[supermotobobby]

Hello all, I am doing a ground up on my 82 GPZ 750 and the only thing I'm modding is the front end and rear end. I have a 98 GSXR 750 Front end and rear end I want to install on the GPZ. I want to keep dual shocks and was wondering if I cut the top bars of the GSXR Swingarm off to weld on shock mounts will the swingarm be too weak?

 

[haybaler]

Okay Bobby, I guess the advantage on the front end is you're getting upside down forks that probably work better than the original Kawasaki forks,
but take care about length and wheel position because if you change the steering geometry you might not like the result, okay?

For the swing arm, sorry, I really don't see any advantage, unless you think it's stronger. Considering what mods will be necessary,
I would just take the original swingarm and weld in some extra pieces for strength.

Just sayin' -- as always, your bike , your choices.

Cheers!

 

[davidr1963]

I wouldn’t chop the bracing off. If I had that swingarm, I’d look at fitting it upside down; lots of swingarms in the 70s and 80s were braced on the underside.

Failing that, and presuming you’re looking to fit wider wheels, a zephyr 750 would probably be quite easy to fit. I’ve also seen zxr400 swingarms fitted too.

I’ve done zxr400, vfr400, vfr750 and gsxr750 k7 for my 83 GPz750s

 

[Kawasakian]

Unless that swing arm is steel, you can't weld it because it will lose it's heat treating properties and crack. They are heat treated after welding. You might be able to make some beefy shock brackets, but taking out the top part will severely weaken the swing arm and defeat any purpose, it wouldn't even be a zero sum gain. IMHO

 

[WFO-KZ]

I would speak with a qualified motor vehicle engineer in regards to your changes.

I don't know about the US but many jurisdictions in Canada require an engineer's stamp for structural changes to any motor vehicle. Even if that is not the case in your state, I would highly recommend it so you don't create a death trap for yourself or anyone who later buys your bike.

 

[supermotobobby]

Thank you all for you advice, since I already have everything off of the GSXR I will look into welding a mount for the single shock to the frame.

 

[davidr1963]

You will need two mounts, one each top and bottom.

 

[supermotobobby]

Also, with the usd forks can I keep the factory GPZ wheel?

 

[Kawasakian]

That will take some careful measuring with a depth gauge.

 

[davidr1963]

Also, with the usd forks can I keep the factory GPZ wheel?

It’s doable but the original wheel is 19” and the GSXR 17”. The GSXR wheel is also wider with bigger discs.

Again, it could be done but it’s a lot of work for an inferior finished job.

 

[Kawasakian]

Your going from a 19" rim down to a 17" rim, could you post a picture of this? Swing arms are so cheap on EBAY, it would be cheaper in the long run to buy a swing arm that fits the wheels. On the other hand, without seeing a picture, maybe you have everything lined up, but it just reads horrible?

 

[davidr1963]

The rear wheel is 18” so going down to a 17” isn’t a problem. To be fair, neither is going from 19” to 17” on the front, in my opinion anyway.

The width on the front is an issue though as the 19” is narrower than the 17” Suzuki wheel. This would require a fair amount of work, particularly with regards to the brakes.

My GPz750 now has 17” front and rear with the rear ride height increased by 50mm.

 

[Kawasakian]

If supermotobobby could post a pic, it would speak 1000 words. Always have been a fan of the GPz750's The only problem I see is steering stem geometry, which may be able to be alleviated by how the rear shock raises or lowers the bike.

I really like those wheel stands, what make are they?

 

[davidr1963]

I’m not sure, I picked them up at the bike show a few years back.

Whilst raising the rear and lowering the front does make the steering angle a little steeper, a combination of a very lazy original steering angle and a much stiffer modern front end, actually makes the bike both more stable and quicker steering than the original.

 

[Kawasakian]

Yes, and No, you've replaced the forks, the location of the axle in relation to the perpendicular steering axis is usually done so that when the handlebars are steered from left to right (or vice versa), the neck of the bike does not raise or lower, it stays the same height. Not only rake is involved, but stem to fork tube offset.

I knew a guy, his name was Frank Strnad, he designed world class racing bicycles. The theories are very much the same. When I was making my 1st recumbent bicycle, he told me, you will have a usable rake when you can turn the handlebars back and forth, and the neck does not rise. This will make the bike neutral. I did this on my CAD program (Rhino3D) and came up with a 12 degree rake to work with what I was making. It was so perfect you could pedal the bike in a smaller and smaller circle till the bike would just roll off the tires, with no scuffing, or any other indication that you were messing up. That is the best way any 2 wheeled vehicle can be, take you right to the edge, and still be stable, if you go off the tires, well you did that, the bike's geometry was not an issue.

You have different forks, an axle with an unknown relationship to the rake of the bike. If you put the front wheel on, and mounted the frame, turn the handlebars back and forth and see if the center steering stem stays the same height, if it does, you should be in a really good spot. That would give you an idea of how your bike has to sit on it's rear wheel.

I have made quite a few recumbent bikes from scratch then moved to trikes. If someone tells me that I am yammering on, well everything I have stated is in relationship to what is being done here.

Frank died in 2019. He sold me his virtually new "Miller Econotig" for $300 bucks, so he could help me get started. I fixed his T.V. for him. That man taught me much about the geometry of two wheeled vehicles. His frames were Chrome Moly and they were only .012" of an in thick wall thickness. Each one he made by hand. No two of his bikes were the same. People came from all over the world, and the bikes were made to that person's frame. Just something to think about, the technical aspect that is.

You don't need CAD to figure this stuff on. A plumb line and 6 foot level will go a long way, or you can just cut and grind your way into something that seems to work. Honda did it for years, that's way the early Honda's are known for the "Honda Wobble", tank slappers that would come at 70 mph for apparently no reason (crappy frame design, spindly forks.)

This is what a $1800 dollar world class looked like back in 1998: