Technical Discussion Topics related to Technical Issues

Rake and Trail

Old 10-16-2010, 03:00 PM
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Rake and Trail

As I've seen a few posts lately about folks making some radical changes to their bikes geometry I though it might be a good idea to post up the basics of how a bikes front end actually works and what you are changing when you raise or lower the forks in the triples or add (or take away) ride height in the rear.

While I will be using bike related examples, these same principals apply to anything with steerable wheels, from cars to jet aircraft or even a shopping cart.

The main thing to look at is "Trail" which is the distance by which the axle "trails" the point at which the steering axis (not the fork axis) of a motorcycle contacts the ground.

Trail is required to ensure a steerable wheel is stable in following the steered direction. You can visualize trail as a lever arm that tends to center the steered wheel behind the steering direction. More trail means a longer lever arm and greater centering force (or stability).

Motorcycle steering geometry is determined by four measurements:

Rake - The angle of the steering axis (where the triple clam mounts to the frame), measured from vertical.

Tire Radius - Half of the tires outside diameter.

Offset - The distance from the steering axis to the fork axis

Trail - The distance from where the steering axis contacts the ground to where a vertical line through the axle contacts the ground.

When you change rake, steering offset or tire radius, you change trail and the relative stability of the bike. More rake angle gives more trail. Less rake angle gives less trail. More offset gives less trail, Less offset gives more trail. More tire radius (a bigger tire) gives more trail. Less tire radius gives less trail.

All pretty simple measurements and their relationship is the basis of a motorcycles steering behavior.

So why mess with the factory settings?

Until recently streetbike manufacturers were pretty conservative in choosing steering geometry, consistently compromising in the direction of stability. Which makes perfect sense, steering geometry may be pretty simple but stability is not. It includes many things including strength and stiffness of the structural parts, tire characteristics, weight placement and many other things. So it is best to stay away from steering geometry that might cause instability in a complex system.

With this information setting up a bike for the track or even getting a streetbike to turn easier involved reducing trail or trying to make the steering geometry less conservative.

Normally this is accomplished by sliding the fork tubes up in the triple clamps (this steepened or decreased the rake angle and shortened trail) Increasing the rear ride height steepened the rake even farther and reduced trail even more (or lowering the rear will increase trail)

So feel free to experiment with settings, just remember the less trail you have, the more unstable the bike becomes. Make small changes, as your life does depend on it......

Also manufactures no longer are as conservative with there steering geometry. Many late model bikes are close to the limit of stability as delivered, so much more care is required in making changes to these bikes (The SH is not one of these, but be careful making changes on late model sports bikes).
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Old 10-16-2010, 05:24 PM
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Personally, I like my "triple clams" steamed with lemon and garlic.

But seriously Mike, great write up. Steering geometry is not something to play with lightly. Look at the rake and trail of a ProStock drag bike, compared to a MotoGP bike. If switched, neither would work for their intended purpose.
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Old 10-16-2010, 06:04 PM
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Hell I'm just an old Marine.... you guys are lucky it's not just grunts and snarls.......lol
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Old 10-16-2010, 06:17 PM
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Yes, truely -Us Marine don't make easy instructors or psychiatrist.. Want a tissue- YA CHUCK WAgon ! ! !
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Old 10-16-2010, 06:20 PM
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Originally Posted by 1971allchaos View Post
Yes, truely -Us Marine don't make easy instructors or psychiatrist.. Want a tissue- YA CHUCK WAgon ! ! !
That's JackWagon you crybaby....... lol
Best commercial ever....... Go R. Lee
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Old 10-16-2010, 07:23 PM
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Good info 8541!

