Tribikenewbie

So on a related note (going for 5 time zombie!! It is almost Halloween), I’m trying to explain to “a friend” (who might be my son) the physics of why he should downshift the chainring (move to a smaller front gear) and back gear (move to a bigger cog) going up long, steep hills. The empirical evidence (practically coming to a stop and falling off the bike) is apparently not proof enough. I’m thinking the simple explanation is that he doesn’t have enough strength/can’t exert enough force to drive the largest front gear when the back gear is on a smaller cog - which would give apx 4 rotations of the back wheel for a single pedal/crank rotation. What are the physics to explain this? I saw a video where a body builder rode up a hill in high gear but his time was slower that an amateur biker (who used lower gears and more rotations) and he was dead tired vs the biker. I saw in this forum that Torque x rpm = power…. I assume more power is needed to go uphill than flat …but that’s as far as I can get…I can’t translate it into the gear explanation. Please help all you physics masters!!

HTupolev

Body builders often deliberately force themselves into catastrophic dehydration and other sorts of malnourishment, so it doesn't surprise me that one would be exhausted after trying to race a bicycle uphill, regardless of the gearing.

PeteHski

Okay so you were on the right lines starting with Power = Torque x rpm

The rider applies an Input Torque (Pedal Force x Crank Length) and an Input rpm (Pedal Cadence). This gives Input Power as defined in the above general equation.

The drivetrain gears convert this rider Input Power into an Output Power at the rear wheel = Wheel Torque x Wheel rpm.

Apart from minor drivetrain losses (which we can ignore for this purpose) Input Power = Output Power and the gear ratio simply determines how the Output Torque and rpm are split. A lower gear provides a higher Output Torque and lower Wheel speed for a given Input Power level. So the rider has to apply less Crank Tiorque (lower Pedal Force) to get the same Wheel Torque (same driving force at the contact patch), but at the expense of a lower Wheel Speed (since Power is a constant).

That’s the best I can simplify it with the actual physics equations included. It may be confusing. A more simple explanation is that a lower gear ratio multiplies your torque input and divides your pedal rpm. So you get more torque at the rear wheel, but at a lower speed.

wheelreason

Just tell him that shifting to the small ring and the largest cog is the best cheat to advance to the next level...

RChung

Just tell him if he doesn't, you'll beat him to the top of the hill and you'll tell all his friends about it.

tomato coupe

There are tests that can determine if he is your son.

unterhausen

I considered having one of those tests done when my son asked me to build him a 'bent.


You can climb in too high of a gear. What I have found is it's really inefficient, and if it's on a long ride you'll probably need to eat afterwards.
Not to mention the danger of cramping.

tomato coupe

If your son wanted a bent, wouldn't that make you over 100 years-old?

njkayaker

The gears are levers. A bigger gear in the back or a smaller gear in the front gives you a longer lever arm.

If you know why you'd use a longer lever (arm), you know why you'd use a smaller gear ratio.

wheelreason

Not lever, wheel and axle....

wheelreason

If he doesn't tell him he has to move out of the basement....

njkayaker

The basic idea is the same.

Both allow a lower power input over a longer distance.

https://www.explainthatstuff.com/toolsmachines.html
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Simple machines-gears-gear ratio and mechanical advantage.
https://energyeducation.ca/encyclope...20gear%20train.


Anyway, it's two wheels.

wheelreason

lol.

terrymorse 

In olden days, we often climbed in too high of a gear, as we didn't have a choice. We may not have been fast, but our legs were very fatigue resistant.

unterhausen

The advantage back then was there was nobody around with more appropriate gearing to show us up. When I started riding seriously again, it took a while for me to change my equipment over to anything more current. More importantly, it took some time to realize I should change my gearing. I don't think basic physiology has changed, it uses more energy to climb in too high of a gear, and there is a price to be paid for that.

tomato coupe

It was my screen name back when I was in the adult entertainment business.

PeteHski

Even the pros are still figuring this out on the steepest climbs. Watch Roglic vs Thomas on this year’s Giro mountain TT. Roglic rocked up with 1x gravel gearing and totally smashed it (despite dropping his chain over a bump!).

unterhausen

Apologies to the OP, but I de-zombified this thread. I came up with the best title I could for a new thread. If OP wants a different title just respond with it here in this thread.

rumrunn6

@Tribikenewbie

fwiw - with my kids, I kept it simple, real simple

I never look at the display on the shifter but my Daughter liked to. it helped her visually understand what numbers to use, when

meaning, fully understanding the mechanics & physics wasn't as important to her, as the basic settings

lots of ppl prefer not to shift gears & just muscle their way thru. everyone is different

how old is your son?

wheelreason

"Everything in moderation"...

Bald Paul

Years from now, when the orthopedic surgeon tells him he needs knee replacements, have the doctor explain why.

wheelreason

LOL. My smallest gear is a 36/25 and I'm hardly ever on it, I've never been able to spin up hills, doesn't feel right, ironically, I spin like crazy on the flats, just what works for me. Now I gotta go watch Hinault blow out his knees...

Polaris OBark

"My son doesn't know what he is torquing about."

Koyote

If your son is smart enough to understand the physics, he's smart enough to choose his own gear.

ofajen

There will certainly be a price to pay in terms of stress and strain. Strictly speaking, though, it will probably use slightly less energy to climb in too high of a gear, but it is likely to require excessive pedal force that will strain muscles and may demand a higher rate of work than the rider can sustain. Total energy use is lower because the lower cadence means slightly less overhead of energy used to move legs, feet and pedals.

Also, if you can sustain the pedaling, you may be slower in the higher gear, which will reduce air drag though it likely isn’t a big factor unless you are a fast climber. Also, some coaches recommend starting long hard climbs in a slightly higher gear that discourages you from going too fast and blowing up too soon.

Otto