Rake and trail for dummies

[Billy Golightly]

This is one area admittedly I've always had a little trouble fully grasping.

I know that there are several different components that account for this obviously. Fork length, tire size, steering neck angle, leading or trailing axle, amount of offset distance between the centerline of the forks and the centerline of the steering stem, etc.

I've Googled and read a lot about this over the years, and have made some changes and adjustments by shortening forks, changing tire sizes, making new triple trees that have less rake built into them, etc. But I've never actually set a front end up from scratch. I'm considering a new project with the Rotax motor, and this would be a necessity.

I know Jason Hall, Louis, Swinehart, a few others have already been down this road with their builds. You guys or anyone else, what kind of range of #s am I looking for if I was to do this from scratch essentially?

[jeswinehart]

My method most likely will be of little help to you Billy. I too looked/studied all the information you mentioned and even applied it.
Since my front end was designed to be a bolt on set up I was able to adjust the rake sevral times (and I did haft to) to find what I felt was the best for me. My biggest concern that I way over thought was to get the goose neck centered. It really is much easier then it sounds using 2 know center points on frame and rear axle. I marked those 2 area on garage floor and then I drew a straight line on concrete floor under gooseneck area.
Using a plumb bob hung from center of goose neck I adjusted until it matched up.
Like I mentioned about the rake, it took me several times using spacer v blocks to get what I wanted. Being able to ride it was key for deterring what suited me best.
I now have my trike up to a weld shop to permanently attach front end. After a year and a half of riding, the front end had loosened up so knowing what worked I decided to make it solid.

[Red Rider]

Billy, If you recall, when I made the welded-up tripleclamps for my 250R, I kept the factory rake built into the tripleclamps, and went from trailing-axle to leading-axle, which was a big mistake. I never measured the resulting trail, but it was much less than factory, if not zero, which made for twitchy handling.

When I drew up the designs for the billet tripleclamps, I wasn't concerned with the rake/trail measurements, and therefore never measured it. I just needed a design that would hold the forks in a leading-axle configuration, with the front wheel back in the same location, as it was with the stock tripleclamps & trailing-axle forks. Based on my rake/trail research, as long as the front wheel was in the same location, with respect to the frame, then it would have the same factory handling traits. (This assumes that there were no changes to the steering head angle or height, fork length, or tire diameter.)

I don't know if the manufacturers published rake/trail specs on trikes, but you might do some research and see if you can find that info. Or, if you have access to other good-handling trikes, I would try measuring the rake/trail on them and see if a pattern emerges. If you do see a pattern, there might be a corresponding sweet spot to shoot for with your build.

[barnett468]

.
I don't understand your question or what you are trying to achieve . . If you want your bike to steer a certain way, I suggest you try a few different bikes as Red Rider suggested and copy the geometry from the one you like the best . . Most, if not all of the steering geometry numbers for the Honda are available, and I do have them stored on my other computer, but you can just measure the geometry of any bike that you like the steering characteristics of and start with those as a baseline.

Steering geometry is incredibly complicated, and I can assure you from first hand experience that what looks good on paper does not always work exactly as planned in actual use, which is why every manufacturer has an R and D facility . . One of our engineers named Randy Hall took a 1983 KX 250 and made his own 3 wheeler which had a trailing axle, and we actually compared that against the Tecate prototype, and imo, it was noticeably better overall, lol.

Since both the Tecate and the Honda won several Championships, one really can't say that one steers better than the other, they just steer a little differently.

One of the most complicated aspects of steering geometry is offset triple clamps . . In this case, I am referring to triple clamps where the lower clamp is a little forward of the upper one . . Systems that do not use offset clamps like these are many times easier to understand and deal with.

[El Camexican]

From my extremely limited experience I think that determining rake and trail is easy if you have a jig and a bit of a pita if you don’t. A spare set of hands is very helpful.
The problem is what to do with the numbers. Everything listed below changes the rake and trail:

- Putting on tires, or wheels with a different diameter
- Chain adjustment
- Moving the forks up or down in the tree
- Suspension pre-load

Like Barnett said, you’d be best served to start by measuring the chassis of something you like riding. It might get you close, but the location of the swing-arm pivot (height and distance from front axle), length of the swing-arm, location of the engine and the center of gravity, seat and handlebar location can all make huge differences to how it will feel when you get it up to speed and start trying to turn.

When looking for happy medium numbers to build around I believe you also need to consider what the numbers will be when the suspension is compressed.
Fortunately all my rakes have been for the purpose of straight line stability, which is a no brainer; just pull out your front end till you either get to 36 degrees, or until you have only 2” of ground clearance.

If you are planning to build something that needs to be steered with dignity over 20mph you may want to build a set of adjustable trees. Sandy Kosman designed one for his Gucci frames. I can’t find an image of it, but it was very trick, perfect for fine tuning.

[barnett468]

If you are planning to build something that needs to be steered with dignity over 20mph you may want to build a set of adjustable trees. Sandy Kosman designed one for his Gucci frames. I can’t find an image of it, but it was very trick, perfect for fine tuning.

. . . .


