This tech article is a contribution from Matt, it tells the story of a swing arm modification, do not try this at home unless you really want to.
Also check out the suspention page in the toolshed
|First things first, finding the parts. For my bike I used a front end off of a 1990 GSX-R 750 and a rear swing arm off of a 1995 GSX-R750. I've heard the newer rear ends are much easier to use because of suspension linkage mount points. 1996 and newer mount below the swing arm similar to the GPz but the 1995 mounts on top of the swing arm which takes a little effort to make linkage that works for it. Once you find the donor parts your job will be easy. GSX-R's had 17" front wheels from 1989 and up I believe. The front ends stayed pretty much the same until they went to USD forks (up side down).|
The front end is the easiest part of the swap. First I attempted to use a late 80's GSX-R1100 front end but was discourage because it had an electronically controlled anti-dive setup and they were much softer than the 1990 750's fork and they did not easily accept 6 piston calipers. The 1100's fork are the same length as the GPz's, but the 1990 750's setup is 2" lower overall (including wheel). This is where we get a more aggressive rake angle. The only thing needed to fit a donor front end on your bike will be the steering stem. For my set up I used a donor stem that was the same as the GPz and custom fitted it to the GSX-R triple clamps. To do this I had to add material because the GSX-R stem had a larger diameter. The lathe I had access to would not have produced a quality stem so I choose to rework an existing one. I used a Tig welder and added filler to the base of the stem, to build up a larger diameter, then milled it down to spec so I could press it in to the GSX-R stem. Once you get the stem you have to make sure the dimensions are right so you wont have any bearing play. I had to shim the lower bearing up slightly to ensure a proper seat. Of course this will vary depending on the front end and stem of choice. There are a few companies out there that make front end conversion kits and stems to match but you are looking at over a grand when my front end only cost $200. Once the stem is in place and you have ensured proper bearing travel you need to decide on the next step. The GSX-R triple clamps have a different style steering stop then the GPz. The GPz had a block in the front of the headstock while the GSX-R's lower clamp had a block in its center. Obviously they are not compatible. Depending on how far you want to go a simple solution is to drill and tap the lower triple clamp and then thread Allen bolts to act as steering stops. Another option is to weld tabs. Whichever you choose I strongly recommend you use steering stops or your new forks will leave nice dents in your tank. Since the forks are shorter then the GPz's the only two options for handlebars are clip-ons under the top triple clamp or you can convert to handlebars. If you convert to handlebars you will need to drill the top triple clamp. I used clip ones from Graves under the top clamp because I was aiming for a better handling bike. By putting clip-ons lower then the top clamp you get more weight over the front wheel, which will help, for control. This is why most sport bikes are in the crouched position. The beauty of the front-end swap is you automatically get better brakes up front. I used Tokico 6 piston calipers, a 6 way adjustable brake lever with remote reservoir, steel braided race lines and full floating rotors, most which came standard on newer GSX-R's and are direct bolt-on's. If you find a set of USD forks you can still follow the same procedure and you will get an even better front end but you will be looking at spending more money. The old GPz front end housed a few things. The information center, ignition switch with steering lock and depending on your bike maybe a few others. I did not retain the upper fairing but from Takashi's swap he kept his so I don't see a problem except maybe with the handle bar or clip on set up you choose. Make sure you have the room if you plan on keeping the upper. You will no doubt have to do some customizing to find a place for your dummy lights and key. The GPz ignition switch will not fit in the GSX-R triple clamp without modification. All these are merely loose ends to be tied up to look however you like. I tried to mount different headlights and eventually went back to a single small bucket so not to offset the new forks. Soon I will be looking to go back to dual round headlights.
|The rear end takes a little more time and effort to do right.
The very first thing you need to do is measure.
The GSX-R swing arm worked out great for me.
This was important to me incase I ever decided to undo my setup. The inner spacer in the GPz swing arm needs to be taken out. The GSX-R swing arm has dual spacers inside its pivot, which works great to your advantage.
If you remove the inner spacer from the GSX-R swing arm you will see that it is the exact outer diameter as the GPz's spacer. This lets you use your stock swing arm axle and thus not having to modify your frame.
The GPz spacer is a bit different though. It is two pieces, one long piece and a shorter piece, which is about an inch long. You will need both of those pieces to be long enough. The smaller spacer has a lip on it, which is the right outer diameter so you must take this in to account when you install it in your swing arm to ensure proper fit.
Since the GSX-R swing arm has a larger diameter it will need to be filed down a bit because it does rub the engine case. What I did was install it and move it up and down a few times to see where it rubs and then remove it and file it down.
There are three places where it rubbed on my bike but it may be different depending on the arm you choose.
Also I used one with a top brace, this rubs the inner fender of the GPz, which will need to be removed and cut or you can choose to make a new one out of a different material. Now that your swing arm pivot is done and you've made sure that it doesn't rub anything you need to see how much play there is left and right. Its maybe ¼ inch thinner than the stock arm so it does need to be spaced.
There is really no way to measure this so it needs to be trial and error. I ended up just using a thick washer on either side and it fit fine. Once that's out of the way you need to start to consider linkage. If you've gone with a 1996 and newer are you may be able to use adjustable 'dog bones' as your linkage, if you have gone with a 1995 and older arm you will have a little work and some calculations to do.
This picture shows my first linkage, you can see how high the rear is off the ground. Refer to the figure below to see how my linkage was done. Because of the mounting point the linkage will have to have a bend in it or at least a notch to clear the swingarm. This is best visualized but all aspects need to be taken into account when designing this linkage because it will set your rear height. I suggest you have the rear wheel on and the front end done then use a jack if possible to adjust the frame telemetry before you design your linkage. I went through 3 different pairs of linkage before I finally got it right where I wanted.
