Quantcast

Prototype Commencal DH V4

hmcleay

i-track suspension
Apr 28, 2008
116
103
Adelaide, Australia
This PB article says the chain is in line with the pivot.
I'm having trouble envisioning how this pulley placement produces anti-squat... Oh wait...

Sorry typed question too soon, am I correct in thinking that the pulley being above and behind the pivot when pedaling (under sag) would produce anti-squat?
I think the PB article means to say that the idler is concentric with the pivot. That's what it looks like to me.

Without having plugged this bike into Linkage, I'm going to guess/estimate some numbers here:
The idler looks to be around 16T.
So, if the bike is also in the 16T cog at the back, then there is zero chain growth contributing to Anti-Squat.
Without chain growth, Anti-Squat is wholly made up of what I call 'Frame Anti-Squat' (which is generated by the upward angle of the swingarm).
In this gear, the chainline is parallel to the swingarm line, and it looks like Anti-Squat will be around 50%.
Also, as the suspensions compresses, Anti-Squat will decrease pretty much linearly. So at sag, it's more likely to be somewhere around 30%AS.

Going to a larger cog at the back will increase anti-squat, and going to a smaller cog will decrease it.

This is the trouble with concentric idler/pivot designs, because there is zero (or minimal) chain growth, then the main pivot needs to be really high (and/or the idler pulley really small) to generate reasonable %AS figures (100%-150%).
 

Sandwich

Pig my fish!
Staff member
May 23, 2002
16,037
1,050
01776
This PB article says the chain is in line with the pivot.
I'm having trouble envisioning how this pulley placement produces anti-squat... Oh wait...

Sorry typed question too soon, am I correct in thinking that the pulley being above and behind the pivot when pedaling (under sag) would produce anti-squat?
View attachment 119485

To me, the pulley looks to be behind and below the pivot, slightly. Not in line with it, which would make for a very robust system, but looking back and forth with both sides of the bike, it seems to be set back a bit.

Hugh might be able to eyeball it, but in this case it looks like the pulley moves while it is below the pivot, so it might have decent AS numbers, but it's going to be variable.
 

no skid marks

Monkey
Jan 15, 2006
2,514
26
ACT Australia
I wonder if it'll thrash clutched mechs and/or cables. Maybe people will just run them clutchless. Not sure if the underside chain will have excessive growth or not. Depends how far clockwise you can rotate the guide I guess.
Looks pretty nice in their video.
 

slimshady

¡Mira, una ardilla!
Dec 20, 2007
2,648
739
La Plata, Argentina
To me, the pulley looks to be behind and below the pivot, slightly. Not in line with it, which would make for a very robust system, but looking back and forth with both sides of the bike, it seems to be set back a bit.

Hugh might be able to eyeball it, but in this case it looks like the pulley moves while it is below the pivot, so it might have decent AS numbers, but it's going to be variable.
If you look carefully you'll see the idler is in the same axis as the main pivot. The pointy protrusion in front of the rear triangle is what confuses you, as it makes it look as it was placed behind the main pivot.
 

hmcleay

i-track suspension
Apr 28, 2008
116
103
Adelaide, Australia
After closer inspection, I would have to agree with Sandwich.
The idler axle looks to be slightly below/behind the main pivot.
(on side note, this would make the design of the pivot and idler axle hardware quite interesting)

It that's correct, then it's a 'swingarm mounted idler' system (like the Corsair Maelstrom, but with a higher pivot).
Calculating Anti-Squat for these systems is much more complicated than usual, but luckily I have provided the algorithm in the Linkage software :brows:
It will have more anti-squat at sag than what I estimated before (assuming it was concentric pivot/idler), but AS will still decrease throughout travel.
Given that's it's fairly difficult to see exactly where the idler is positioned in relation to the main pivot, it would be hugely inaccurate to attempt a mock-up model in Linkage. So hopefully someone can measure one up soon!