..........
.
...........................................
.
..........
.
...........................................
.
.................
.
..................................................

.

[El Camexican]

The one I saw on the Gucci frame had a helm joint looking thing on an adjustable threaded rod. Cool as the ones you posted, this was MEGA cool I think Kosman is shut down. All the website links take me to Harley sites.

[Billy Golightly]

Thanks for the all the feedback and replies gentlemen. What I'm assuming is that I would be building and setting a front end up from scratch essentially. I'm currently exploring this little fantasy I have that I may be able to take a 250R frame and make a front and rear end that could bolt onto it to make a 2 wheeler cafe racer type machine, but then go back to trike parts also if desired. So my challenge is to figure out some front end rake and trail #s that I can work with, without having to reangle the steering neck on the frame itself. 2 wheelers in general have a much much different type of rake and trail from what I've observed, so this might not be really feasible, but that's what I'm working on trying to get a "range" of measurements basically I can work within the confines of. As far as trikes, I love how the 3rd gen 250Rs handle, and even better with a smaller front tire (I'd prefer a slightly twitchier/quicker turner than not for my style on a trike) but I have my doubts that if I just bolted on a new front end with similar dimensions and a street bike front wheel and tire it would behave in a way I'd like on the road.


Sounds like some good old fashioned trial and error will be on the agenda

[Jason Hall]

Billy here are some of the things I have learned over the years. First is a 350X is leading axle with 0 rake in the clamps. A 250r is trailing axle with 6° of rake built into the clamps. If you think about 3 wheeler front axle working like Caster in a normal front spindle. It will help you see the difference in the 2 front ends. When you turn the front end the side of the axle that you are turning towards is actually getting closer to the ground. That is what makes the handlebars want to return to the center position like the angle of a spindle of a car or quad. More rake in the front end of a 3 wheeler will change how close the axle gets to the ground when you turn. So, more rake will turn harder, and will want to return to center faster. Now there is leading and trailing axle. If you think about that, it affects the way the front end puts weight on the axle. So I explain that like this. With a leading axle, there is more tire in front of the axle. Imagine picking the front end up by the way the weight is pushing down on the front tire. If you were picking up a leading axle the way it pushes against the ground, you would pick up the front wheel. If you picked up a trailing axle front end by the way it was pushing against the ground you would pick up the forks. This gets confusing, but it all ends up changing the way the handlebars feel in your arms. The trailing axle actually has a harder time turning because the forks are sitting on the axle, so the front end will want to return to center faster because of the way the forks are pushing down on the center line of the axle. With the trailing axle the forks are on more of a angle in order to have the correct amount of trail because of the built in rake in the clamps. With a leading axle the axle center line is in front of the forks, and there is more tire in the front of the forks, so the axle is being pulled up by the weight of the bike. With 0 rake this gives you an easier feel at the bars because of the way the weight is pulling up against the forks. With a leading axle you can have less rake, which means less fork angle, and less caster angle which will make the bars turn easier. Man, I hope I didn't confuse you by talking around in circles lol.. It would be easier to explain this in person with a 350x, and 250r to look at so you can get what I'm saying

[Billy Golightly]

Jason,

Could I summarize - that if the rake angles and everything else being the same, I would in a 2 wheeler application prefer something probably with a TRAILING axle design, so as to offer more straight line stability?

[Jason Hall]

Every motorcycle built for the last 20 years is leading axle. Also
from my experience with the Missle 250r that Kasey built and I rode in the dunes I would stay as far away from trailing axle as you can!! Lol.. Leading axle is the way to go IMO for anything! Adjust your rake angle to give yourself hi speed stability. You can always slide your forks up or down in your clamps to change the rake angle. You can also adjust rake by adjusting your rear ride height.

[83ATC185]

Sorry to hijack your thread but i just have a few questions that i hope make a little bit of sense. Say if you took a 200x and flipped the forks around to make it a lead axle bike, what effect would that have on handling? Would you be able to adjust for the difference by moving the tubes down in the tree? Would you even want the same rake with the lead axle? If a 350x has no rake built into the clamps, what differences are there in the angle of the frame neck, compared to the 250R with 6 degrees of rake in the tree? Also how does changing the height in the rear to get more or less rake affect handling compared to just changing the rake on the front? Just trying to wrap my head around all of this information

[Jason Hall]

I have ran 250r's with the axle flipped leading for drag racing at the dunes. The front end will push and not want to carve and turn. Trail is measured like in the pics I posted below. With rake built into the clamps, the forks are on more of an angle than the steering head. By flipping the axle leading with rake you would make the trail measurement to long. The first pic shows the difference in the steering head angle and the fork rake angle. So imagine what a leading axle with rake would do to your trail measurement. Not only would the trail be to long, but it would lengthen the wheelbase and make a 3 wheeler more tippy.

[Jason Hall]

Raising and lowering the rear suspension will change the angle of the steering head/forks.

[Jason Hall]

One thing I keep forgetting about with a motorcycle is a lot of turning a 2 wheeled motorcycle is done by Leaning the bike, so that would probably change quite a bit of my thinking as I seem to concentrate more on 3 wheelers. Lol..