BB drop - fact or fiction?

[Sinisterminister]

Between 2 bikes, the same except for bottom bracket drop, one is 65mm, the other is 75mm, could you tell the difference when riding?

[Letmyinspiration]

(remove clothes to reply)

[kemical]

I had a bad experience with my jekyll's settings. One day I decided to try the full freeride position as we were heading out to a very technical area with lots of big rocks to climb up and down. The big ring took such a beating ( did not have a rock ring on).. that I stopped and took a minute to put it into full X/C position. The I rode down a very steep rock face and at the bottom, the front wheel just dug in and sent me flying. The head tube angle in the X/C position makes it so the front wheel does not want to ride over stuff that normally i would just float over. After that, I put it in the middle, took off the big ring and put a 32 bash ring on in it's place. I've been happy since.

I'd like to go back to the full freeride position, but I think I would have to re-learn pedalling .. otherwise I'd hit every rock that i pass by <G>

I have the "5 & 5" jekyll.. Max TPC+ shock in front and 5" travel in the rear - I love it. I have a scalpel for racing.

[Tans]

I'm fascinated by the idea that a higher center of gravity is more stable for anything. While dubious,
I'm hoping for more explanations.

Years of trials riding hammered home the unhappy notion that the lower the center of mass, the better the machine would handle--this was one of the chief problems of the four-stroke engine with its heavy valve train sitting up on top of the engine, which in turn tended to sit up on top of a wet oil sump.

The usual analogy was that most people prefer to sit low on a rickety chair than up on top of a rickety step ladder. Or, to use the step-ladder alone, it's easier to balance while standing on the first rung than on the top rung.

Whenever anyone objected that standing up on the foot-pegs obviously raised the center of mass, the explanation was that it dramatically lowered and widened the actual point of control from the seat to the foot-pegs and allowed far more body movement.

As far as I know, racing street motorcycles still struggle to keep the center of mass as low as possible, just like race cars.

Elsewhere, the Navy during the War of the Pacific was forever compromising with the demands for more and more
40mm anti-aircraft tubs placed up high, versus the captain's concern that the ship would capsize with so much weight up high.

Walking on stilts is usually considered tricky.

In this thread, I noticed Chalo's comment that his tall-boy bike can go much slower without falling over, but I'm puzzled, since even an aging klutz like me can roll a touring bike back and forth while heading up a gentle slope. I doubt that I could do this while perched up on one of his oddball frames.

So if you can swallow your first exasperated reaction, help me understand what I'm missing here--is it something to do with handling versus stability, or balance, or what?

[Letmyinspiration]

Not so. The center of gravity stays the same. It relates to cornering ONLY.

(remove clothes to reply)

[kemical]

My mountain bike ( cannondale jekyll) has a rear shock adjustment that changes the BB height 1+ inches .. and also changes the head tube angle.... so it is hard to tell which has the greater impact.

in the cross country mode.. the BB is at it's highest point and the head tube angle is 70 degrees.. good for navigating switchbacks, etc.

in the freeride mode the BB is at it's lowest and the head tube angle is about 68 degrees... greater stability on rocky downhills.

I set mine "in the middle".. because in the freeride mode, I hit the pedals on rocks, etc and in the cross country mode... I endo when going off steep drops,e tc.

[BRITNEYLUVA]

OK, my Jekyll is currently 'in the middle' as well. As a matter of interest, is there anyone out there with a Jekyll _not_ in the middle?
I certainly would not want my BB any lower, because, as you say, you'd spend too much time bouncing pedals off rocks.

[Teran]

Stable may not be quite the proper term, but the bike falls over more slowly, and your corrections to dynamically balance the bike don't need to be as quick.

I'm no expert on trials bikes, but from what I've seen they're handled more like pogo sticks at very low speeds than like conventional bikes at much higher speeds. I suspect the high CG is harder to throw around with body english.

This isn't the same. Here you have a fixed-size base, and varying CG height. When the CG is high, it falls over at a lesser angle from the vertical. The bike is not statically stable, but is dynamically balanced.

Not true of roadracing motorcycles. Aside from the fact that stability isn't the prime goal, if the bike's CG is too low, its moment of inertia about its roll axis will be greater, and the tires will have to generate higher side forces in the fast transition to a bank, so it won't be able to make the transition as fast.

The ships needed static stability, and a high CG would make them easier to capsize. It's a little like the ladder, but not quite. If the center of gravity shifts outside the center of bouyancy when the ship rolls, it capsizes. The center of bouyancy shift depends on hull shape, the CG shift on its height, if nothing shifts around and makes it even worse.

A little, but you weren't born with them, and they aren't that well connected to your legs. You also can't rely on the length of you feet for forward/backward balance, so it's much harder to stand in one place. In spite of this, they're pretty easy to learn, much easier than a unicycle, for instance.

If you can go very far backwards, I'm genuinely impressed.
This aging klutz wouldn't try it.

[Teran]

When a bike is allowed to passively fall over, it pivots on its contact patches. When it is forced into a bank by moving the contact patch of the front tire sideways, the instantaneous center of rotation is well above the pavement. Although it's not perfectly analogous, you can see this effect when balancing a yardstick or equivalent on your hand. Move your hand quickly back and forth and see which point remains stationary. For a slender rod, the point is 2/3 of the way from your hand to the top, not at the bottom, and actually above the CG. For the motorcycle it's more complicated, only one end steers, the instantaneous rotation axis isn't horizontal, the moments of inertia about the roll and yaw axes both come into play, there are gyroscopic reactions and suspension reactions and tire reactions, and nothing is really rigid. I don't claim to understand it thoroughly, but lowering the CG is not the ultimate goal of roadracing bike designers. Many 4-stroke exhaust systems are now higher than they would need to be simply for cornering clearance, and gas tanks are still pretty high.

It's true that too high a CG makes wheelies and stoppies more likely. Current practice seems to be to put the engine far forward to more nearly equalize weight on the tires. I've never seen an accidental stoppie in a road race, and I've watched hundreds, although some riders do get the rear tire a few inches off the ground in hard braking.
Controlling power wheelies seems to be more of an issue, especially with bikes like the Aprilla MotoGP, but I don't think it's been looped so far.

Anyway, if I implied that minimizing moment of inertia about the instantaneous roll axis was the only consideration, that was wrong. It's one of many different and sometimes conflicting considerations.

[hypermush]

Well, you can test it fairly easily on the same bike.
Let's say you usually use 175 cranks. Get some 170's or shorter.
If the difference is 5mm, then that is close to raising your BB by 5mm for cornering.
Try cornering and you will have more stability and can carve a tighter line, though only a little bit. I used to race BMX bikes over 20 years ago, and this was a fairly big issue. On road bikes, I usually ride in a straight line for long periods of time and the tires are relatively skinny, so the benefit, while real, is probably not going to cut much time off your trip.