trail verses rake

[Dennyz]

I am playing around with steering geometry numbers and have come to one of those areas of compromises.

I am looking for a trail of just over 4" using a particular wheel size. I can change the triple clamps or the steering angle(rake) to achieve this. The steering angle is much easier to change over the triple clamps.

So, the question is: What would be the side effects of one over the other. Or, is it all about the trail and how you get there is less important.

All comments welcome.

[Pat Kelly]

Increased trail will make the bike more stable at speed. Increased trail is easiest achieved on a bike by increasing the rake.
This increase of rake/trail will make slow-speed manuvers difficult.
You can change the rake/trail ratio by using a "raked" triple-tree. This makes the steering angle more or less than the fork angle.
Remember the old stingray bicycles with the curved forks? they had very little trail for the given rake, but then you didn't go very fast (without cards in the spokes)
A unicycle has no rake and no trail and can spin on a point. You can actually change the rake and trail on a unicycle by leaning in the direction you want to go (been riding them since I was about 8 years old). Shopping cart wheels have negative trail so the wheels follow where you're going. OCC choppers have lots of rake and trail so they don't make parking-lot manuvers very well (the wheel flops from side to side).

[prometheus578]

What you say is true, Pat, but Denny's trying to decrease the trail, not lengthen it.
Trail on these bikes is 5.12 in... he wants 4 inches.
Denny... I tried last night to figger out what you were explaining. Obviously, I couldn't.
Less rake will give you a quicker response when steering... also increases the tendancy of high speed wobble.
Four inches might be a little too drastic?
Raising the fork tubes in the tree will shorten the trail, but this is accomplished by lowering the front of the bike which you probably don't want.
Just how do you plan to make this modification?
Fit a triple tree from a different bike that has the numbers you want?
Cut off the steering head and reweld it at less of an angle?

[TopCatGr58]

Hey Prom,
I don't think 4 inches of trail is that drastic! After my impact rake adjustment technique I ended up with only 2 inches of trail and I've had Godzilla up to 110mph before experiencing any wobbling, and then it may have been due to aerodynamics more than the trail

ps: I,m typing this reply using the web plugin for the Wii system, a bit of a pita, a real vs. virtual keyboard would be better.
T.C.

[Dennyz]

Most modern sport and sport touring bikes have a trail around 4". The R1 is 4.0" (w/ 24 degrees rake) and the FJR1300 is 4.3" (w/ 26 degrees rake). I am working with R1 front forks, triple clamps and wheel.

If I run the forks at the stock rake (caster angle) of 29.5 degrees, I get a trail of 5.35". If I bring the rake in to 26 degrees, I get a trail of 4.47". The trail I am looking for is 4.30", which I can achieve with a rake of 25.25 degrees.

Trail can also be decreased by increasing distance between the steering center and the fork centers, requiring custom triple clamps. The relationship between trail and fork to steering pin centers is inversely proportional(almost). I can achieve a 4.30 trail on the stock rake by increasing that distance by an inch.

Changing the rake is obviously a bit simpler than making custom triple clamps.

What I am wondering is what are the bike handling pros and cons of one method over the other?

[Pat Kelly]

I can describe this with a comparison riding my XS's vs Wifes Virago.
My take:
The XS handles ok for a 700 lb bike that's 30 years old. Semi-sporty yet stable at/over 100 mph.
Her Virago (steeper rake/more trail) takes more effort to turn (lower CG has an effect there too). After the turn the bike wants to straighten out on it's own.
Her take:
The 1100 Virago is just fine though seems to be under-powered on the freeway (hey, it's a cruiser V-twin after all).
The XS has plenty of power but the steering is too twitchy and takes more effort to go (perfectly) straight.

[Dennyz]

There are some that believe more trail does not always mean more stability at higher speeds. Certain trail configuration can have harmonic anomalies with tire side wall and fork flex.

I am wondering if the XSs 5.12" trail is a contributor to the wobble some of us have experienced in high speed corners.

High trail also requires the rider to lean harder to get the bike to turn, resulting in less "feel". The XS has some of this.

I recently rode a '03 Honda Shadow with 6" trail. The bike was stable in the straights. On corners, it was very challenging to maintain a smooth line in the turn. It wanted to dive in and shot out with very little balance change. It took a lot of fun of running the twisties.

Lower profile radials and shorter trail numbers are typical on modern good handling bikes.

[81xsproject]

You may find some good info HERE . It is a link to a download of Tony Foale's Motorcycle Handling and Chassis Design: The Art and Science. I got my copy from this link on the UK site. It is a big file, a very good read.

[Dennyz]

Thanks 81xsproject,

Good information for what I am working on.

[xschop]

fork extenders

I will start this weekend on some ADJUSTABLE fork extenders that allow for 1-4 inch extension of the hayabusa front end on my xs11. I took it out for a spin last night and it's like I don't even have to lean but slightly and I'm cutting the corner, This has made up for the longer swingarm while trying to maneuver out of a parking space. The bike is stable up to 80mph so far (lot aerodynamic too!), but the foot pegs are too low. So, I will make the fork extenders. It seems like I am crawling on a pocketbike, it sits so low. Think this will create a problem at high speeds, since it will be the equivalent of putting a 17" tire on the stock xs11 fork length?

[xschop]

fork stems

IIRC the Busa trees had the forks pulled in by @ 3/4" more than the xs11's

[81xsproject]

Did mean 2 3/4?

[xschop]

3/4

No, I meant 3/4" that the forks were pulled in parallel to the steering stem, the forks are about 2 3/4 shorter and the Busa wheel with tire is about 2.5 to 3" shorter (dropping the bike another 1.5')

[fredintoon]

"I am looking for a trail of just over 4" using a particular wheel size. I can change the triple clamps or the steering angle(rake) to achieve this. The steering angle is much easier to change over the triple clamps. "

Hi Denny,
Like you say, 2 ways to reduce trail.
1) Gimmick the 'trees/fork tubes/front wheel diameter.
2) change the steering head rake angle.
Now (1) is easy, mix'n'match Special & standard 'trees & fork legs but for (2) you must cut & re-weld the frame or perhaps drive the bike into something solid.
And I'd suppose that uncertain high speed steering on an XS11 had less to do with the fork's geometry than with it being too skinny for a big fat bike.