Rear suspension forces analysis

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UpWrong
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Rear suspension forces analysis

Postby UpWrong » 4 Jan 2020, 11:06am

I posted this on a rather old Bentrideronline thread, but would be interested in any comments here. Assuming single pivot design and a single idler, I think these are the 3 main forces at work:
  1. chain pulling the rear hub towards the idler
  2. reaction force of tyre on the road
  3. chain deflection force on the idler

With HP Velotechnic's "no-squat" rear suspension design these forces produce the following reactions:
  1. chain line constrained to always pass above the pivot point, lifting the rear hub to bring it nearer the idler
  2. tyre reaction force lifting the rear hub
  3. chain deflection force lifting the rear of the trike, assuming the idler is behind the centre of mass of trike and rider

This is why I find it hard to believe that HPV rear suspension is particularly efficient. Of course the forces are changing all the time with pedaling and suspension response.

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[XAP]Bob
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Re: Rear suspension forces analysis

Postby [XAP]Bob » 4 Jan 2020, 11:48am

There will be negligible tension between the hub and the idler (unless your idler is on the power side?)

The primary chain tension is forward from the sprockets, which is designed to create a torque in the rear wheel.
It will also pull the whole wheel forward, but that is resisted by the compression strength of the chain stays.

If the power side pulls through (or close to) the suspension pivot) then there will be no (little) effect on the suspension from that tension.

The chain tension cannot lift the trike, since all forces on the chain are balanced within the vehicle. It could provide a bending moment, particularly around a pivot, but that is why the power side chainline (which is the only tensioned chain segment) generally passes alongside the suspension pivot.


I haven't seen the HPV design, but the above is generally true.
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UpWrong
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Re: Rear suspension forces analysis

Postby UpWrong » 4 Jan 2020, 12:14pm

[XAP]Bob wrote:There will be negligible tension between the hub and the idler (unless your idler is on the power side?)

The primary chain tension is forward from the sprockets, which is designed to create a torque in the rear wheel.
It will also pull the whole wheel forward, but that is resisted by the compression strength of the chain stays.

If the power side pulls through (or close to) the suspension pivot) then there will be no (little) effect on the suspension from that tension.

The chain tension cannot lift the trike, since all forces on the chain are balanced within the vehicle. It could provide a bending moment, particularly around a pivot, but that is why the power side chainline (which is the only tensioned chain segment) generally passes alongside the suspension pivot.


I haven't seen the HPV design, but the above is generally true.

Yes, idler on the power side.
The chain torque on the sprockets is pushing the rear wheel into the ground, and the ground pushes back up.
The distance of the power side chain line from the swing arm pivot changes as the suspension operates. With the HPV design it is constrained to always pass above the pivot.
The chain forces are trying to pull the rear hub and bottom bracket down in line with the idler, and the idler resists this with an upward force on the trike.

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[XAP]Bob
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Re: Rear suspension forces analysis

Postby [XAP]Bob » 4 Jan 2020, 12:29pm

Can you grab a pic?

Chain tension will never force the wheel down, because there will be an opposite reaction force at the axle (yeah, I'm simplifying the dynamics slightly, but you can't increase the vehicle weight by applying internal tension)
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UpWrong
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Re: Rear suspension forces analysis

Postby UpWrong » 4 Jan 2020, 12:33pm

scorpion_nosquat.jpg

Brucey
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Re: Rear suspension forces analysis

Postby Brucey » 4 Jan 2020, 12:37pm

if the upper chain run does not pass through a (single) suspension pivot then the suspension will either squat or rise when you pedal. The chain forces most certainly can cause the whole machine to rise or fall because you will have an additional force which compresses or extends the rear suspension whenever you pedal hard.
HPVs are arguably simpler than upright bicycles because there are not usually such gross rider weight movements when pedalling which promote 'bobbing' directly. Even so most tadpole designs use front suspension which is stiffer and has less travel than you might expect, and this may be part of the reason why.

