[Closed] Has VPP had it's day?

The acceleration of the bike and rider is proportional to chain tension...

I just read http://www.dw-link.com/reasons.html#
There is so much nonsense on that page I don't know where to begin.
Do people really spend £1000+ on a frame based on this pseudo-science ?

hmmmm...you're right [edit]...hang on...

what I was referring to was the variation of chain tension in one rotation of the pedals, not overall working harder

... so your acceleration varies like a sine wave, and so does the resulting bobbing of your suspension, caused by this variation in acceleration.

hmmmm...you're right.

it's ok, I editted... 🙂

I just feel that the vertical force of the rider lunging on the pedals with each stroke is going to have far more effect on the suspension than the variations in the horizontal force of acceleration.

yes i think that's the elephant in the room here and probably what cynic-al was referring to in his bit about smoothing out pedalling technique. you can't do much about that in suspension design I don't think.

Is it not the case that pivot placement is the most important thing rather than the system used?

i.e. pick any system, vpp/horst/sp, and design it well and it will work but put the pivots in the wrong place and it will be terrible.

have you ever noticed that when you get to a certain cadence you can make a hardtail bob,, i dont think it's speed related but cadence related

but you can use chain tension to inhibit this

on a single pivot bike it's basicly the height of the pivot if the pivot is above the point where the chain meets the front ring then the rear suspension will tend to extend

if it is below the point where the chain meets the front ring then the suspension will tend to compress

now a vpp bike can be summed up as a single pivot bike with a pivot position that moves around so the virtual pivot position moves depending on the swinging arm position and the postion of the linkages supporting it

"on a single pivot bike it 's basicly the height of
the pivot if the pivot is above the point where
the chain meets the front ring then the rear
suspension will tend to extend
if it is below the point where the chain meets
the front ring then the suspension will tend to
compress"

sorry mate this is just incorrect!
family visiting shortly but I'll try to explain if I get a chance...bet you can't wait!

TT...happy to agree!

sorry mate this is just incorrect!
family visiting shortly but I'll try to explain if I get a chance...bet you can't wait!

you right i cant wait to see your explaination

oh jesus...this could get messy 😯

if the pivot is above the point where the chain meets the front ring then the rear suspension will tend to extend

if it is below the point where the chain meets the front ring then the suspension will tend to compress

I get this and sort of agree with it.
If you imagine the points where the chain meets the chainring and sprocket and the pivot point as the three corners of a triangle, then the tension on the chain in isolation will tend to raise or lower the suspension as described. Shortening one side of a triangle affects the angles.
I still think this force is negligible in relation to the rider's weight on the pedals though, so it makes no difference..

chain tension is proprtional to load on the pedals and depends on which gear you are in ,,

It only depends on which gear you are in with derailleurs, which is why I mentioned earlier that I used a Rohloff on a VPP Blur.
Hub gears are an advantage in that situation.

It only depends on which gear you are in with derailleurs

this has got me thinking,, i have an alfine on my whyte preston fs bike,,

as the front and rear rings are constant with hub gears and hammersmit then the amount of feedback are the same in respect to the position of the chainpull and pivot position
but the amount of feedback is also a result of chain tension and wouldnt this depend on which gear you are in

I was going to say that chain tension is entirely dependant on pedal pressure, but a Hammerschmit would alter that.

As quoted earlier:

http://www.dw-link.com/physics.html
." When a bicycle accelerates forward, the rider's mass is transferred rearward. Without something to counteract this mass transfer, the rear suspension on most bicycles will compress under acceleration. This mass transfer as a reaction to acceleration is what riders have come to know as "bob."

It's badly worded IMO because mass doesn't really shift rearward if until the suspension moves (usually and then only by a small amount) but really he's talking about how momentum is important to consider, not just simple chain/braking forces.

The rider and everything bar the rear suspension have the majority of the mass and therefore momentum. What DW is talking about is that if you drive the rear wheel even on a design that has no chain forces having ANY effect on the suspension (eg main pivot concentric about the bb) you'll still get bob on most designs because the rear axle will drive the frame forward and this will activate the suspension one way or another. Imagine fixing a frame solidly fixed at the main frame so it can't move. Remove the shock and then push the dropout horizontally - almost any design will either compress or extend the suspension and even if it was a single pivot in line with the rear axle or four bar it'd still only be completely unaffected in one position through it's travel.

To eliminate the effect of chain tension,the pivot would need to be at the point where the chain meets the chainring, not at the centre of the BB.
At least with a hub gear the difference between the two is constant.
Any linkage that is designed to take chain tension in to account can't work properly on all three rings of a derailleur.

I see your point about the rear axle driving the mass forward, but under load the majority of the mass will be concentrated on the forward pedal travelling downwards in an arc, first forward and then back. I still think this is by far the greatest force on the suspension and will not be affected by the driving force its self.

Mtqg nope bb concentric will do it and work the same in all rings which is why I used that example rather than in line with pivot examples.

I still think this is by far the greatest force on the suspension and will not be affected by the driving force its self.

But that force is also driving the rear wheel isn't it... You're suggesting that pedalling forces are much bigger than drive forces which can only bs true if the efficiency is very low. Bike drivechains aren't very inefficient so it can't be true...

pivot concentric to bb gives zero chain growth so should have no effect on suspension

pivot concentric to bb gives zero chain growth so should have no effect on suspension

I'm sure you have done the trials, but for a chainring & cassette setup with a swingarm pivot at the bb and the chain big ring/small sprocket for example, dropping the rear axle will tension the chain, they form a triangle. Or am I getting terms confused?

