Suspension Travel, Ride Height, and Other Normals

[mathil]

Hey all,

I'm in the midst of building a monoshock XS650, and I have a few questions...

As I have it set up now, the "ride height" at full height (no rider or bike sag yet) is 5". I can limit the travel to whatever I like, so the bike could have a minimum, suspension compressed ride height of 2", 1" 1.5", or any other conceivable number. "Ride Height" is measured from the lowest point of the frame rails to the ground.

The more I walk by the bike in my shop as I'm doing my daily work the more I think it should probably be a bit higher. I mean, one COULD probably live with a bike that has such suspension values, but how FEASIBLE is it?

Are numbers like 7" full droop, 5" travel and 2" fully compressed a better number to aim for? All it takes for me to change this at this time is to move the top shock mount, a 15 minute job. I'd rather take input from more experienced people now while its easy, rather than spending much more time down the road to fix it...

Obviously it's a balance between form and function... Who doesn't like a low bike? I'd also like to be able to lean this thing a little, and take the odd bump once in a while without worrying about the frame bottoming out.

Thank you for any and all input, this is a great place to hang out and learn!

 

[teazer]

Of all the numbers to think of in terms of suspension and more important handling, the least important is what was referred to as ride height and is just the ground clearance between the lowest part of the bike and the ground.

Most important are Rake and Trail, followed by swing arm droop and center of gravity.

For example, the frame could be modified to that the motor sits higher but the rest of the dimensions and angles remain the same. That will change the C0f G and if the sprocket were in the same place, that would be all that changed.

The best places to learn about this stuff is probably Tony Foale. His book isl available from his web site http://www.tonyfoale.com/. John Robinson's book on motorcycle Chassis is another good source of information.

Start with the chassis as it is and with teh foorks yand wheels you want to use and then raise and lower the center of the bike and measure rake and trail and swingarm droop. Then decide what handling characteristics you are looking for and start to lock in teh data. For example, fats sporty handling - say 24.5 degrees of rake, say 100mm of trail for stability, say 10 degrees of droop. Can your chassis get there or do you need to change forks or triple clamps or are those numbers not possible.

If handling is secondary top a low mean look, it really doesn't matter (within reason) what you get. The lower the bike, the less suspension travel you can use. So for example if you start with say 5 " of ground clearance unladen, and the suspension has say 4" of travel, it will be 1" off the ground fully compressed. When yoiu try to go around a corner at any sort of speed and teh suspension is partially compressed by centrifugal force and you hit a slight bump, 1" is not enough to allow more than a few degrees of lean angle, so teh wheels get levered off the road and the bike crashes. Sub optimal.

For a super low bike that is not ridden, 2" is a good clearance number to work towards. It looks tough but is not practical. For a more sporty ride, lean angle is more important than a low stance, so the width of the bikes as well as the height start to come into play.

And just to complicate matters, a low C of G requires a greater lean angle than a bike with a high C of G under the same conditions.

That's a long winded way of saying that asking for opinions on what magical ground clearance numbers sound right, probably will not get you to where you want to go. Read and research from people like Foale and Robinson or Bradley. You were smart enough to ask the question and I'm guessing that means you are smart enough to do more research and to come up with real answers.

Good luck with the project and please post a build thread. It would be most interesting.

FYI, here's a link to one of our projects - a 1975 mono shocked, reed valve GT750 Suzuki 2 stroke. http://pinkpossum.com/GT750/phattrakka/part4a.htm

 

[Swagger]

to the OP: Buy Tony Foale's chassis book. Seriously.....don't kill yourself.....

Teaze...cool site man! Love the little animation too.

 

[mathil]

Rake and trail will be within reason, as I'll be cutting the neck anyways. Tire size changed. It's not going to be a "turner" but I would like to be able to hit a bump on a long sweeping highway turn and not drag the frame.

Motor is staying where it was originally intended. I could move it, but that might be a project for down the road.

This isn't a "cafe racer" build, but rather a bastardization of several custom styles that appealed to me. 21" wheels front and back, and a tailsection (go figure...). It's not going to be a performer so chassis performance takes a back seat to rideability and aesthetics. If I wanted a turning, bike, I would have kept the smaller wheels and a near stock wheelbase.

I'll start up a build thread. I'm kinda leery of posting unfinished work, especially since i get a whole lot done, then let it sit in my shop while I do my day job to mull things over. Ideas perculate in the back of my head until I've decided what to do. I'll definitely start a thread though...

 

[chrisf]

There is so much more to this than just wheel travel. If you're just looking at wheel movement--which is wildly different that damper movement--I'd just measure what you have and keep it at that for the reasons the gents outlined.

--Chris

 

[mathil]

There is so much more to this than just wheel travel. If you're just looking at wheel movement--which is wildly different that damper movement--I'd just measure what you have and keep it at that for the reasons the gents outlined.

--Chris

I realize that. I am using a Yamaha R1 shock. It has a common sized spring so I can swap out a different rate once the bike is put together, assembled and weighed. The leverage ratio is quite high. I haven't measured actual wheel rate VS spring rate, but my guess is that as it stands its in the 2.5:1 range atm.

Keeping the factors close to stock is not possible. Putting 21" wheels on, as well as a monoshock means a complete rework. Nothing from the backbone back is the same.

 

[hillsy]

As far as the rear goes, make some adjustable mounts for the shock so you can experiment without having to re-weld the chassis.

The front can be lowered / raised to suit by sliding the forks through the trees.

I don't know where you are planning to ride this bike, but 5" static clearance where I live would be pretty limiting (crap roads).

 

[teazer]

to the OP: Buy Tony Foale's chassis book. Seriously.....don't kill yourself.....

Teaze...cool site man! Love the little animation too.

Thank you kindly. I was taking pictures at each step to see if I could see the differences and then it occurred to me that if I shrank the pics I could use a GIF animator. I had fun yesterday making my first C/F part - a set pan which will be hidden - that's why it was an ideal first project. I used pre-preg before on a TZ faring modification but this was a first for me. I'll take some pics and update the site some time"soon"