Low Speed Wobble - Your thoughts?

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Dec 13, 2012 | 06:38 AM

AusMongrel

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From: Australia

Low Speed Wobble - Your thoughts?

Ok... So I have changed the geometry of my ride, there is no mistake in that, but I wanted some opinions.

First... my setup.

2010 Fat Boy.
4 Degrees Raked Hawg Halter Trees.
19 inch Front wheel (was 17inch)
18 inch Rear Wheal (was 17inch)

So...
At 60kph (35mph) I get a wobble in the bars. I have checked the fall away - its within spec

Tyre pressures are fine - 40 in the rear 39 in the front.

I know bad rake will induce high speed wobble - but what about low speed wobble?

Your thoughts?

Cheers
John

Dec 13, 2012 | 07:00 AM

Scrmnvtwins

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From: Indiana, USA

Found this online may help

Copyright 2001, by Dick Van Hooser

411 on Rake & Trail

The 411 On Rake and Trail

For decades many motorcyclists have liked the look of raked and extended front-ends on their bikes. Lots of different ways of achieving that look have been tried and the quality of results has ranged from terrific to deadly. The best successes have been the result of good engineering. There are a few things you need to know before you chop your ride. One important area of knowledge is the combination of rake and trail.
“Rake” refers to the angle between the front forks and the ground or, in case we are talking about just a frame, the angle between the head tube and the ground. In most stock bikes the triple-trees hold the forks parallel to the head tube so rake is the same for the frame and the bike. Changing the rake by modifying the frame, the triple-tree or both can make dramatic changes in the trail and, therefore, in the bike's handling.
“Trail” is harder to explain and the diagrams below will help clarify it. Trail is the fore-and-aft distance between the pivot axis of the fork and the center of the contact patch of the front tire at the ground. Trail is one of the most important determiners of your bikes handling characteristics.
A bike with lots of trail will be directionally stable. It will tend to go straight and be easy to ride hands-off. It will not have its direction changed by every tiny bump in the road. Such a bike will take more physical effort to steer than a bike with less trail.
A bike with only a little trail will be livelier (some would say “squirrellier”). It will take very little effort to change its direction whether that effort comes from your hands on the bars or from a bump in the road.
Cruisers and touring bikes tend to have large trail dimensions and sportier bikes tend to have less. If you picture these kinds of bikes you will realize that manufacturers tend to combine large trail dimensions with wide handlebars and small trail dimensions with narrow bars. As a result, the personal level of effort to steer these bikes may be similar. If you get the opportunity, try riding a stock cruising bike then riding an identical bike with narrow drag bars. You will definitely feel the difference in effort required for steering.

Trail is the horizontal distance between the pivot axis and the center of the contact patch. If the pivot axis intersects the ground ahead of the center of the contact patch, trail is positive.
Trail causes the front wheel to act like a caster. The greater the trail dimension, the more forcefully the wheel tries to align itself with the direction of travel. Or put another way, the more stable the bike is. Nobody wants a bike so stable it can not be turned but we all want a bike that can be ridden no-handed for at least a few seconds without going out of control at the tiniest bump.
Generally, a trail dimension of about 4 inches provides a nice balance between stability and responsiveness.

When we increase the rake angle by raking the NECK 10 degrees we INCREASE the trail.

If we rake the front by putting 10 degrees all in the TRIPLE TREE we REDUCE the trail.

The best solution is to use a little of both. We start with stock rake of 33 degrees and add 5 degrees to the neck then add 5 degrees to the triple tree and get a pretty good result. Trail is reduced but positive. There is another design step we can take that is fairly simple and improves the overall result. It involves another change in the triple tree.

Triple trees have a dimension called off-set. It is the distance from the center of the fork tubes to the center of the neck pivot axis. It is an important factor in trail. The better manufacturers of raked triple trees reduce the off-set in order to increase trail in the final set-up.

With a rake of 5 degrees added to the frame and 5 degrees added to the triple tree and a reduction of off-set in the triple tree we have a bike with a longer wheel-base, a prettier front-end and a trail dimension close to the original.

DISCLAIMER:
The drawings here and the numbers used are close to scale but not exact. This is just to explain the principles, not to serve as instructions.
It should be noted that we have not changed the length of the fork tubes in any examples and as a result the bike is lowered. The pictures and numbers assume the rear was lowered to compensate. Using these pictures, one should be able to figure out how the use of longer tubes would affect the outcome. A lot of careful measurement or consultation with an experienced builder will be needed to get a predictable result.
Copyright 2001, by Dick Van Hooser.

Dec 13, 2012 | 07:09 AM

AusMongrel

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From: Australia

Thanks,

Great info there.

Nobody talks about low speed wobble though when they talk about rake and trail.

The bike is super stable at high speed. The wobble comes in at 60kph and is gone by 75kph and its only slight. Cant feel it unless I let go of the bars.

I am starting to wonder if its a tyre.

Dec 13, 2012 | 10:14 AM

Scrmnvtwins

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Joined: Aug 2012

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From: Indiana, USA

I have always thought stability comes with speed so assume you might be broderline with reduced trail as a result of your raked trees and larger wheel, a few calculations could show you where you are.

Dec 13, 2012 | 05:00 PM

AusMongrel

Thread Starter

Cruiser

Joined: Jul 2010

Posts: 203

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From: Australia

Yep Thanks!

I am trying to find out the outside diameter of my tyres, as I know all of the other measurements. But might have to physically measure it.

My tyres are 200X50X18 and 120X70X19

Cheers
J