I raised the rear via 6mm shim and raised the fork tubes lowering the front effectively changing the geometry quite a bit. I left it like that for a while, but then got JD's f4i modded longer shock and raised the front end accordingly to approximate oem front-rear stance. I'm happy with that set up of about a year.
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Old 10-16-2010, 11:15 PM
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I am still tweaking my geometry as the SSS really needs a longer shock which I havent put in yet, and the 1000rr forks are shorter than stock. Its not bad now but will improve with the adjustable length penske (I hope). I think the swingarm should not be so parallel with the ground. (my theory)
Attached Thumbnails Rake and Trail-7.26.10-vtr-002.jpg  
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Old 10-17-2010, 10:11 AM
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I just took out 4mm out of my rear fox shock to lower with the 1000rr front and found out that the guy who owned my bike before had the shock all the way raised and actually put an aluminum block between the top of the shock and the frame to get it even taller. That guy was 6'6'' though but having the rear that high severly affected the geometry.
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Old 10-17-2010, 10:55 AM
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The Al. block is the common way to raise the rear ride height on a SH.

The whole point of writing this was to get guys to think a bit about what they are actually doing when making changes.

I guess it's my "Gray Beard" moment from hearing people talk about Major geometry changes (i.e. Oh I raised\lowered the front\rear 1.5 inches ) and trying to pass some information along which might help keep someone out of the hospital, or worse.

Which lead to, might as well get all the basics and terms posted as a reference for talking about chassis set up and what you are actually doing.

Personally, I make changes 5mm at a time and only 1 change at a time.
Anything more and you are asking for trouble.

So don't be afraid to make changes to get the bike to handle the way you like. Just understand the importance of Trail and what happens if you end up with to little. It's not a pleasant experience.

Also there is no "set" number to look for. Everyone one rides different, so one persons set up numbers might not work for you. There are upper limits to stay in though before things have a good chance of getting "interesting".
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Old 10-17-2010, 11:41 AM
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The easiest way to understand trail is thinking about a shopping carts front wheels. Because the "axle" trails the "forks", the wheel is self-centering. Next time you go to the market look at one and think how it works. If you pop a wheelie with the cart (imagine increasing wheel size to maintain contact patch) you have changed the rake to a point where it negates the self-centering affect of the trailing axle setup because you have decreased the angle of trail.
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Old 10-25-2010, 07:56 AM
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It's been a while since we've had this discussion. It's a good topic to bring up and keep in mind.

I prefer to keep the front end at the stock height and raise the rear. My rear shock is 10mm longer than stock, which is about 30mm at the rear axle. I have an SP1 fork, with the normal sag numbers, which is longer than the SP2 fork with a long top-out spring. The steering is very light and much improved over the handling when I got the bike.

The longer shock length increases the swingarm angle, increasing the anti-squat character, which tends to extend the rear supension and helps keep a load on the front under acceleration. Keeping the front end high maximizes cornering ground clearance.

I am at the point where the front end wags down the front straight. That's partly because the thing wants to loft the front wheel, but also because I have reduced the trail. If I keep my weight forward to keep the front end down, it is better. I also had to add more steering damping. New triple clamps, with 1mm less offset are next, to get back some of the trail I lost when I steepened the head angle.
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Old 10-25-2010, 10:49 AM
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Originally Posted by RCVTR View Post
It's been a while since we've had this discussion. It's a good topic to bring up and keep in mind.

I prefer to keep the front end at the stock height and raise the rear. My rear shock is 10mm longer than stock, which is about 30mm at the rear axle. I have an SP1 fork, with the normal sag numbers, which is longer than the SP2 fork with a long top-out spring. The steering is very light and much improved over the handling when I got the bike.

The longer shock length increases the swingarm angle, increasing the anti-squat character, which tends to extend the rear supension and helps keep a load on the front under acceleration. Keeping the front end high maximizes cornering ground clearance.

I am at the point where the front end wags down the front straight. That's partly because the thing wants to loft the front wheel, but also because I have reduced the trail. If I keep my weight forward to keep the front end down, it is better. I also had to add more steering damping. New triple clamps, with 1mm less offset are next, to get back some of the trail I lost when I steepened the head angle.

That's pretty much how I have mine set up also. I am using SP2 forks but I have cut top out springs so the length is close to the same as an SP1.

Though I believe the way mine are valved does make a difference as I can still run without a steering damper and no noticeable wag in the front end but I also haven't taken it to top speed with the new front end, so things might be different at those speeds.

I still have to finish (though I am real close now) getting the suspension sorted before I'm ready for a top end run. I have a decent base line now, just need to take some time and fine tune it a bit.

