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sammyd173

Swingarm Theory

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I'm wondering about how moving the rear wheel forward and back in the swingarm can affect what the bike does, and how noticeable it might be?

It seems that the further back the rear wheel is, the more lift you would get when trying to blast up a rock. The cantilever effect would be amplified the further back the rear wheel was. Imagine the rear wheel a foot back (for mental illustrative purposes), and how much more that would lift you up as the rear wheel shot forward, sending you up. I wonder if Bou runs his rear wheel further back to get that amazing lift. 

It would also make sense - maybe - that the further back your footpegs are, the less of a lift effect you would get. Imagine your pegs all the way back to the axle of the rear wheel  - you'd get no lift at all. 

Conversely (if anyone is still reading this blather), the closer the rear wheel is to the front of the bike, the easier it should be to balance on the rear wheel as you are not lifted up as high. Think of trials bicycles. And the further back the pegs are, the easier to control the bike on the back wheel? 

Then there's all the other implications of increased/ decreased rear traction, softening of rear shock when the wheel goes further back, and lots of other things I haven't even considered. So my question is, do any of you actually set up the back of your bikes with these effects in mind?

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The manufacturers choose the location of the rear axle to give the best handling overall. If you only ever jumped up things, the ideal location would be different. So would lots of other things on the bike be different.

As far as the slot goes, it is there so you can adjust the chain tension. Yes you can change the handling by having it at the back compared with at the front, but it is only a very small difference and 99.9% of riders would not be able to tell the difference just by riding the bike.

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Moving the wheel forwards shortens the wheelbase which sharpens the steering and the bike would lift slightly easier and obviously the opposite applies. This is why you see hill climbers with long swinging arms. I am a poor rider and would never notice the difference but i imagine a factory riders mechanics will have the perfect axle position for the bike and replace the chain rather than adjust it until the wheel adjusters are at their maximum range. 99.5% of the riding population may never know the difference but at the highest level of sport every adjustment is noticeable.

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It does make me laugh when a new bike is announced with a 15mm longer / shorter swinging arm. Surely nobody can actually tell, that's probably two clicks on the wheel adjusters ?

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There is a vid of casales talking about how he adjusts his for different terrain, put in casales bike set up and it should be there. 

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Yes 99.3% of riders will not notice anything.   However what actually happens when you apply lots of power..?

The chain becomes tight on the top run, which tends to lift the rear wheel, and make the bike squat.   The push from the tyre, at ground level, acts on the centre of gravity of the bike, and causes the front wheel to lift...  Wheely...!

Some old bike had Plunger rear suspension, that moved the wheel vertically, as that was considered to be better.   Some had rigid rear suspension, and could only be ridden by Yorkshire Men.

.

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Typical rear wheel adjustment is about 16mm with standard snail cams. Wheelbase is usually around 1295mm so 16 mm is about 1.2%. Although this is small 1.2% is sufficient to make a change to rear wheel grip and front wheel propensity to lift.

If you generally ride slippy sections shorten the wheelbase, if you ride steep grippy stuff lengthen it.

I have never noticed the difference between rear wheels positions on a trials bike but could feel it on an MX

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17 minutes ago, dadof2 said:

 

If you generally ride slippy sections shorten the wheelbase, if you ride steep grippy stuff lengthen 

Sure casales said complete opposite !

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I suspect something has been lost in translation and maybe a mix up between stability and traction.

On any surface the closer the driving wheel is to the centre of gravity (shorter swing arm) the more traction there will be in the direction of travel. However in the same situation a long swinging arm / wheelbase gives more stability laterally, this is probably what is meant in the video.

Perhaps the clearest example is in F!. The long wheelbase Merc is stable and dominant on faster tracks / high speed corners like silverstone because it has more lateral stability. The shorter Ferrari tends to be better on tracks that need low speed speed traction and more responsive cornering like Monaco. On these tracks the Merc has less traction and its rear tires tend to slip and degrade faster than the Ferrari.

Edit - quote from Honda "A shorter wheelbase is better, especially if the traction is poor. The shorter wheelbase transfers more weight to the rear to help the bike hook up better and wheelie easier"

Hook up is american for finding grip / drive / traction.

Edited by dadof2

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