[Speedfreaks101]

After looking for a few days I have figured out this much:

( from this thread http://forum.svrider.com/showthread.php?t=77286&highlight=Dog+bone )

5 3/4(stock) 5.75
6 1/8 = 6.125 = .375 longer = 1 inch lower
6 5/16 = 6.3125 = .5625 longer = 1.5 inches lower
6 7/8 = 6.875 = 1.125 longer = 3 inches lower

So this means that the dog bones ratio works on a 2.66667:1 scale.

What I need to know is what is the ratio of lowering the forks in the clamps vs the effect on ride height.

Can anyone help?

thanks,
Bart

 

[andyauger]

Fork rake is 25 degrees. So if you raise them a measured 1" higher in the triple clamps it lowers the front ends by 1" X cos(25 degrees).

 

[Speedfreaks101]

Thank you.

If I understand correctly that means that 1 inch = .906307787 inch , so we are not even getting a 1:1 ratio at the forks.

Now that brings up a new issue. If you drop the rear 1.5 inches (actual suspension drop is .5625) and can only drop the front end 15MM which equals .591 inch (or roughly 19/32 inch). This means that you are effectivly lowering the suspension height at the forks by .535627902117


So even though you might be able to lower the forks the same amount as the rear shock the impact is much greater on the rear.

Am I understanding correctly?

 

[andyauger]

Numerically you are correct. In practical terms, there is no way to be sure that your sag measurements are anywhere close to 0.01" accuracy.

It is true that you can lower the rear more than the front due to mechanical limits. It is also true that changing front and rear ride height does affect handling very noticeably and directly. This is due mostly to the slight change in front end rake (trail doesn't change meaningfully).

So, for example, if you have your front and rear preload set correctly and you then lower the front end 15mm then you will notice quicker turn in, some loss in straight line stability.

If instead you lower the rear end 15mm you will notice slower turn in, more straight line stability.

If you lower or raise both ends equally then the handling stays basically the same, except that the higher the bike is the slower it will transition from left to right (increased polar moment of inertia). The lower the bike is the less lean angle you have before stuff starts dragging.

There is no "right" answer. It depends on the roads you ride and your personal preference for how the bike feels.

 

[Speedfreaks101]

Thanks for the help.

 

[DibDubDoo]

This is due mostly to the slight change in front end rake (trail doesn't change meaningfully).


If you lower or raise both ends equally then the handling stays basically the same, except that the higher the bike is the slower it will transition from left to right (increased polar moment of inertia). The lower the bike is the less lean angle you have before stuff starts dragging.

Can you explain this to me, everything I have read said otherwise.
If rake is changed so is trail.
I read race teams lift bike (COG) to make it more flickable left to right.

 

[RichDesmond]

Can you explain this to me, everything I have read said otherwise.
If rake is changed so is trail.
I read race teams lift bike (COG) to make it more flickable left to right.

Typically, when converting a street bike to track use you raise the rear and maybe lower the front. This is done for geometry reasons though, not because you want to raise the COG.

Trail matters a lot more than rake. People get confused on that, I guess because if you change one you change the other (unless you change triple clamp offsets at the same time) and the rake is easier to see and visualize.
Tony Foale's book on chassis design has a good section on it, he built a front end with a normal amount of trail and 0° rake, it handled fine except when on the brakes. Then the fork tubes tended to bind up. Which is why bikes have some rake.
The trend over the last 20-30 years has been to less rake. Larger diameter, stiffer fork tubes have made that possible. The benefits are a shorter wheelbase and more weight on the front end. Steering response isn't an issue, that's governed by the trail which is set by choosing the right offset.

 

[Veee]

Tony Foale's book on chassis design has a good section on it, he built a front end with a normal amount of trail and 0° rake, it handled fine except when on the brakes.

same book say higher COG will make bike easier to flick from side to side.
I wonder how easy would be to flick Harley if you put forks with 25 degree on it?

 

[cain]

same book say higher COG will make bike easier to flick from side to side.
I wonder how easy would be to flick Harley if you put forks with 25 degree on it?

I have an old Harley-Davidson FXR. They came with a 28 degree rake and have a reputation of being the best handling bike H-D made until recently. It is a fun bike to ride on twisty roads and can hold its own for a 560 lb. cruiser.

Of course Buell has made Harley powered bikes with 21 degree rake that are supposedly very easy to flick around.

 

[andyauger]

Raising the COG slows transient handling (increase in polar moment of inertia). That's why racing motorcycles try to get as much mass as low as practical.

On the front end there are four different dimensions that change handling feel. Steering axis (the angle of the headstock relative to vertical), rake (angle of the forks relative to vertical), offset (perpendicular distance from the steering axis to the wheel axle), and trail. There are good articles available on how each one affects steering.

Rake and trail are indeed tied together on any given bike. Unless you are going to modify one or more of the four parameters above you will feel quicker turn-in, less straight line stability any time the front ride height is lowered or rear ride height is raised. You will feel slower turn-in and more straight line stability any time the front ride height is raised or the rear ride height is lowered.

It is true that trail with zero rake can be made stable, but the steeper the rake the more subject to head shake.

 

[Veee]

Raising the COG slows transient handling (increase in polar moment of inertia). That's why racing motorcycles try to get as much mass as low as practical.

slow or fast is not same as easy.

 

[andyauger]

I only have two questions about that reply, Veee.

Huh? What?

 

[Veee]

it is easier to initiate turn in with higher com. it is longer to full lean.
easy-fast
higher com is longer lever for inertia.
lover com less lean needed for given speed.
you are mechanical engineer, does that sound right or wrong?

 

[tcl]

Oh, this one.

 

[tcl]

I'll play.

When you initiate counter steer, around what axis does the bike pivot? It is not the line between the tires and the road.

 

[Veee]

is com fulcrum?
center line of com to contact patch of line between front and rear wheel.

 

[tcl]

is com fulcrum?

Could be...

When you turn right, the top of your bike does not tip to the right but rather the bottom of your bike moves to the left.

Andy's pendulum is upside down.

 

[Veee]

I heard Harley is really stable at 150mph, if you can get it to that speed.

 

[Veee]

Andy's pendulum is upside down.

maybe he drinks by sucking on straw, I like to just tip bottle over my head and let it flow ;D

 

[DibDubDoo]

Typically, when converting a street bike to track use you raise the rear and maybe lower the front. This is done for geometry reasons though, not because you want to raise the COG.

Yes, I do understand that part. I was replying to his statement about raising or lowering whole bike. This is article I saw.
http://www.motorcycledaily.com/22dec00apriliarsvmille.html