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Old 08-12-2013, 07:45 AM   #1
boatpuller OP
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Why does a longer swing arm give better traction?

Been reading the reviews of the new water cooled GS, and most talk about the 2 inch longer swing arm, and that that provides better traction in the dirt or off road.

Why?
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Old 08-12-2013, 10:26 AM   #2
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It shouldn't. A shorter swingarm will put the rear wheel closer to the center of gravity, and that would physically put more weight on the rear wheel giving it more traction. A longer swingarm will transfer that weight to the front wheel giving the front more traction and the rear less.

So unless they picked up some additional rear wheel travel and used that to soften the rear suspension to make it easier to load the rear before it breaks loose, then the longer did nothing for traction. But even then ~ a softer shock would do that reguardless of the arm length.

I think they need to explain their posistion a little bit better. Because a longer arm will give you more stability but not more traction.
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Old 08-12-2013, 11:50 AM   #3
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He's absolutely right.
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Old 08-12-2013, 01:23 PM   #4
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I think it is to reduce "Axle Tramp", I think that is what it's called. Traditionally the rear suspension on GS's would pack up, would not rebound over wash board surfaces. After having ridden the Tenere, with a longer swing arm, it does not do this so I hope on the new GS this problem is eliminated.
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Old 08-12-2013, 03:46 PM   #5
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BMW did not move the rear wheel to make the swingarm longer, they moved the pivot forward. Not a longer wheelbase.

The longer swingarm gives a more vertical movement to the wheel and less angle changes to the arm, driveshaft, etc.

To answer this imagine the extremes: a bike with 12" of travel and a 6" long swingarm, crazy angle changes. It would have really weird traction problems.
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Old 08-12-2013, 05:11 PM   #6
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Some of this idea may stem from the old competitive hill-climbers ... but the reason they lengthened the swingarm was to keep the bikes from wheelie-ing unnecessarily; by controlling that, they could run big paddle-lugged knobbies that got better traction so the two different ideas became linked in "common knowlege" which is neither common nor knowledge. In dirt the ideal traction is, for most I've asked, enough of a controllable wheelie to place the maximum weight on the driving (rear) wheel, period. When you start working out the force vectors associated with different angles, riders, tread patterns and tire materials it becomes an exercise in mathematical masturbation that's more easily solved by the subjective judgment of an experienced rider. I've never heard anyone in that kind of game ask for a longer swingarm, but many have asked for more horsepower, torque, or better tires.
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Old 08-12-2013, 06:58 PM   #7
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The only way this picture could have more ADV topics in it is if she were holding dino oil in one hand and synthetic in the other.

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Old 08-13-2013, 01:36 AM   #8
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For the same wheelbase, a longer swingarm will do provide better traction, since the for-aft movement of the wheel is less for given suspension movement, ie the wheel moves more vertical to the ground.
If a longer swingarm now means also longer wheelbase, then maybe yes, maybe not, depending on weight distribution.
Interestingly enough the longest swingarms ever put on bikes, I think are the ones on R1 sportbikes. BMW paralever depsite its problems has the quality of mimicking a very long swingarm, something like 2m I beilieve.
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Old 08-13-2013, 10:16 AM   #9
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yes... and...

Yes... it is valid to use a longer swingarm to enhance traction for the rear wheel.. my reasoning to follow...

But first, increasing traction for the front wheel is a good thing! (when needed.) Longer wheelbase moves center of gravity forward resulting in a more planted steering feel.

If you ride off road and are the kind of rider that gets physical, transfering weight to which ever wheel you want to dig in... I can see where a little more wheelbase could enhance your control of shifting weight and hence transfering traction to the rear wheel. Just a thought...

A longer swingarm is useful for increasing suspension travel... longer softer travel increases traction.
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Old 08-14-2013, 07:13 AM   #10
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Thanks for the answers.
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Old 08-14-2013, 01:22 PM   #11
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Sorry, can't agree on that in the water cooled GS's case.

The engine/transmission is redesigned, allowing the swing arm to mount 2 inches forward. The rear wheel's placement was not changed (at least as reported in the articles I referenced).
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Old 08-14-2013, 02:04 PM   #12
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Let's not forget what happens when you hit the accelerator.

1. Torque applied at the rear wheel pulls the tire down and tries to pull it under the bike
2. Center of gravity moves backward relative to acceleration

A balance between friction and over-torque (wheelie) is what is important if pavement traction is the concern.

As mentioned, for maintaining traction over unimproved surfaces, a vertical to trailing pivot would be best, just like the steering angle, but not practical. So, a longer swing arm would certainly help. However, there's a lot more to setting up proper geometry then just the swing arm length. There's a lot out there on the topic containing: Rake, Trail, swing arm angle, wheel base and steering head to swing arm pivot.

Most relate to how the bike handles. Traction is a small variable in the relative scheme of things.

So, if the trail and wheel base stay the same with a longer swing arm, that's a good thing.
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Old 08-14-2013, 07:59 PM   #13
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As I lack the knowledge to agree that your statements are fact, the best I can do is offer that for the purpose of this discussion let's assume they are facts.

Usually, when someone goes too far in the wrong direction, they get called on it here on ADVrider. As that has not happened to you, you are likely correct. From a basic concept of leverage. your statements seem reasonable.
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Old 08-14-2013, 10:40 PM   #14
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we can let it ride (give it some time) for others comment also.

member btao has brought up a lot of other factors, and he gives very well comments for them too.

a lot of items contribute to, and effect the handling and the control of a motorbike.

very small changes, very very small changes as little as say 5mm can have a large effect.
that is why so much testing happens on MotoGP motorbikes. A small 5mm change may effect the handling and control just enough to be the difference between winning, and a 5th position finish.

now back to swingarm increases in length (without other changes)
the reason a longer swingarm put on a very powerful sportbike does at least 2 changes.
the major change is it actually REDUCES tire grip (traction)
and then, as a result of less traction, the motorbike will have a less tendency to wheelie quite as easily.

and therefore, the motorbike may be able to get on the gas harder, thus ultimately it may make a faster run.

sounds backwards right?
going faster by having less tire grip?

it is very complex, all of those items that cause traction and control, thus speed and manueverability to increase or decrease.

that is why so much time AND money are spent on something like MotoGP motorbike development.
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Old 08-15-2013, 03:18 AM   #15
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Quote:
Originally Posted by boatpuller View Post
Been reading the reviews of the new water cooled GS, and most talk about the 2 inch longer swing arm, and that that provides better traction in the dirt or off road.

Why?

All current trials bikes have swinging arms 45-50mm longer than those used on old twin-shock bikes. The modern bikes grip much more effectively than the older machines, and one reason for that seems to be the longer swinging arm.

Shorter swinging arm seems to mean losing traction happens much more suddenly and is less predictable. Obviously the front/rear weight bias is different between early and current bikes, with modern tyres meaning more weight on the front, which provides greatly improved steering.
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