The thread about “bike for a big chap” got me thinking…

A 20 stone guy sat on a bike exerts 20 stone of ‘weight’ on it.
A 20 stone guy stopping from, say, 20mph relies on his brakes “x” amount to stop.

I weigh 11 stone, but for simplicity sake say I weigh 10 stone.

Doing a drop-off from ‘what height’ would exert 20 stone of ‘weight’ on my bike? Assuming I don’t use transitions/down slopes etc.
Also, from what speed would I be exerting the same amount of ‘braking requirement’ on my brakes?

I have no clue about the first one, and surely the second one isn’t just 40mph?

I’m sure I could have answered this when at college, but I’ve gotted dumberer since then….

DrP

“Doing a drop-off from ‘what height’ would exert 20 stone of ‘weight’ on my bike?”

Genius!

The speed one is 100 times faster.

So i would say the height one is the same but then I’m a bit thick so no doubt a 5 year old will be along in a minute to prove me wrong 😳

I concur.

don simon – Member
I concur.

Not with me surely 😯

“The speed one is 100 times faster.”

Oh, I want to see your workings on that one.

This is going to be bloody hilarious.

(or is it, as I hope, a joke?)

My workings involve 1/2 a bottle of Whiskey which is why i covered myself with the 5 year old comment.

So what is it then Bez ?

. “The speed one is 100 times faster

Absolutely,
100 times higher too

Good, good.

What is it? Depends on what the question actually is. It’s so vague there’s no way of answering it at the moment.

There is enough info to solve both questions

So WTF is “braking requirement” in precise mechanics terms then?

There’s enough info in the parameters, sure. There’s just not enough clarity in the question.

Is the question “From what speed would a 10 stone person have the same stopping distance as a 20 stone person, given an equal retardation force in both cases and ignoring all other variables?”

I assume you can work on the amount of energy required to bring the bike and rider to a stop?.

Well, you know, a 20 stone chap coming to a stop from 20mph (you choose the distance) requires a certain amount of ‘braking force’ doesn’t it…(or put another way, has a certain amount of kinetic energy that needs to be ‘gotten rid of!)
What speed would a 10 stone chap require to be travelling/stopping from (same distance) to require the same braking force?

DrP

That’s it Bez – of course part of the skill involved in answering the question is deciphering the question (like most exams!)

DrP

Can I just check what it is you’re a Doctor of, DrP?

1/2 mv^2

Ok, I’m tidying the kitchen and using a phone here, but

Work is force times distance, both of which are the same in both cases, so you just find the speed where kinetic energy is equal, and that’s half the mass times speed squared, which leaves you some simple working to do, I’ve got no envelopes to hand.

Not a doctor of physics! People and pains are more my thing!

DrP

Yes 1/2 mv^2 = 1/2 mv^2
You know the masses, so solve for v/v

28mph

So the braking one is basically 28 and a bit mph?

The drop off one would involve gravity and acceleration, no?

DrP

I didn’t peek sir, Honest!

DrP

I would be shocked if at least o level physics wasnt compulsory for medicine 😯

Prepare to be shocked…..

Also, this far into my career I’ve not needed this particular knowledge train, and have thus forgotten it and replaced it with more relevant information.

DrP

o-levels are beyond degree level these days, so I doubt it.

Must be why im shit at my job then, I seem to retain this superfluous information 😉

as regards landing forces, try putting the weights into the mojo shock spring calculator, the realtive spring weights should give you an idea of the relative amounts of force the diferent riders will apply to the bike.

In freefall (ie off a drop) the 10st and 20st rider will fall at about the same rate, the heavier rider may leave the takeoff faster (inclined plane acceleration) and travel further (momentum) in the real world this may lead to a bigger drop. The big rider will be more likely to damage the bike, but given that they’re used to weighing that much, and the shock spring should be right for their weight, the landing wouldn’t feel appreciably harder.

As for the braking, see also the 180mm rotor thread. For twice the weight, in the same distance you need twice as many frictions. The rotors won’t be twice as hot. The brakes won’t work while airborn.

I’m sticking with my original answer 🙄

What if the bike is on a moving conveyor belt?

too easy!
give a 9stone lad/lass a piggyback on your bike then let her down before dropping off gradually higher kerbs/walls until your legs (not pants) replicate the feeling.

give a 9stone lad/lass a piggyback on your bike .

Just getting a “backer” on a full suss bike does funny things to your mates tyres and shock…

I weigh 11 stone, but for simplicity vanity’s sake say I weigh 10 stone

FTFY