If you can adjust the bike while you are designing you can see visually how things will look and at the same time you can take your measurements.
To make the linkage you will have to make a diagram. Take a measurement from the pivot where the linkage will mount on the uni-trak and the point on the swing arm it will mount to. You can approximate this dimension but the closer it is the better off you will be. Then you will have to complete a triangle with this dimension making sure your dimensions will fit. As you can see in the figure you cannot go directly from point to point. Remember that if you dropped your front end you need to raise the rear end to compensate. If you drop the front end and don't raise the rear your bike will be a dog in corners and it will run wide and turn slow.
As you can see on my bike the rear end is very high. It is higher then it needed to be to compensate for the front end but I wanted to get my weight as far forward as possible to increase its handling potential without loosing rideablity.
As you can see with the drawing I've overlapped the stock linkage to make the point. I made my linkage out of aluminum flat stock because the clearance between the shock and the swing arm isn't much. The aluminum linkage broke so I then fabricated a set from steel. This piece of linkage will only be in tension so you don't have to worry about it buckling but you should still make it as strong as possible. Because of the drastic change in linkage and the angle at which it pulls at the suspension feels stiffer. This combined with the Fox shock make for a solid rear end. Once the linkage and the pivot are done the rest is simple. The rear wheel and brake will all fit fine since they more than likely belong to this swing arm. The rear brake stay will need to be altered. I used the stock rear brake stay mounted to the center stand mount because I no longer have a center stand, if you do it probably wont reach the ground anymore. This is not a straight shot though so it will need to be re-engineered. I have a section cut out of the middle and I welded it off to the right so it clears the tire. This way the stay mounts without adding anything to the frame and it also clears the tire and exhaust system. The last three problems you will find with this will be the chain, exhaust and the rear sets.
First I made a set of custom rear sets
but it was very difficult to design a rear brake that would work and look good
so I opted to use the stock rear sets. They need to be shimmed out because they will rub the wider swing arm.
You could also buy a set of rear sets and custom fit them to your bike using custom brackets.
Depending again on the swing arm you may need a spacer for the stock rear sets anywhere from ¼" to ½".
This is best done with a piece of bar stock that is cut and drilled so you don't loose any leverage on the foot pegs. Because the rear end is so much higher and the front is lower you don't have any clearance problems on this setup. If you opt not to heighten the rear end as much as I did you may need to move the foot pegs back and up to get the optimal lean angle.
Keeping the stock rear sets you can retain the stock brake pedal and master cylinder and will be much easier since it already mounts.
Again because of the wider swing arm you more then likely need to adjust the exhaust. I have a kerker 4 in to 1 system, which rubbed the swing arm so I had to cut the outer portion and simply bent it out and re-weld it.
I would eventually like to make a full system, maybe under tail exhaust but this will do for now. Again if you retain the stock foot pegs you can still use your stock exhaust mounting positions but if you go with rear sets you will need to make a muffler mount of some sort.
Obviously you can't get a 630 chain for a GSX-R wheel and even if you could it wouldn't work. Stock most GSX-R's came with 532 chains so you will more then likely need to get a rear sprocket and a front sprocket. I used a PBI aluminum rear sprocket that had a small offset. I mounted it opposite the way it should go to make it have an inset so I would have less to shim the front sprocket out. The front sprocket I got for a 79 KZ1000 530 conversion. Depending on the front sprocket and its offset you will need to shim it out indefinitely. This will of course vary on the sprocket but will usually be substantial if no other alterations take place. I think mine was less than ¼" but really this needs to be done to each bike to make sure its right. Another thing you can do to lessen the offset is to machine the sprocket carrier. I did this a small amount and used the rear sprocket to adjust more of the offset. If you do this you will need to make up for it with an additional outer wheel spacer to ensure the wheel is centered. Also remember the tire will be there. My chain is very close to the tire because I felt that was better then shimming the front sprocket out more. I highly recommend you safety wire the front sprocket. Because it will be offset you run a higher risk of it coming loose.
In the end you should get a superior handling machine if you take the time to do it right and ensure that all the loose ends are taken care of. My recommendations are to take your time. Research all you can before you do your swap. I used what I had access to and made it work but I don't doubt that there are easier parts out there to swap. You can do a front-end swap without doing the rear but you will still need to adjust the rear end to suit. You can make a 180 section tire fit in your stock swing arm but it will be a tight fit and will probably take a lot more custom work then to swap the whole rear end off a newer bike. Always remember what you are changing and what it will affect in terms of performance and handling. Study newer bikes and their dimensions and how you can translate them. The GPz was the top dog if its time and there is no reason that it has to be outdated. If you have all the parts a full swap could take as little as a month or less but leave time for unforeseen problems to arise. I started my swap by gathering parts first. Once I had the parts I started removing my stock rear end and making sure it would work first. The front end will work off any bike really but the rear and needs to be a little more specific and tailored to your needs. It took me 8 months to have a working bike because I ran in to electrical problems that arose after I started. I spent close to 4 months studying different bikes, reading handling articles and jotting down dimensions that could possibly suit. In the end my frame dimensions are very close to a Yamaha R1, with a little less power but a 750 turbo motor is in the works to even that field. A full first ride report is in the works but the bike on the whole feels much better. There is more confidence going into a turn, through a turn and coming out of a turn. If you can ride your stock GPz fast then there is no doubt you will benefit. If your bike dances around going into a turn or the rear end brakes loose coming out of a turn then you no doubt need an upgrade like this.