Monoshock help

[Ngreen]

So I have been fiddling around the idea of doing a monoshock instead of the semi stock rear end I have planned. In theory it seems like I could put a brace in the swingarm before the tire and a place to connect on the backbone just under the front of the seat? Am I mistaken and not calculating in some important feature? I have been looking and sEea lot of stuff about different linkages and stuff. Please tell me if I'm about to mess up big before I do this. As always thank all of you for lending some some of your much saught after knowledge!

 

[Yamaha_chop]

It's a fairly straightforward process, but you need to decide a couple things.

If you want to go with a linkless setup you need to use a shock that's setup for that already. If not, you'll want to get the links that go with the shock you choose.

Once you have the shock in hand, you need to place the shock at various points along the swingarm to find the correct ratio. This generally requires tacking it in place and sitting on the bike with the motor in it and bouncing up and down a bit. Not very scientific, but it'll get you close. I'm sure there's a mathematic way of doing this, but I'm not sure what the formula is.

 

[Ngreen]

What does the linkage do? What shocks don't need a linkage? Thanks for the help

 

[Yamaha_chop]

If you're going to jump into this, those are the sort of things you need to do your own research to figure out. If you don't understand why things are the way they are then you're not going to get it right.

 

[member3480]

You need to have the linkage if you are doing a true mono shock or you can just mount a shock on the bike but its not going to ride like a mono. There are other lings with same question. Here is a easy one to do GPZ550 MONO on XS FRAME and second picture is kind of what you want to do. This was a KZ 1000 with mono shock.

 

[1974jh5]

There's a lot of engineering which has to go into this. The #1 reason for using a link setup: if the shock is direct mounted at the very front of the swingarm, the spring will have to be STUPID stiff or the rear suspension will be so soft it won't even hold itself up. I'm talking 600-800 inch pounds!

For those who care about the numbers: it's about 4" from the center of the swingarm pivot to the only real place you can put the bottom shock mount. The swingarm is ~18 1/2" center to center (it varies a bit because of the axle movement for chain adjustment). That leaves 14 1/2" of 'lever' sticking out. Divide the short measurement by the long measurement and there's the motion ratio (14 1/2" divided by 18 1/2") =.783 motion ratio. Square .783 = .613 and use this number to calculate the 'wheel rate', which is what holds you and the bike off the ground. So if you use a 800 inch pound spring and multiply it by .613, there's your actual 'wheel rate' of 490.4 (and that's probably about where you need to be for a firm suspension that will allow the bike to handle without wallowing). 600 inch pounds becomes 368 at the wheel.

So if you grab a shock with, say, a 500 inch pound spring it's about 315 at the wheel, meaning most average guys would bottom it just by sitting on the bike. Add your GF and you are toast. Cranking in a ton of preload does not fix this problem.

Next is the amount of travel. An XS will stand about 3 1/2", maybe 4" of travel before weird things happen. (Not the least of those is the chain will get so loose it could come off easily, that's due to the swingarm pivot being so far behind the countershaft sprocket.) So if we take that .783 motion ratio and subtract it from 1.0, we get .217. That's the ratio of shock travel to wheel travel. So if we take 4" and multiply it by .217 we get .868 inches of shock travel. To direct mount a shock at the only real place to do it would require a shock with a max of that .868" travel. That's a rare beast. Also, the shock shaft speed is so fast and moves such a small amount due to this ratio that the internal damping mechanism becomes very hard to engineer properly. The Formula 1 engineer guys do it in a few mm of travel, but chrissakes look at their budgets!

The motion ratio also makes it VERY important that the swingarm be properly braced. As osteoderm has shown in his well thought out setup, it's certainly possible but not real easy. In fact, I've been looking at early '80's Honda XR swingarms, they seem to be very similar dimensionally to a XS arm. I need to get my hands on one that's off the bike so I can get accurate measurements.

The linkage setups are designed to 'speed up' the motion ratio so that the 3 1/2-4" at the wheel becomes more like 2- 2 1/2" at the shock. Now you can use a much softer spring (FWIW a stock R1 has a 500 inch pound spring, a R6 a 550 inch pound) and a shock with longer travel, meaning the damping is much easier to attain. That's the main reason for adapting existing linkage; taking advantage of what the factories spent millions of $ designing.

 

[Jack]

So I have been fiddling around the idea of doing a monoshock instead of the semi stock rear end I have planned. In theory it seems like I could put a brace in the swing arm before the tire and a place to connect on the backbone just under the front of the seat? Am I mistaken and not calculating in some important feature? I have been looking and sEea lot of stuff about different linkages and stuff. Please tell me if I'm about to mess up big before I do this. As always thank all of you for lending some some of your much sought after knowledge!

It's not all that difficult but if you stick with Yamaha accessory like FZ or FZR it's a straight up modification. you need to find the stock steering angle of 27 Degrees or lesser for quicker steering,then depending on type of swing arm being used,you'll be mostly limited to 8 degree S/A angle(Depends on S/A arm height) ,that's about as steep as you can go before chain interference to S/R becomes a problem. After you establish S/A A,you'll need to locate a mounting point for the top shock eye(If FzR design you want the shock centered between the V that connects to the backbone but the mounting location depends on s/A A( The shock needs to mounted straight up and down),once that's all established,you want the bottom pivot link leading to the shock eye to be around 60 to 62 degrees angle,nothing less. All these degrees and mounting points are critical in allowing free movement without binding up for the XS. On the old style FZs(Aluminum Arm)there's some cutting and welding required of the Dog Bone linkages for fitment but other than that,it's not to bad of moc-up that you could'nt handle,at least I would hope so

 

[osteoderm]

What does the linkage do? What shocks don't need a linkage? Thanks for the help

Just in case 1974jh5's (entirely correct and thorough) post has left your head spinning:

The linkage makes up for the leverage that a conventional shock looses when it is moved from the far end of the arm to the near end of the arm. There's also piston speed/travel and variable spring-rate concerns, but it's the leverage issue that kills most linkage-less monoshock schemes.

If you're just trying to get the general idea into your head without doing a bunch of math, take a good look at as many different stock linkage suspensions as you can. Google is great, but better yet, go out and find a few modern bikes and check out the linkages. If you can find someone willing to let you watch as they cycle the suspension a few times, that's even better. Seeing these things working in person really helps!

 

[Ngreen]

First off wow. I didn't think I needed to pay attention that much in highschool. Haha. But this has been getting my mind spinning and this is the stuff that got me into this in the first place. Getting an understanding. So I'm very much considering taking this road. I have been trying to get the bike mocked up to try to approximate how long of a shock and what not I need. I understand that a little movement at the shock will translate to much more movement at the axel. The way I have my seat rails set up (still not welded so could change) I can only have maybe 3-4 inches travel in the axel. I don't mind if it is a rougher ride. Is this feasable? Also it may make a difference that I'm a lil on the hefty side so will be running a bit of preload to compensate. (at least that's how my understanding of this new field of suspension seems to lead). Once again all of you have been so helpful. I think I have started getting my head wrapped around this stuff. Hopefully

 

[osteoderm]

Ngreen, you'll need more than "a bit of preload to compensate"; with 20" of leverage, it would take the spring off a truck to keep you from bottoming out. Offhand, I can't think of a motorcycle coilover that is stiff enough.
Some people use two conventional shocks at the front of the swingarm, but even if you have enough spring, those shocks are not valved to perform as intended, making for a poor or possibly uncontrollable ride.
The linkages can appear complex and intimidating, but they're not just some fancy extras; they're critical to making a typical monoshock setup work.
Many well-intentioned folks have gone down this road before you, done a ton of great welding and fabrication, only to have their bike effortlessly bottom-out the first time they sat on the seat, no matter how much preload.

 

[1974jh5]

KTM has a linkless rising rate monoshock system. But there were some compromises, such as where the shock was placed on the swing arm. That shock placement is VERY critical; KTM engineers worked on the problem for something like 3 years before it was ready for production. Along the way, they had to throw out all the conventional damping mechanism and start from scratch. It's like anything, it can be made to work (and KTM's setup works VERY well) but it may be difficult for the average guy to reproduce in a home shop.

On springs: once you get over about 600 or so inch pounds you start getting into race parts territory. The springs on most of these monoshocks are 2 1/4" ID. It is TOUGH to find a really stiff 2 1/4" spring that's short enough to fit them, you would probably have to have them custom wound. 2 1/2" is pretty easy to find but you will have to make your own spring mount parts or adapt them from something else. Plus, if the shock reservoir is in line with the shock body there probably won't be enough room to run the 2 1/2" spring. I learned that one the hard way.

 

[member3480]

You got a lot of good input on this and everyone has gave some great ideas. When you build it you got to take into fact that weight will be the final out come. I have done about 15 mono conversions most on Drag Bike builds i have done on older KZ or GS that i changed to mono for look and performance. Look is one thing but when you are trying to put as much power to the rear wheel and you want a long stretched frame it all has to work because power in the long run will make or break you. So the easy ones are the complete switch SWINGARM/WHEEL/SHOCK and when you get it all setup with motor in and rider sitting on it you will know what you got. Its great to add all the math in and figure out how much travel you got but sometimes all the work you do it still doesn't turn out how you thought it would. I have done it and had to start all over. I tried to use two Harley shocks the same way a soft tail is done on a bike but didn't do my homework and found out that the motor weight / angle / stretch all didn't match up and the bike dropped 3" when you sat on it. So i am not downing anyone for doing the math but make sure you take all the unknowns into play and i think that is what everyone here is telling you. I have done two GPZ550 setups and both work good and i was not the first to do it. I found one done on another site and its great for a cafe bike HEIGHT/TRAVEL/HANDLEING all good. I have also made some crazy ones but ride wasn't there . I was after look not ride. So take all that was said and build a kick ass bike BUT!!! i want a picture posted.

 

[Ngreen]

Haha I'm waiting until I have something that resembles a bike again and then I will be doing as much of a full writeup as possible. Can't leave all of the guys who have helped me out so much just hangin!

 

[1974jh5]

If I can find another cheap bike, or a cheap titled frame... (heads over to the Classifieds)