I am also looking at changing the offset but those Attack triples are kind of pricey......but that 1mm change in offset does sound like a real good thing with this set up, so I guess I need to save up for a while.
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Old 10-27-2010, 08:27 AM
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Check your alignment!

Well...
I solved my little front end instability problem.
I hate to admit it, but I had not taken the time to do a precision alignment of the rear wheel. I always thought it was a good idea, but figured close was good enough for now.

So I set up the string last evening and the rear wheel was misaligned by a country mile! I was using the marks on the swingarm, but I didn't even have that right.

I like the string method. It takes some time, but I got the wheel aligned within 1/2mm at the leading edge of the front rim. Well worth the time and effort, IMO.

So my triple clamp offset is fine, for now.
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Old 10-27-2010, 09:16 AM
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Originally Posted by RCVTR View Post
Well...
I solved my little front end instability problem.
I hate to admit it, but I had not taken the time to do a precision alignment of the rear wheel. I always thought it was a good idea, but figured close was good enough for now.

So I set up the string last evening and the rear wheel was misaligned by a country mile! I was using the marks on the swingarm, but I didn't even have that right.

I like the string method. It takes some time, but I got the wheel aligned within 1/2mm at the leading edge of the front rim. Well worth the time and effort, IMO.

So my triple clamp offset is fine, for now.
well that's big of you to admit being stupid. It also is good for me because it reminds me I'm not the only one.haha

I'm surprised you never take your hands of the bars going downhill or comin up to a stop to see if you're tracking straight. Tell you later what i did recently with my dual sport.

Anyway, once you have everything lined up as you do now, and have verified tracking with hands off, stick a rule into the swingarm slot behind the axle to see if each side measures the same. After you are able to verify that equality, you no longer need a string unless you mess with the front or have a tip over. I try to remember to always check myself on because it's too easy to screw up. So when you adjust the chain, you can measure the slots and then see how the marks align. Once you have established a relationship, you have a couple easy ways to check for alignment and then verify with hands off.

Now for some of my stupid ****. I bought an xr650l to ride(in '02' mind ya) when i totaled my SH. Well after 8 years and some 12,000 miles, i finally took my hands off the bars and guess what.............tracked off to the left, and with authority. haha. Well i've been trying to find out why for a dam week now and decided that i can't find a problem. Checked the wheels for true, forks, removed fork brace, wheels, swingarm, and steer bearings, and of course aligned the wheels from front to back and back to front, all to no avail.

The only thing I can figure causing this trailing off to the left is that the front wheel is offset between the forks to one side about a 1/2"(brake on one side and speedo drive on the other.
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Old 10-27-2010, 10:23 AM
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Yeah, I tend to learn by doing stupid s#!^.
I have taken my hands off the bars and it tracked ok. It was something that only comes in to play at high speed and hard on the gas. As soon as there was some load on the front tire, it went away.

It sounds like your XR650 frame, or swingarm might be twisted.
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Old 10-27-2010, 05:36 PM
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Originally Posted by RCVTR View Post
Yeah, I tend to learn by doing stupid s#!^.
I have taken my hands off the bars and it tracked ok. It was something that only comes in to play at high speed and hard on the gas. As soon as there was some load on the front tire, it went away.

It sounds like your XR650 frame, or swingarm might be twisted.
I don't know but a friend has an KLR 650 and it drifts off to the left also. The KLR has an offset front wheel like mine.

I don't think we have much choice. If we're to dumb to figure things, we'd better learn from the trial and error method or perish huh..haha
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Old 10-27-2010, 10:10 PM
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Or just soldier up, like Casey Stoner and ride around it, rather than fix it.
If the *** end of my bike acted like his, I would just park it.

Sounds like your friends frame is twisted too. Maybe it happened during heat treatment.

I'm going to have my bike measured, so I know if my frame is straight and what my trail is, with the chassis geometry and tires I'm running. You can't be smart, if you don't have enough information.
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Old 10-28-2010, 04:34 AM
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+1
I should have learned this long ago when I had a high side resulting in a broken collar bone and a bent frame. Unfortunately, i didn't find out about until a month later during the next race when my ducati went into a speed wobble on the last lap ejecting me like popcorn being blasted out of the pot. painful ****!

