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Moulton vs Brompton

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Moulton vs Brompton

Old 01-27-24, 11:26 PM
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Responding to many posts above:

I have not seen the design in detail at the bottom bracket/pivot. But in general...

If the bottom bracket is part of the rear triangle so top chain length does not vary, it could still bob from pedaling. First, the chain "top", under tension, is still above the pivot point, so even if at the bottom bracket, will exert a moment, which is the tension in the chain multiplied by the perpindicular distance between that top chain and the pivot point. Another way to visualize this: Picture everything as completely rigid; The top chain a rigid rod connected to the chainring, and to the rear triangle, everything one-piece; Under pedaling force, the chainring exerts a torque on the entire rear triangle and rotates it upward. The only way to eliminate this is to have the pivot point in-line with the top chain, because then that chain tension has no moment arm to act through, it just pushes on that pivot point, in line with the chain, and nothing rotates. However...

The above neglects ground thrust. The rear triangle is a "trailing arm" suspension; On cars with trailing arm suspension on a driven axle: Under braking, the trailing arm pulls the chassis (frame/body) down, and under drive thrust it pushes the chassis up. Applied to the rear axle on a rear wheel drive chassis, this results in "anti-lift" under braking and "anti-squat" under acceleration. On the same vehicle, front axle, *non-driven*, suspension reversed for a "leading arm suspension", you have the converse, but only by half; The leading arm results in front "anti-dive" under braking, pushing the chassis up, but there is no "anti-lift' under acceleration because *there is no torque applied to the suspension*. (This is why, original VW Beetles, trailing arm front suspension, the front would dive hard under braking, the suspension was "PRO-dive.") SO... applied to the bike swinging rear triangle, that ground thrust forward when pedaling will want to rotate the triangle front UP, while chain tension wants to rotate the triangle front DOWN (unless pivot point in line with the chain, as mentioned). So these two need to be balanced. HOWEVER, because the radius (moment arm) of the rear wheel is larger than crank radius, the ground thrust will be lower than pedal force, as those two total torques are equal (force x moment arm). Thus, I *think* the ground thrust has less influence on bobbing than the pivot point with respect to the chain top line (EDIT: if the pivot point is relatively LOW; The higher the pivot point above the ground thrust, the greater the moment arm, and greater effect. I *think*, I haven't Free Body Diagramed this all). Braking is a different story, sort of; There is no chain tension to balance against, so braking ground thrust should pull the front of the triangle down, that's "rear anti-lift", EXCEPT, to pull the chassis (frame) down, you are also pulling the triangle/rear tire UP, which unloads it, limiting your braking thrust at the ground, the rear tire wants to lock up. This is why, with early front drive cars, short trailing-arm rear suspensions, and no ABS, wanted to lock the rear brakes like crazy, so they had to bias the brake balance way forward, reducing total braking power (and sometimes they still locked the rear brakes, I vaguely recall a 60 Minutes article about GM X-bodies (Chevy Citation, et al), locking up the rear and spinning on a wet test track). ABS solved that, and better suspension design. It also meant for those older cars, you stop facing uphill on a steep uphill in San Francisco and apply the parking brake (rear axle only) and let your foot off the brake, the car rear end jacks up to its limit, it's applying a forward ground thrust that is normally not present in vehicle operation with a front-wheel-drive car (higher thrust than just backing out of your driveway).

Last edited by Duragrouch; 01-28-24 at 08:05 AM.
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