going to just slide out, it's nervous (and so am I from it).

I look at endurance style bikes like Roubaix (7.15 BB drop) and Domane (7.8cm drop) and wonder how much difference is needed in BB drop to make a bike feel nice and stable descending.

Any first hand experience?

 

[cxwrench]

There's obviously more to it (that stable feeling) than just bb drop. Trail plays a huge part. How you fit on the bike. Tire pressure. Wheel/tire weight or lack thereof. I think it would take a centimeter or so before it'd really notice it, but i've never really ridden bikes back-to-back that had that much of a difference. Hard to say, for me.

 

[JoelS]

A smaller BB drop like your new bike leaves me feeling sort of perched up on top of the bike, rather than down in it. However, there is less chance of pedal strike if pedaling through corners. And yes, the higher center of gravity will result in a less stable feeling. But the rest of the geometry comes into play as well.

My personal preference is 75mm of BB drop. But that's after riding a lot of bikes.

Good question too.

 

[cda 455]

There's obviously more to it (that stable feeling) than just bb drop. Trail plays a huge part. How you fit on the bike. Tire pressure. Wheel/tire weight or lack thereof. I think it would take a centimeter or so before it'd really notice it, but i've never really ridden bikes back-to-back that had that much of a difference. Hard to say, for me.

Good point there.

 

[aclinjury]

First of all, I'm pretty sure your chainstays are not 417 cm!

for straight line stability, you want low and long. In this case, you want the bb as low as possibly, and wheelbase as long as possible. Since chainstays are a part of wheelbase determinant, longer means more stability.

For cornering agility, you want low bb and short chainstays.
For acceleration, you want short chainstays.

 

[Fredrico]

I have a question about bottom bracket drop in regards to the feeling of bike stability, and how much difference in BB drop is needed before a rider can feel the difference (subjective I know, but any experience would be appreciated).

My old road bike had 7cm BB drop, and 417 cm chainstays. My newer bike has 6.75 BB drop and 407 chainstays. The new bike feels less stable descending, I can't describe the feeling other than it feels like the bike is going to just slide out, it's nervous (and so am I from it).

I look at endurance style bikes like Roubaix (7.15 BB drop) and Domane (7.8cm drop) and wonder how much difference is needed in BB drop to make a bike feel nice and stable descending.

Any first hand experience?

Yep, as Joel says, "I feel perched atop the bike" with the higher BB. I thnk they were designed for criteriums, so riders won't scrape pedals cornering, as Joel says. My preference is also 7.5 cm drop. That's what's on my trusty old DeRosa, a smile on my face every ride. But you better pull up the inside pedal on a corner! I've lost those little tangs on its rat trap pedal cages that you flip up with the toe of the shoe before inserting foot into toe clip. The pedals clear the ground by a scant two inches.

We've always theorized low BBs bring the center of gravity down on the bike. That's why you feel more stable. The old DeRosa has steep angles and handles quick, to say the least, but is still very sure footed in a straight line because of the low BB. It also doesn't rock side to side much in an acceleration or hard climb. And coming down mountains at 45-50 mph, it stays in line like a motorcycle, inspiring absolute confidence. :yesnod:

My other bike has 6.5 cm. drop. It wanders much more easily, even though it has shallower angles. But I've never scraped its pedals in corners.

 

[Peter P.]

That feeling that the front tire is going to slide out on descents is caused by a low trail factor more than a certain BB drop.

The steeper the head angle, when combined with a low trail, makes things worse, in my opinion. Tom Kellogg at Spectrum Cycles has one way of explaining the phenomenon.

 

[wim]

My old road bike had 7cm BB drop, and 417 cm chainstays. My newer bike has 6.75 BB drop and 407 chainstays. The new bike feels less stable descending, I can't describe the feeling other than it feels like the bike is going to just slide out, it's nervous (and so am I from it).

My money is on something other than BB drop. I say that because "stability" of a bicycle is not improved by lowering the center of mass. Were it so, recumbents would be the most stable bicycles , which they are definitely not. The notion of lower = more stable holds true for four-wheeled (non-leaning) vehicles, but not for in-line vehicles that can lean in turns. I think this lower = more stable belief is so widespread because we live in a car-centered society, but that's just a theory of mine.

 

[looigi]

I say that because "stability" of a bicycle is not improved by lowering the center of mass...

Correct. Think of it like balancing a pole on your palm. A tall pole is a lot easier to balance than a short one. Also stand a long pole and a short pole on end and let them fall over. The long pole takes longer to fall over than the short one.

The CG of the rider and bike is barely affected by the height of the BB. What is affected is how/where the mass of the rider is connected to the frame. Standing, the bike is really floppy between your legs. Seated the bike is much more stable and you can even ride hands off.

 

[Vitamin G]

First of all, I'm pretty sure your chainstays are not 417 cm!

for straight line stability, you want low and long. In this case, you want the bb as low as possibly, and wheelbase as long as possible. Since chainstays are a part of wheelbase determinant, longer means more stability.

For cornering agility, you want low bb and short chainstays.
For acceleration, you want short chainstays.