Good HPV designs often have a stub axles on the swingarm and a large opening in the swingarm structure so that the upper chain run can pass coincident with the swingarm pivot axis. Because the idler is normally set a long way further forwards (so the chain clears the rider), relatively small changes in suspension ride height can affect the tendency to squat or lift under power. However when you change sprockets at the back, there is still a change in the chain height unless there is a second idler near the swingarm axis. This second idler (if present) usually needs to be able to float from side to side if a derailleur is used.

Good bicycle designs often have a more complicated linkage to the rear wheel and this generates a virtual pivot point and/or an axle path which isn't likely to be interfered with by changes in the gear used. There is always some effect rather than none though.

Another point to note is that chainring size dictates chain tension and therefore the severity of any squatting/bobbing if the chain height isn't perfect w.r.t. the pivot. Thus on many machines they work fine on the big ring (low chain tension) but might bob or squat noticably when using smaller chainrings, even if the chain is passing through the swingarm and onto the rear sprockets in the same way.

In the photo you can see the upper chain run passes above the swingarm pivot. This means that it will certainly squat (there is a resultant force which compresses the rear suspension), just not as badly as one with a more badly positioned chain.

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[XAP]Bob
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Re: Rear suspension forces analysis

Postby [XAP]Bob » 4 Jan 2020, 12:51pm

The chain in this case will indeed cause the suspension to compress/relax as your tension varies during a pedal stroke - they have minimised this in high gears, but will will be more evident in lower gears.

The 'rise and fall' is a function of bending the vehicle rather than a function of chain forces moving the vehicle up and down overall - maybe my verbiage has been unclear, but that was what I was trying to say.

The chain on my sprint runs pretty close the pivot - closer, IIRC, than that one does - but an offset rear triangle allows a bit more lateral leeway for the chain.
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There are two kinds of people in this world: those can extrapolate from incomplete data.

UpWrong
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Re: Rear suspension forces analysis

Postby UpWrong » 4 Jan 2020, 1:06pm

Thanks Brucey. That's a concise description of suspension squat and rise with a single pivot point (force number 1 in my list).

Sounds like tyre to ground reaction force will have no effect of suspension compression or extension in itself.

I'm still trying to work out if the deflection force on the power side idler has an additional effect. With two wheel bent designs the idler is usually positioned under the rider to stop the frame from bowing. If it's not under the centre of mass then one end of the vehicle will have a tendency to be lifted more than the other.

nigelnightmare
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Re: Rear suspension forces analysis

Postby nigelnightmare » 4 Jan 2020, 2:48pm

[XAP]Bob wrote:The chain in this case will indeed cause the suspension to compress/relax as your tension varies during a pedal stroke - they have minimised this in high gears, but will will be more evident in lower gears.

The 'rise and fall' is a function of bending the vehicle rather than a function of chain forces moving the vehicle up and down overall - maybe my verbiage has been unclear, but that was what I was trying to say.

The chain on my sprint runs pretty close the pivot - closer, IIRC, than that one does - but an offset rear triangle allows a bit more lateral leeway for the chain.


The chain line is supposed to go through the center of the pivot according to ICE's setup instructions, with the chain under moderate tension and the rider sitting on the trike.

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[XAP]Bob
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Re: Rear suspension forces analysis

Postby [XAP]Bob » 4 Jan 2020, 3:57pm

Yes - the lateral offset allows it to run alongside the pivot in medium gears - so with smaller sprockets it slightly pulls the suspension "out" and with larger ones it slightly pulls "in" (larger/smaller cf+ your reference sprocket
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.

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[XAP]Bob
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Re: Rear suspension forces analysis

Postby [XAP]Bob » 4 Jan 2020, 11:22pm

UpWrong wrote:Thanks Brucey. That's a concise description of suspension squat and rise with a single pivot point (force number 1 in my list).

Sounds like tyre to ground reaction force will have no effect of suspension compression or extension in itself.

I'm still trying to work out if the deflection force on the power side idler has an additional effect. With two wheel bent designs the idler is usually positioned under the rider to stop the frame from bowing. If it's not under the centre of mass then one end of the vehicle will have a tendency to be lifted more than the other.

Missed this post earlier.

The tension has to act on both sides to cause any compression in the suspension , you can’t separate the two ends of the chain run.
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.