Single pivots and pedal bob?

Stevo_666

Question time: following on from this thread, it looked like single pivot set ups set lower down in the frame would suffer more from pedal bob (other things being equal). But when I consult WMB's 'Complete guide to suspension' from Aug last year it says that "A lower pivot gives a more active feel and less pedalling-related bob."

Which one is right? Or is the bike in the thread above a 'Faux bar' set up and I'm barking up the wrong tree?

I'm trying to understand a bit of this suspension lark and it's doing my head in :?

supersonic

The article is wrong in that place, I'll get it changed - the web version seems to have been edited from the WMB article I wrote, with a bit added at that point.

Generally a low, rearward pivot will have little pedal kickback through most gears, but increased bobbing.

Anonymous

Chain length extension is used to counter bobbing. This means that as you pedal, it resists the suspension wanting to compress. It also has the unwanted side effect of creating feeback into the pedals.
if you have too much chain length extension during suspension compression, the pedal kickback will be very noticable.

If you have too little, there will be lots of bobbing, caused by a few fundamental things, Of which, the most important ones are:
your weight pushing down on the pedals will cause the bike to squat.
your pedalling being a low-frequency relatively intermittent force (compared to an engine, say*)
Accelerative forces will cause the bike to squat at the rear end.

*An engine revs much higher, and delivers power in smoother strokes. Our pedalling is akin to a slightly unbalanced two cylinder engine running at the same RPM as the drive sprocket


Each bike has a set amount of anti-squat, and some bikes do not have a linear suspension compression to chain length relationship.
Each bike's setup will be optimal for a certain gearing, a particular rider weight, and a particular rider's dimensions (center of gravity, height, leg/arm length etc).

Stevo_666

Thanks lads.

Yeehaa - good explanation as even i can understand it, ta Starting to get the hang of this now.

nicklouse

Stevo 666
read

http://raystrax.com/PathAnalysis/index.htm

Anonymous

Nickouse's link is exhaustively produced, and is an excellent read on the matter.
The only thing I take a slight exception to is the debynking of the "pivoting at the chainline" section. Although it may well have been claimed by someone, somewhere, I do not recall any of the manufacturers listed claiming that they were pivoting AT the chainline for effect.
In fact, most of what are rregarded as "pivoting at the chainline" examples, actually had the pivot, crucially, slightly ABOVE the chainline.

supersonic

The article is getting on: the physics are good, but when wrote many manufacturers were claiming that. Can't think of names off the top of my head, but in the late 90s and early 00s, read it a lot in mag adverts.

Anonymous

Hmm, adverts may not be written by people who understand the concepts though.

Stevo_666

nicklouse wrote:

Stevo 666
read

http://raystrax.com/PathAnalysis/index.htm

Bedtime reading :P I feel like a student again...

supersonic

yeehaamcgee wrote:

Hmm, adverts may not be written by people who understand the concepts though.

Also many magazines were claiming it too, hence why I wanted to write a piece that tried to dispel a few myths. Of course, never enough room to go into everything, but in the space given I was happy with what we wrote. Except that hiccup that someone added when put on the web ;-)

Anonymous

You know I still disagree wholeheartedly (well, mostly) about braking though, right?

supersonic

Lol, I agree with the link Nick provided ;-)

We have seen that Floating brake systems give a bike the breaking character of its linkage geometry. There is nothing special in this beyond what is noted in the “Braking.” section of Chapter III.

The false claims are from those who think four bars and floating linkages 'isolate' braking forces somehow.

Anonymous

Ahem, Language, Sonic. "Braking"
Yeah, it's the isolation nonsense that I don't believe. But, without resorting to another million page thread, I also believe that traction effects will have a greater effect on rear braking by wanting to extend the wheelbase, than pretty much anything else. I mean, once your weight is thrown forward, the grip is reduced on the decelerating wheel, so it should reach some balanced state pretty fast, where the rearwards pull becomes the strongest effect.
But that is simply my theory, based on observation and first principles - I may well be wrong.
(and dammit, I WILL get round to testing this in some kind of phys-sim one day )

supersonic

I was going to write 'sic' in their lol, that is direct from his own page haha!

Observation in the end, and what you feel is the best thing to go on. Which is what I pretty much summed up in my last paragraph. You can have all the theory and diagrams in the world, if it don't feel reet, try summat else ;-)

Anonymous

Nah, what I mean is, I don't have any cold hard maths to back up that theory, so I may be well off the mark.

supersonic

That's ok, 90% of the sus manufacturers don't either hehehe.

Anonymous

Lol, very true! If they DID truly and fundamentally understand the dynamics, then we'd probably see everyone settle on a very similar design.
As it stands, it seems that someone brings out something better, then everyone coppies that for ages, then somebody finds something better, then everyone copies that for ages and so on.
Well, apart from Specialized and Kona!

supersonic

Specialized just add the brain - to fix problems is supposed to not have in the tech...

It seems a lot of manufacturers look heavily at one point of the design it does well, then make the rest up. GT and Ellsworth are the worst for explaining stuff. Boardman don't even bother, just a fourbar.

Anonymous

And a lot of people (ahem) make spurious claims, rather than just saying "basically, what we've done is find a way around patent infringement

supersonic

Very true!