I'm really looking forward to seeing how this bike goes this weekend!
Hooray for idlers!
 

no skid marks

Monkey
Jan 15, 2006
2,514
26
ACT Australia
After closer inspection, I would have to agree with Sandwich.
The idler axle looks to be slightly below/behind the main pivot.
(on side note, this would make the design of the pivot and idler axle hardware quite interesting)
Possibly slip a fancy nut or bolt into swingarm before installing ilder and cover. They could use the idler bolt hole maybe to do it up or hold it in place.
In the last photos posted on here does the top opening near idler look wider without the cover on?
 
Last edited:

hmcleay

i-track suspension
Apr 28, 2008
116
103
Adelaide, Australia
Possibly slip a fancy nut or bolt into swingarm before installing ilder and cover. They could use the idler bolt hole maybe to do it up or hold it in place.
In the last photos posted on here does the top opening near idler look wider without the cover on?
Or maybe the pivot axle is a Maxle type arrangement, so that it can be inserted and tightened from the LHS only.
 

no skid marks

Monkey
Jan 15, 2006
2,514
26
ACT Australia
Or maybe the pivot axle is a Maxle type arrangement, so that it can be inserted and tightened from the LHS only.
Possibly, hope not.
Non proprietary part would be nice.
They should be able to have a fancy deep enough nut(or bolt) that they angle in through idler bolt hole or top slot. Shouldn't be hard to make something work.
 
Last edited:

Vrock

Linkage Design Blog
Aug 13, 2005
263
35
Spain
IMHO The main pivot is in line with the seat tube and the idler is one inch behind and a bit lower. Pivot hardware is going to be a bit different than usual, but nothing too complicated. Accurate Linkage models are going to be almost impossible to do, unless Commencal is kind enough to share Blueprints or someone take measures in real life.
 

no skid marks

Monkey
Jan 15, 2006
2,514
26
ACT Australia
IMHO The main pivot is in line with the seat tube and the idler is one inch behind and a bit lower. Pivot hardware is going to be a bit different than usual, but nothing too complicated. Accurate Linkage models are going to be almost impossible to do, unless Commencal is kind enough to share Blueprints or someone take measures in real life.
Im guessing you can see the pivot from the other side of bike. Might be wrong.
 

fluider

Monkey
Jun 25, 2008
440
9
Bratislava, Slovakia
Interesting! Good old concepts are having great come back.
From the last pictures I'd say, it's a swingarm mounted idler that will probably always render the chainline right at the main pivot, or somewhere very close to it. Being a swingarm mounted can have various reasons, ease of construction and manufacturing being one. My guess is, the antisquat will be determined mostly by the pivot location and the two chain lines will have insignificant effect on AS. Since the pulley is with high intention located very close to the main pivot, I gues that the two chainlines will render at a point that lies on swingarm slightly in front and over the mainpivot. So, it seems to be high pivot with very low pedalkickback (that's the purpose of pulley location) with anti-squat starting somewhere at 100% and rapidly falling throughout the travel.
 

4130biker

PM me about Tantrum Cycles!
May 24, 2007
3,904
442
Lizard Town
Super clear the way you break things down. I think it was awesome! You should make a thread here and post more videos like this!
 

4130biker

PM me about Tantrum Cycles!
May 24, 2007
3,904
442
Lizard Town
Hi guys! Thanks very much for the comments! :) From anti-squats & suspension experts like you that means a lot for an non-engineer amateur biker :D.

I have more videos about suspension theory (for begginers). Yes I will make a new thread :)

Bye,
Sorry my english, I'm from Portugal :)

EDIT: I already did the new thread, here:
http://ridemonkey.bikemag.com/threads/videos-mtb-rear-suspension-series.275496/
Very cool!
I think you are referring to Hmcleay, but just to be clear, I am a total amateur when it comes to this stuff, but find it very interesting. :)
 
Last edited: