I hear people saying that chainring size is important with rear suspension, in particular single-pivot suspension. I used to have a standard triple, changed that to a 22/36 double, and am now planning on moving to a 1×11 setup in the new year at which point I was told by my LBS that I should sick to a 32 or 34 tooth chainring for suspension reasons. Can anyone explain what these reasons are?

The closer the chainring diameter is in line with the pivot point, the less that pedalling effects the suspension, thereby reducing pedal bob.. or suttin…

….

I think.

Did you notice massive issues with suspension in your 22 or 36?

If not, I wouldn’t worry.

The chain is under tension when you pedal. The tension comes from the top of the chainring, down the chain to the cassette, which will try and move the rear wheel.
The size of the chainring affects the position of this force.

If the force is above the pivot your suspension may tend to compress (big chainring).

If the force is below the pivot the suspension may tend to extend (small chainring).

If the force is directly through the pivot the force effect is neutral.

Playing with chaingring can give different traits.

Bigger ring pulls the chain from further up, usualy causing the suspension to squat slightly (or not extend as much) depending on the design. Small rings have much higher loads, but are pulling the suspension out, ballanceing against its tendancy to squat when you mash the pedals.

Most designs manipulate the axle path so that the chain tension at the correct sag is pulling the suspension just enough to stop it squatting. On a single pivot this is usualy just above the chainring. On a virtual pivot/4bar design the same happens it’s just not obvious, the ‘virtual pivot’ is still in line with the chainring, its just often wayyy out in front.

Hence most frames will be designed to work best with a certain chainring.

From the other end, too small a chainring will give too much feedback from the trail as the suspension compresses the chainlength grows, this is amplified the bigger the gap between chainring and pivot.

5 pages, loads of arm chair engineers, a post to a chainless downhill run video and a reference to chris porter,

So one reason I ask is that I’ve noticed that if I run my shock at 25% sag I can easily bottom out over rough ground, and I’m now wondering if that’s due to my 36t chainring. Interesting. I don’t suppose anyone knows what the ideal chainring on a 26er Santa Cruz Superlight is? 🙂

How do you think it works with a 3×9?
Its all about compromise. You may notice a small change, but really it wont make a massive difference.
I would have thought the LBS recommending 32 / 34t rings is more to do with retaining a decent set of usable gear ratios than affecting the suspension significantly. Most bike’s suspension will have been designed with the suspension optimised when riding in the middle ring, because thats where you spend most of your time.

I guess I was expecting the compromise being poor suspension performance in the big ring as that’d generally be used on non-technical surfaces.

One day in the future some will flog a bike with 3 chain rings

The marketing blurb will that it uses three chainrings to optimise the suspension performance uphill, on the flat and downhill…….

TBH it all depends on where you stand. One person’s horrible pedal feedback is another’s anti-squat

I don’t suppose anyone knows what the ideal chainring on a 26er Santa Cruz Superlight is?

Have a look on linkagedesign, might be listed there.

Probably a 32 though.

Before coming into this thread I knew it was gonna be some bs thread. Guess what…!

Like honourabke george said in a sentence.

I can easily tell the difference between 34 and 32t up front on my bike but I notice my tyre pressures being 10% out too!

I know the cool kids will make out that chainring size makes no difference but it does. Whether you notice it or not doesn’t change that. The shift to 1×11 on so many bikes has left a lot reliant on shock tunes to not be a bobby mess uphill, when running a granny ring would let them pedal efficiently whilst running a shock tune that isn’t over-damped.

If you want to be able to stomp uphill without much bobbing you’ll need a 30 or 32 on the Superlight. If you tend to climb in the lowest sprockets a 34 or 36 will be fine because the chain angle to the big sprockets gives more anti-squat than when in the smaller sprockets. It’s more complicated than just chainring upper edge vs pivot location, it’s the force vectors between contact patch, rider centre of mass, chain tension and pivot position.

This all comes from the explainer pages posted by Dave Weagle, right? The ones where he’s trying to sell the virtues of DW link / whatever his new thingy is.

I can’t really get my head around this though. Each time I think about it, I imagine the forces all act as a moment around the axles (ie the BB and rear axle). You could use a 20t chairing front and rear, or a 40t front and rear. The moments are the same whichever size ring you use.

So anyway, if there are any actual engineers who can explain why this doesn’t happen, I’d be interested.

If all the forces were just moments about the axles then your bike wouldn’t move forwards. There has to be a force vector pair where the tyre pushes against the ground and the ground pushes bike and this accelerates the bike forwards. These forces act in pulses, hitting maxima near level cranks and minima near vertical cranks, so the bike accelerates near the maxima and decelerates near the minima.

The centre of mass of the rider is a long way above this force vector pair, so every time the bike accelerates the mass of the rider is left behind and every time the bike decelerates the mass catches up. This upside-down pendulum causes the suspension to bob. With careful design of the suspension kinematics the forces causing the suspension to bob can be met by equal and opposite forces which are a result of the chain tension and driving forces.

http://linkagedesign.blogspot.co.uk/2012/03/santa-cruz-superlight-29-2012.html

I can’t really get my head around this though. Each time I think about it, I imagine the forces all act as a moment around the axles (ie the BB and rear axle). You could use a 20t chairing front and rear, or a 40t front and rear. The moments are the same whichever size ring you use.

Chiefgroovey makes some excellent point

I’ll make another one

The chain tension may have a moment about the pivot.

So in you example 40 40 may have the chain line running through the pivot (as viewed from the side). Chain tension neither extends or contracts suspension

20 20 would move the chain line down. The chain tension will try and extend the suspension as the force now has a moment about the pivot…

I get what you’re saying, completely. I think it’s probably correct, too. I just can’t help thinking it might be an oversimplification.

CGG’s point is excellent techono-babble but it doesn’t address my question.

I dunno, I’m just not sold by all of this. People that purport to know the answers often don’t when challenged which makes me think no one really understands suspension hoodoo.

CGG’s point is excellent techono-babble but it doesn’t address my question.

It would if you understood what I was saying! 😉

How else would you like me to address it? Could you give me a list of words that cannot be used due to them rendering it ‘techno-babble’ to your eyes?

There was a time when dw was touted as befitting from three chainrungs and the designed argued that without 3 rings, bouncy bikes would never have caught on.

However, all that seems to have gone quiet since everyone’s bike has a single chainring.

I understood what your paragraph was talking about, it sounded very clever – well done. I didn’t read the Spanish link. But it doesn’t really address my question. Maybe I need it simplifying further 😉

(You were basically explaining why a bike bobs. This doesn’t explain why a 40:40 setup is different to a 20:20 one.)

(You were basically explaining why a bike bobs. This doesn’t explain why a 40:40 setup is different to a 20:20 one.)

40:40 shifts the chain tension force vector higher up, so although it’s in the same direction its leverage about the pivot is different, usually significantly reduced, hence less anti-squat.

(You were basically explaining why a bike bobs. This doesn’t explain why a 40:40 setup is different to a 20:20 one.)

But mine did, Where does it fall short

To be clear the word pivot refers to the suspension pivot of a single suspension bike

I dunno, I’m just not sold by all of this. People that purport to know the answers often don’t when challenged which makes me think no one really understands suspension hoodoo

I might not but in the world of finite element analysis some one will really get all this stuff