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A couple of things I've been thinking about which may or may not be relevant. Could someone check my maths to see if I'm somewhere near right.
It was claimed earlier that a cyclist and a motorcycle engine produce about the same torque.
Using my example of a 200kg load on an 85mm radius chainring, I make that 17Nm of torque for the cyclist.
50Nm would be typical for a motorbike, so that's three times the cyclist.
A cyclist only produces maximum torque when the cranks are horizontal, and near enough zero torque when they are vertical, so the average will be about half the maximum..
A multi cylinder motorcycle engine with a flywheel will produce fairly constant torque.
So now the motorbike is producing six times the torque of the cyclist.
A motorcycle has a primary drive as well as a gearbox so that the final drive (chain and sprockets) rotates at between 1/2 and 1/10 of the engine speed, or has between 2 and 10 times the torque.
So, all together the motorcycle is putting out between 12 and 60 times the torque of the cyclist at the gearbox output sprocket/chainring.
So, again, we come to the bicycle suspension being designed for 1/3 of the weight of the motorcycle while dealing with 1/12 to 1/60 of the tension on the chain.
I still can't see how chain tension has a significant effect on bicycle suspension.
The other thing is that no one has mentioned torque reaction at the rear wheel yet.
As the torque at the rear sprocket tries to rotate the wheel forward, it is also trying to rotate the swinging arm backwards. This is why bikes wheely under power.
It's hard enough getting a bicycle to wheely solely under pedal power without deliberate weight transfer and pulling up on the bars.
It would be impossible for a cyclist to wheely something as heavy as a motorbike using pedal power, but it's easy for a motorbike engine.
You never saw me trying to wheelie my Gixxer6.. easy my arse!
Graham, you've got your maths/mechanics a bit iffy.
i'm not sure where you get 200kg from but i weigh 75kg, so let's round it up to 100kg.
100kg = 981n = 1000N (give or take)
i use 175mm cranks = 0.175m
torque = 1000 x 0.175 = 175Nm
(the diameter/radius comes in if you want to calculate chain tension)
(tension = torque / chainring radius (= 0.065 metres for a 32t))
chain tension = 2700N
i've never done the maths before, that's loads! - that's roughly the 'weight' of 3 people.
so you design a suspension system to cope with the weight of 1 person, and then pull it up/down with a chain pulling with the equivalent of 3 people.
when i've looked at motorbikes i've seen that some clever engineer has done a lovely job of getting the tiny output sprocket almost concentric with the swingarm pivot.
because a concentric pivot won't cause any chain 'growth'/ swingarm compression/extension under chain tension.
i'm an engineer, just trust me...
(yes, i can do the maths)
I should have looked that up first shouldn't I ?
I thought 1N = 1Kg
Its 10N = about 1Kg
So my calculations are out by a factor of 10. 😳
easily done.
power is torque x revs i think,,
so if two cyclists give the same power the one doing the least rpm must be producing the highest torque and chain tension ,,
can one of our tame boffins please help
power is torque x angular velocity
torque in Newton metres.
angular velocity in radians per second.
(derived from first principles: power = energy / second, and energy = force x distance)
I think that's a yes 🙂
I've worked out all that chain angle whatsit, just need to find a bit of time to explain it all. I'm sure you're all on the edge of your seats. 😯
Re. power and torque, what ahwiles said. Convert from rads/sec to rpm by multiplying by 60 to get rads/minute then divide by 2 pi to get revs/minute (there are 2 pi radians in one revolution).
so if two cyclists give the same power the one doing the least rpm must be producing the highest torque and chain tension ,,
chain tension depends on gear choice Shirley?
Only with multiple chainrings.
1x9 or hub gear, it depends entirely on pedalling effort.
correct.
MidlandTrailquestsGraham - Member
Only with multiple chainrings.
1x9 or hub gear, it depends entirely on pedalling effort.
PEDANTRY ALERT!
How many full suss bikes are being ridden with single chainrings or hub gears?
Oh and you forgot to tick me off re crank length, nor the reaction from the suspension!
Every full sus DH bike ever runs a single chainring?
Ok, here we go...
I realised the other day that something was wrong with my analysis as I started to wonder whether the concentric pivot example was a special case. Consider a horizontal swingarm but with the BB directly below the pivot and a front chain ring that puts the chain line right at the middle of the pivot. If my statement above about chain line running through the pivot were true (chain force will not extend or compress the suspension), this scenario would produce no anti-squat effect as I had also stated that a horizontal swingarm has no anti-squat capability. It is clear however from drawing a standard anti-squat diagram that this setup will have a certain degree of anti-squat capability.
To explain what actually happens, you have to go a bit further into the force vectors and whatnot. When you are pedalling, there is obviously a force acting along the line of the swingarm in a forward direction, into the pivot. At the same time, there is an opposing force in the chain pulling backwards. Each of these forces can be separated into their vertical and horizontal components. The horizontal component of the chain line force must obviously be less than the horizontal component of the swingarm force, otherwise the bike would not move forward. The vertical components are what are important for calculating and understanding the anti/pro-squat.
Consider the scenario where the swingarm and chain line are parallel and both angled down towards the rear wheel – not an unlikely setup. The chain line force will have a vertical component acting down pulling the main frame (sprung part of the bike) down and so producing a pro-squat effect (compressing the suspension). The vertical component of the swingarm force however will be acting upwards pulling the main frame up and so producing an anti-squat effect (extending the suspension). I have already shown that the horizontal component of the swingarm force must be greater than that of the chain line force so it follows that the vertical component of the swingarm force must also be greater. It doesn’t take a genius to therefore work out that there will be an overall anti-squat effect as the vertical swingarm force component is dominant, outweighing the pro-squat effect of the chain line force.
In the first example I mentioned above in this post, the swingarm force is horizontal so has no vertical component. It therefore has no anti/pro-squat capability, as I mentioned in my original biblically long post. The chainline force however has a vertical component acting upwards, pulling the main frame up and producing an anti-squat effect.
So in quick conclusion it IS possible to have a chainline that lies above the main pivot but below the point where it would be parallel to the swingarm line and have an anti-squat effect (i.e. extension of the suspension rather than compression). Having the chainline coincident with the main pivot does not mean the chain force won’t act to compress or extend the suspension, it just means that the chain extension (and so pedal kick-back) and variation of antisquat throughout the range of travel can be minimised.
That is basically a simplified version of what Tony Foale states in his book so I'm counting on him being right!
Diagrams would help explain this and I could probably do some if anyone is interested 😯