Yeah... that would be a real truck! I fixed the original post...

 

[Vitamin G]

Correct. Think of it like balancing a pole on your palm. A tall pole is a lot easier to balance than a short one. Also stand a long pole and a short pole on end and let them fall over. The long pole takes longer to fall over than the short one.

The CG of the rider and bike is barely affected by the height of the BB. What is affected is how/where the mass of the rider is connected to the frame. Standing, the bike is really floppy between your legs. Seated the bike is much more stable and you can even ride hands off.

Well, that's good -- there's hope for me to learn to descend on this bike yet...

Head angle is a nice 73 degrees, so no problems there. Fork rake is 43, should be a nice amount of trail.

 

[JoelS]

Well, that's good -- there's hope for me to learn to descend on this bike yet...

Head angle is a nice 73 degrees, so no problems there. Fork rake is 43, should be a nice amount of trail.

Assuming that's 700x23mm tires, that yields a trail of 58mm. That's pretty short. These days, most OTS framesets have more like 65-68mm of trail, which is a ton more stable. And the smaller the size of the frame, the more trail. They do this to help prevent toe overlap. Which bugs me to no end. I ride a pretty small frame (52mm top tube) and can't find an OTS frameset with the geometry I like. I LIKE a low BB and 51mm of trail!

Here's a little online calculator: Bicycle Trail Calculator | yojimg.net

 

[wim]

And the smaller the size of the frame, the more trail. They do this to help prevent toe overlap. Which bugs me to no end.

Same here. In part, they do this to accommodate the vast numbers of people out there who absolutely have to have a race-type bike, but then complain about some of the typical attributes of those bikes. And I can tell you from experience that a lot of those folks are unwilling to learn how to deal with toe overlap even if given a clear explanation of why it exists and step-by-step instructions on how to deal with it.

 

[JoelS]

Same here. In part, they do this to accommodate the vast numbers of people out there who absolutely have to have a race-type bike, but then complain about some of the typical attributes of those bikes. And I can tell you from experience that a lot of those folks are unwilling to learn how to deal with toe overlap even if given a clear explanation of why it exists and step-by-step instructions on how to deal with it.

One of many reasons for going custom.

 

[wim]

One of many reasons for going custom.

Well, that can put a custom builder into a real bind when dealing with an insistent customer. Should he accommodate a no-toe-overlap request and build a crappy-handling frame? People have refused frames with toe overlap, so it's not an easy question to answer.

 

[JoelS]

Depends on the customer. I knew that I would have to overlap, a lot. But I wanted a frame that would handle. I got exactly what I wanted. And I'm thrilled with the bike. Er.... both of 'em

 

[danl1]

One thing that hasn't had a good vetting here is the chainstay length. It's a widely ignored part of how a bike handles and feels, but has a lot more impact than folks realize. Longer wheelbases make a bike feel more stable, by slowing the steering effect slightly. But they also change the weight distribution - longer stays putting a bit more load on the front end, again making things a bit more predictable-feeling. That cm can make a surprising difference AEBE. That'd be fully consistent with the sense that the back end was a bit more 'lively' with the new bike, that it might want to 'slide out.'

And, there's an element of perception. Especially with it being shorter in the rear and higher, if it happens to be a bit stiffer, you'll feel the rear wheel interfacing with the road more than on the other. Even if there's no real difference, the additional 'information' coming up through your butt will tell your brain that the tire is nearer it's traction limit. That's just a factor of riding enough to calibrate the signals.

 

[laffeaux]

IMO a lower bottom bracket does make a bike handle better (at least at higher speeds). However, since you're comparing two different bikes there are more difference than BB height/drop, so you may be feeling many different changes.

I have a mountain bike with an EBB which allows for a bit of BB height adjustment (as well as a bit of fore/aft adjustment of the BB). The EBB is used to create tension for different gear combinations on a single speed set up. However, I run the bike with gears - which means that I can set the BB where I wish. I've run it high and low, and feel that the bike takes high-speed corners much better with the lower BB height (more drop). Why that is true is not completely clear, but I can feel a difference in the 2-3cm change that I can create on the bike.

But as other have said, rake, wheel base, chain stay length, and weight distribution all affect handling. What you're feeling could be a result of any combination of these changes.

 

[Fredrico]

Very interesting.

Reminds me of someone mentioning walking on stilts. Easier to balance when the foot pegs are lower, harder to balance when pegs are higher.

Similarly, the rider transfers weight down to the pedals with each stroke, in effect lowering the center of gravity and making the bike easier to balance. When out of the saddle and climbing, ALL the rider's weight in on the pedals. My low BB bike climbs considerably smoother than the high BB bike. :idea:

There's a similar effect of lowering the center of gravity on the bike when cornering, by lightening up on the saddle and pressing down on the outside pedal, right? :yesnod:

 

[danl1]

There's a similar effect of lowering the center of gravity on the bike when cornering, by lightening up on the saddle and pressing down on the outside pedal, right? :yesnod:

Well, no. Unless your body moves down, your center of gravity doesn't change. By lightening up on the saddle, if anything, you are raising your center of gravity.