It's amazing how easy it is to do stupid **** like this. A more recent example: I rode the xr650l for months and finally thought the front fork oil must be to heavy. In the process of changing it as I was pumping the forks to extract as much as I could, the left fork stuck fully compressed. The only way i could get it to extend was to loosen the fork brace which, by the way, really wasn't cranked tightly.

The point is that I was experiencing bump induced wobble on high speed curves and I just kept riding for months until finally I had enough of it to do something about. Guess I don't learn very fast somtimes. doh!
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Old 10-28-2010, 08:09 AM
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It shows how important the details of chassis setup are, when your goal is to ride near the limit of the chassis. Not that I can claim that ability, at this point.

But as I continue to tip it in harder and get on the gas earlier and more aggressively, I can't afford to be spit off by some lurking instability, let alone tucking the front end because of not enough trail-induced feedback to the bars.

There are lots of people out there going pretty fast, with crappy setups, but the truly fast guys all have their bikes dialled.
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Old 10-30-2010, 09:00 AM
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Here's a simple, sraightforward method for measuring chassis alignment and determining whether your frame or swingarm are twisted:

http://www.tanoshi.net/motorcycle/chassis_part1.pdf
http://www.tanoshi.net/motorcycle/chassis_part2.pdf

I've been searching for frame measurement ideas. My original intent was to have the bike measured by a Computrack dealer, but the closest one was 2 hours away and they have sold theirs. It would have meant a day off work and a bunch of $.

I then started looking for precision work tables for taking measurements - nothing. So I've been trying to come up with a design for one that I can build and set up in my garage. I don't need ultra high precision, but I would like to be able to measure to +/- 0.5 mm. It needs to be dead level and pretty damn flat.

I have a friend who is a cabinet maker. He has a big tablesaw that we can use as an assembly table. My plan is to build a box out of marine-grade plywood and assemble it upside down on the table. I'm going to put ribs inside to hold everything dimensionally stable. There will be extra ribs where the tires sit. I'll seal everything with polyurethane to keep it form absorbing moisture.

I will set the assembled box, with the top off, on the floor and shim it up so that it is precisely level, then grout (or bondo) the gaps at the base of the ribs and sides, so that it is rigidly attached to, and supported by the floor.

Then I can check the top surface with a straightedge and sand down any high spots and fill any low spots, but it should be pretty close. When I attach the top, I should have a servicable work surface to measure from.

I'm going to use screw-adjustable jack stands (trailer leveling stands) to support the bike under the footpegs, without taking the sag out the suspension. Once I get it set up and the wheels aligned, I can take reasonably precise measurements of rake angle, swingarm angle, etc. Once I have the meaurements, I can see where I'm at, in terms of trail. I think I need to take 2-3 mm off my offset, to increase my trail, but it will be best to know where I'm at, where I've been and where I'm going.

Any suggestions?

I'll post up plans and photos, as I go, if anyone's interested.
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Old 10-30-2010, 09:18 PM
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Get that sum bitch ready. I'm riding out there next summer to be your first customer. The only deterrent might be when I get into those big mountains out there past mile high, I might start going north and south till I run off one of those mountain cliffs from sheer joy. Always wanted to do the rockies up into canookville and back down.

My SH seems pretty good right now since I replaced the triples, But the XR650L is giving me fits. I spent hours so far with the strings, wood, bars, and rods, rules and other various measuring devices and all i do is confuse myself(not hard to do..haha).

those links you posted have restored my hopes. thanks.
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Old 10-31-2010, 12:54 PM
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Originally Posted by RCVTR View Post
Any suggestions?
Yeah, I have one..... let us know when it's done......

I am also curious what the real numbers are on the set up I have right now.

The one thing I do wonder though is if the guys making radical chassis changes bother to read this stuff.

The latest example of someone not clear on the concept was this guy.
https://www.superhawkforum.com/forum...ad.php?t=24093

When you first read his post it sounds like a simple, on the brakes too hard crash.

