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Full suspension FWD MBB - some advice needed.

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Yanto View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Yanto Quote  Post ReplyReply Direct Link To This Post Posted: 22 August 2018 at 6:23am
Long time since I calculated force vectors and wouldn't know where to start now, so I will say I have no idea, but if it works Clap
Ian, racing again No 6.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 22 August 2018 at 10:52pm
Hi Oleg, bike looks great - love the baroque rear mudguard!

Are you familiar with 'free body diagrams'? The free body can be anything you like but you only consider the forces acting on it, so no internal forces.

I would start with a FBD of the crank/chainset assembly. The forces on it are the pedalling force (say 300N), the chain tension and the reaction at the bottom bracket. The chain tension will be the pedal force multiplied by a ratio of the crank length to the chainwheel radius. From this you can find the magnitude and direction of the force at the bottom bracket.

Now an FBD of the wheel. You have the chain tension, the vertical force at the ground, the horizontal force at the ground and the reaction at the wheel axle. You can find the horizontal force (traction force) by taking moments about the wheel centre. Using this force on a FBD of the whole bike will give you the weight transfer. I would use just the weight transfer figure (acting upwards on the wheel) because the static weight has already been absorbed by the spring). So now you can work out the reaction at the wheel axle. You can also calculate the force on the spring and the reaction at the swing arm pivot.

Now you can do a FBD of the swing arm as you have the forces at the bottom bracket and wheel axle - remember to reverse the forces in the last two FBDs (Newton's Third Law). If you take moments about the swing arm pivot you will find which way and how strongly it will want to rotate.

Hope this helps?
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Balor View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Balor Quote  Post ReplyReply Direct Link To This Post Posted: 23 August 2018 at 7:54am
I think you think too highly of me, I'm more of a tinkerer than an engineer, and was never good at math and physics beyond intuitive grasp of an 'order of magnitude' of forces involved. :(.
 
More than that, my current problem is more 'conceptual':
Does force AT THE PEDAL contribute to extending or compressing the suspension? This must be a fairly simple question for someone mechanically inclined (which I am really not).
 
Let's disregard tire and weight transfer force components for now.
As you see, this is URT suspension, with BB located on the swingarm and if you push at BB the force should extend the suspension, but again - I'm not pushing at BB, I'm pushing at the pedals, and when in 'power phase' they are ABOVE the pivot and resulting lever arm should contribute to compressing the suspension, right?
Hence, counteracting weight transfer and tire forces that both extend the suspension and should, indeed, make the bike bob by extending suspension pretty severely unless there is a lot of sticktion - but the suspension is quite compliant YET there is almost no bob unless climbing and even than pretty little!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 23 August 2018 at 10:57am
Try sitting on the bike, holding the front brake on and pushing against the pedal. Does the suspension extend or compress?
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Balor View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Balor Quote  Post ReplyReply Direct Link To This Post Posted: 24 August 2018 at 9:26am
Good idea! I'll try that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Balor Quote  Post ReplyReply Direct Link To This Post Posted: 29 August 2018 at 9:00am
Forgot to write up, indeed it compresses under pedal pressure. Unintuitive, but makes sense anyway.
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