I see your point about the rear axle driving the mass forward, but under load the majority of the mass will be concentrated on the forward pedal travelling downwards in an arc, first forward and then back. I still think this is by far the greatest force on the suspension and will not be affected by the driving force its self.

The majority of the mass is normally on the saddle. However, if you stand on the pedals what you say is true. Needs to be borne in mind though that it's change in force that causes bobbing, and the suspension is already supporting your body weight, so simply standing on the pedals does not immediately apply more force to the suspension. It might cause that force to vary more as you lunge about, but that's a different and problem. You can only push down on the pedal buy your own body weight plus how hard you can pull up on the bars and up on the other pedal after all.

A BB concentric pivot means the suspension has no effect on chain growth.
Chain [i]tension[/i] will still have an effect on suspension though.
The line of force through the chain is above the pivot, so it will pull the swinging arm in that direction.

You're suggesting that pedalling forces are much bigger than drive forces which can only bs true if the efficiency is very low

Gears multiply force [i]or[/i] speed.
In any gear higher than 1:1 the downward force on the pedals will be greater than the forward force on the frame.

I'm sure you have done the trails, but for a chainring & cassette setup with a swingarm pivot at the bb and the chain big ring/small sprocket for example, dropping the rear axle will tension the chain, they form a triangle.

No it won't! How could it? (ok, other than a tiny change due to the mech jockey not being concentric too)

MidlandTrailquestsGraham - Member
A BB concentric pivot means the suspension has no effect on chain growth.
Chain tension will still have an effect on suspension though.
The line of force through the chain is above the pivot, so it will pull the swinging arm in that direction.

Nope. You need chain growth for chain tension to matter...

They form a triangle mate. The chain goes from the top of the chainring back to a point very near the rear axle, the chainstay goes from the alxle to the bb and then add a line from the bb to the top of the chainring. you get a triangle. drop the axle and you tension the chain. no?

You know what concentric means, right. I don't mean a pivot near the bb, I mean around the bb. Again, concentric means no chain growth so chain tension is irrelevant.

trailer - no.

the wheel is free to rotate independently of the swing arm oh forget it i give up.

The majority of the mass is normally on the saddle

I think there's some confusion over the use of the phrase "chain tension".
A concentric BB pivot won't affect tension as regards slack in the chain.
I'm referring to the tension, or force, in the top run of the chain.

You know what concentric means, right. I don't mean a pivot near the bb, I mean around the bb. Again, concentric means no chain growth so chain tension is irrelevant.

I do, and I don't agree I'm afraid. No being awkward. If you draw my diagram you will see what I mean. if you put in a fixed length of chain and fixed the chainring in position you could pick the bike up and the suspension would not extend. so that pivot position will cause pedal feedback because of chain growth as the suspension extends. that's how I understand it. rolling along is more complex than this obviously but...

I guess I don't understand what you mean by chain growth in that case...

No awkwardness inferred though you are wrong 🙂

If you had that swingarm free to move with no shock/etc to stop it extending too far and you picked the bike up or got air, the rear wheel would end up verically under the bb. That would still be true if he pedals were being turned the whole time. To put it another way, what would stop the rear wheel dropping under gravity? There is no force to resist it other than friction in the pivot maybe.

...and the chain would have had to grow by approximately one quarter of the outside circumference of the chainring?

Chain growth = bb to rear axle measurement increases through suspension travel.

...and the chain would have had to grow by approximately one quarter of the outside diameter of the chainring?

No, think about it 🙂

As the swingarm falls the chain also unwraps from the underside of the chainring...

got you. you are quite correct. thanks for bearing with me 🙂 bit of a misleading term though as it doesn't actually involve the chain 😀

As the swingarm falls the chain also unwraps from the underside of the chainring...

no clubber, the definition of chain growth is a feature of a suspension system in which the distance from the bb axle to the rear axle varies with compression/extension - it doesn't involve the chain at all. my error.

and actually in all this consideration you can ignore the loose bit of the chain, it has no relevance at all. the only bit that matters is the taut bit from the chainring to the back wheel. I'm getting there.

I'm not quite clear if you agree or not, now 🙂

😀 I agree that there is no chain growth with a concentric bb but i think that if you tension the chain (the bit between the chainring and the rear wheel) on concentric bb pivot bike, that tension will cause the suspension to compress. its the same effect as for a vpp bike with the pivots below the bb, both times the pivots are below the line of force being applied to the rear axle, so the axle is pulled up.

we actually cam into this via a misunderstanding re:chain growth. I don't think there is any disagreement here 😀

Sorry, it's just not. I'm trying to think of a good way to explain without just repeating myself. Anyone else?

I'm not sure either,but I think I'm with trailertrash.

Ignore the chainring for now.
If you've got a concentric pivot and you imagine a chain wrapped round a fixed sprocket with the brake on.
With the chain held vertical, it takes very little effort to lift the swingarm.
Bring the chain down to almost horizontal, but still above the pivot, and it takes a far greater effort.
Far more effort than is actually available to have a significant effect on the suspension when you take the riders weight in to account.

Besides I guess it's OT as I was only using it as an example of why chain tension/growth isn't the only thing to consider which is what DW is all about - in his marketing at least 🙂

My attempt at explaining it.

[img] [/img]

The jib is the singing arm.
The rope is the chain.
Pull on the rope,the jib rises.
Put tension on the chain, the suspension compresses.

Will you chaps please slow dooooowwwwn, I'm attempting to catch up but I read slow you post fast!