Then if you look at the pics of his bike (I can't find that thread now) and read some of his other posts you see his forks pulled way up in the triples.

The you read that because he is a shorter guy, he has pulled the forks up 2". and is looking for a way to lower the rear around 1"

I don't know if he ever got around to lowering the rear but either set up is rather poor to keep this nice.

I did try to get him talking about his set up when I replied to his tread but have not seen any reply.

I just wonder way it took so long for the bike to throw him on the ground. pulling the forks up over 50mm??? I can't even begin to think of how small a trail number he was running. Even if he did get the rear lowered that is still 25mm, which is still a very small trail number. I will say with the testing I have done that 15mm is the max you can go on a SH (even then you sacrifice way too much ground clearance) before the bike gets "iffy" when pushed hard. Then if you raise the rear 5mm with the forks pulled up, your trail is about as small as you would want for a street set up.

I was running the forks pulled up 10mm and a 6mm ride height spacer with the stock suspension parts. It did work well (as I don't like to run a steering damper on the street but that is a different discussion) now with the new suspension bits I have gone back to the stock height in the front (or as close to stock as you get with a set of RC51 forks) and 10mm of ride height in the rear.

Right now it feels good. Then again I haven't finished testing it out yet. It does take a while as you have to start out slow and then work up to speeds. As another point that is hard to get folks to understand is, yes it might work well at 40-50 mph but that does mean something weird won't happen at 120mph hard on the brakes......

Once again, when you change things, make small changes and test them out. If you don't do this, then buy some damn good gear, because you will be testing it out.

So with that it is easy to see that when he slammed on the brakes and compressed the front end, steepening the rake and reducing the trail even more, that he went *** over teakettle down the road.

I'm still trying to figure out a way to get through to some of the newer riders. While messing with suspension geometry is a good thing to get the bike set up for how you ride. You also need to understand what you are changing and also the consequences of making those changes.

After you have a idea how things work, then make small changes. The rule I go by is 5mm at a time. Then go out and test the bike. Not just a run around the block but go out and really work it. That is the only way to know if you are still safe.

On this bike, if you end up with the forks pulled up over 15mm and the rear up over 10mm or any combination of the 2 (like I stated earlier, my set up was 10mm front & 6mm rear with the stock components but I believe that leaving the front stock and going up 10mm in the rear is a better set up as you give up too much ground clearance with the forks pulled up and have the rashed header to prove it..... ), you are playing with fire.
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Old 11-12-2010, 09:42 AM
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I've been studying and learning about racebike setup and have learned a fair amount so far and have a lot more to learn.

I have had hints that swingarm angle was a very important consideration, but have discovered that chassis setup pretty much begins there. So you start with the tire radius and set the swingarm pivot height, to give the desired angle. This defines the anti-squat behavior, for a given torque applied to the rear wheel and fina drive sprocket sizes. That sets the position of the rear suspension at maximum acceleration, when the front wheel is unloaded. I think, but I don't know for sure yet, that you want the loaded swingarm angle to be near the neutral point between squat and anti-squat, so that the rear suspension has good compliance.

From there you can work on rake and trail numbers - since not everything is adjustable, you are basically finidng the best compromise.

I purchased some software by Tony Foale (www.tonyfoale.com). It is available on ebay for $59. It is a beautiful piece of work. You can enter all of the geomtery info, including rear suspension linkage position and geometry and plot things like rear wheel force, vs. suspension position, rake a trail under full acceleration and full braking, etc.

As soon as my work table is finished, I'm going to compare stock rear suspension geometry, with the Moriwaki linkage and increased shock length, spring rate, etc.

If anyone wants to take measurements of the Super Hawk, I can provide a list of the necessary measurements and create a model of it. It may help to better define the necessary geometry for the rear suspension dogbone for stock and modified swingarms, as well as establishing a good baseline for people making modifications.

I will be happy to do the modeling. It will add to my understanding and hopefully increase the knowledge base of everyone who is interested.
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Old 11-12-2010, 09:55 AM
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RCTVR, I have a single sided swingarm on my vtr with a cbr1000rr front end. A race mechanic friend told me I need a longer shock to create a downward angle on the swingarm. Can your geometry gismo account for this? I have been slowly tweaking the bike for handling but it still needs work. Any first impressions?
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Old 11-12-2010, 10:47 AM
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My first impression of the software is that it's frickin' cool. Very complete and has great animated tutorials. Worth way more the it costs.

We can get as detailed as you want with your swingarm angle. The basic measurement is:
SIN(swingarm_angle)= (swingarm_pivot_height - rear_axle_height)/swingarm_length.

If you want to quantify anti-squat, you will need details of all of the rear suspension dimensions, including linkage lengths, attachment points, front sprocket location (from swingarm pivot), sprocket teeth, chain pitch, etc.

Most of the time, people just shim the rear shock, to get some swingarm angle. But to quantify it requires some detailed measurement.

I was building a spreadsheet to make all of these types of calculations, but the software does it all and a lot more.
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Old 11-12-2010, 11:18 AM
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Well you know I'm interested......
I've reached a point where things are starting to work pretty good but it really would be nice to know where I have ended up at so I can make some informed choices on what to do or try next.

Now you also mentioned Mori linkage..... were you talking about the RC or the SH?

SO now it's off to the web site you posted to do some reading.
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Old 11-12-2010, 01:21 PM
  #27  
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The users manual for the software is available in PDF for download. It covers the main features of the software.

I have the Mori linkage for my RC51. They made some for the SH, but I think they must have made about 5 of them. I've only seen 1.

But my thought was, if I can understand the behavior of the RC linkage, I can design one for the SH, if I have all of the rear suspension geometry. There is a need for retrofit SP2 swingarm linkages as well. And a VFR SSS linkage.

Smokinjoe - the downward angle for the swingarm on an RC51, with stock TC offset that was recommended to me was about 10.8 degrees from horizontal. I'm not to the point where I can say why, but I trust the source.
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Old 11-12-2010, 01:47 PM
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Something to keep in mind...

when you install a shorter fork, or raise the fork in the triple clamp, it drops the nose of the chassis, and it also drops the swingarm pivot, which decreases swingarm angle. To get the swingarm angle back, you have to raise the rear ride height, not lower it, thereby decreasing the rake angle. It all ties together.
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Old 11-14-2010, 10:52 PM
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I have a almost stock setup. just a shim and droped the forks other than springs and valving in the front and a new shock in the back I would be glad to give you any mesurements you want as i am planing an going with a sp2 swingarm instead of the braced swingarm if it is true that the longer swingarm will put more weight on my front wheel. The hawk does not drive to bad now but if it can be better when I have to make the swingarm stronger becouse of the rebuilt engine might as well go in the right way to make it handle better at the sametime.
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Old 11-15-2010, 09:34 AM
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First of all, here's a better answer about 1st impressions:
The software is a very good and flexible modeling tool. It allows you to analyze various rear suspension linkages - which basically multiply the suspension travel over the shock travel. You can tune the ramp rate, by changing rocker geometry, compare with other known geometries which are known to work well and design the mods which will get you close to that.

In order to model squat/antisquat and attitude under braking and acceleration you need to measure the center of gravity, which you can get by measuring the F/R weight distribution with the motorcycle horizontal and again with one wheel up on a pedestal.

You can get as detailed as you want, as long as you can take measurements accurately. I may end up dropping the engine out of the chassis to measure distances from the swingarm pivot to the suspension mounting points. I can also measure the unloaded geometry of the rolling chassis at that time. Something you can't do with the SH chassis, because the swingarm mount to the engine case only.

Stumpy, if you can take all of those measurements - I can send you a sheet to fill out - we can look at the suspension rate and modify the rocker geometry for better track performance, based upon how the RC51 rear works.

This is assuming that you will never carry a passenger. The rising rate of the stock suspension rocker has increasing leverage at the top of travel, to prevent bottoming when riding 2-up. The modified racing linkage has more initial leverage and less rising rate.

This is all more detailed than most people need or want to get. For me, the knowledge and understanding is a worthwhile pursuit.
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