Rake

[G-Man]

I have a pretty good idea of rake and trail.  What I don't have is an understanding of how that will affect the bike's handling.  Corrrect me if I'm wrong after I type outloud.

By increasing the rake with that aftermarket raked triple tree from VTXtreme, I predict that there would be understeer in turns.  Therefore, I would have to take the curves slower.  Not much of an issue with sweeping two lane roads at 65mph but an issue in the twisties.

I really like the look of the bike with the fairing and raked trees posted recently.  I always thought there should be more room between the fender and belt cover and radiator.   Form over function is the old debate I guess.

[Black Dog]

Give Daryl a call (He is VTXtreme 414-517-9546).  He knows the math/angles/nerd stuff about these products.  He will answer your questions with no BS, and will not pressure you for a sale.  He only wants happy customers, and knowledge about your purchase, before the fact, helps keep you happy.

He's my friend, but I don't work for him.

Black Dog

[The Purple Haze]

Your front suspension geometry is defined by the following six variables which are defined as:

OFFSET: Centerline of the top steering neck to the centerline of the top of the fork tubes.

RAKE: The angle in degrees of the steering neck from the vertical cord.

FORK LENGTH: The distance between the top of the fork tubes to the centerline of the axle.

DIAMETER: The diameter of the front tire.

TRAIL: The distance defined by the vertical line from axle to ground and the intersection of centerline of the steering neck and ground.

RAKED TRIPLE TREES: In order to bring trail figures back into line, triple trees with raked steering stems can be used. Usually adjustable in 3, 5, 7 degrees of rake.

HOW TO MEASURE CORRECT TRAIL

Raise the bike to an upright position, using a tape measure, hold the tape straight down from the front axle to the floor. Put a mark on the floor at that point. Then place the tape parallel to the steering neck, following the angle of the steering neck all the way up to the floor. Put a mark here also. Now measure the distance between the two marks and you have your trail measurement. It should read between 2 and 4 inches. Note: If your bike is equipped with a rear suspension, have someone sit on the seat when you make the measurements to simulate your actual riding condition.

TOO LITTLE OR NEGATIVE TRAIL

With too little or negative trail (steering axle mark behind the front axle mark), the bike will handle with unbelievable ease at low speeds, but will be completely out of balance at high speed. It will easily develop a fatal high-speed wobble. EXTREMELY DANGEROUS!

NORMAL TRAIL

Normal trail is somewhere between 2 and 4 inches. The bike will handle easily at both high and low speeds. Flowing smoothly through curves without swaying or wobbling. If you use a very fat rear tire, you should keep the trail as close to 4 inches as possible.

TOO MUCH TRAIL

If the trail is more than 4 inches the bike will handle sluggishly at high speeds. It will seem almost too steady. You will have trouble balancing the bike at lower speeds or on winding roads. It will feel generally sluggish and clumsy.

[vanagon40]

If you use a very fat rear tire, you should keep the trail as close to 4 inches as possible.

Hey Purple Haze, can you explain this a little more?

The reason I ask, I have a "fat rear tire."  With the OEM front tire, handling at low speeds was sluggish (i.e., counter-steer), but no high speed problem.

After I installed a front tire that was not as wide, low speed handling improved, but I have high speed instability.

Perhaps "fat" is relative term, comparing the rear tire to the front tire?

[The Purple Haze]

I am no expert by any means.  Unless you significantly changed the diameter of the front and rear tires, you should not have changed your trail dimension.  Changing to a narrower front tire will quicken your steering due to the smaller contact patch.  Add a wide rear tire that tends to "plant" itself and your steering can get squirrely.  Your "high speed instability" is most likely just a more touchy steering at high speeds.  PLEASE ask an expert for advice on how best to cure the problem!

[vanagon40]

I don't need an expert to solve the problem, it would simply be a matter of putting the correct tires back on the bike.  No, I did not significantly alter the diameter, just the width.  You are correct, it is most likely just touchy steering at high speeds.