Wheel size modifications/variations

So I'm in the process of doing a 06--08 GSXR front end conversion on my KZ750 LTD. I have the bearings, stem, triple details sorted out. My questions is specifically around varying wheel sizes. This is a pretty heavy custom build so i know geometry changes around going to happen. That being said I'll be changing over to a 17x2.15 or 2.75 front and 17x3 rear wheel to maintain a close to stock size. In the rear I'll probably be running a 130/90-17 tire. i'm attempting to find if anyone has handled a 17inch front conversion and if so what tire combinations they are running. Any help would be awesome.

Ive done a full front/back conversion on my CSR1000. Ive fitted a complete ZX9R front end and on the back,a GSXR600 swingarm with a Bandit600 wheel. I have the original tyres that came with the wheels when I bought them as Im still busy with the build. I will fit new rubber before hitting the road. Both ends are 17inch.
My geometry has totally changed from the original and I have no idea how its going to behave on the road. Ive been told that Ill have quicker steering!!
and ground clearance is a concern with the upsy dupsy forks.
With the major changes to the geometry from the conversion,I dont think tyre size is going to make a big difference.If youve got a GSXR front,I would say just put the GSXR wheel in it. Itll save you a lot of work.Same with the discs and calipers. If you try to mix and match,youll just be giving yourself more work.
My back tyre size was dictated by chain run and tyre/frame clearance. I think Ive got a 150 or maybe 160. Cant remember off hand.

Aha. just saw your build thread. You want to go for spoked wheels,forget what I said.No idea how you go about that. Good luck with your build. It looks good so far.

You could still run the 17x3 up front in a spoked wheel. Just have to do some searching for one. Same with the back. I'm running a zx9r USD front and GSXR750 rear that has been converted to dual shocks on my Honda CB750 ( I know not a kz) and with the 120 tire up front and a 180 out back it handles like its on rails in the straights and corners great. You will lose a bit of ground clearance since the forks will be several inches shorter as well as running a smaller rim but if it becomes too much of a concern you can always have a custom set of drop triples made.

Why not just lace up a 19in. front and 18in. rear wheel?

Based on the donor fork I'm using, which already has the lower triple, it's easier to just mate a new upper, stem and bearings than it is to have a custom triple made. Recommendation is 30mm offset triples work ideally with 17" wheels plus I like the look of a similar sized front and rear.

If anyone is interested in triples for a conversion these are the guys i'm using. cognitomoto.com/collections/fork-convers...-for-fork-conversion , they've been really easy to work with on finding the right fitment and helped me find a kz650 stem that should work with my conversion.

I figured since geometry was changing so much it really didn't matter, i'm just looking to preserve some of the vintage look and not go with a complete new rear as well. Something about meshing new tech and old tech is interesting to me, not to mention replacement rotors/pads are way more affordable than the stock offerings. I'm thinking either a 17x2.15 or 17x2.75 front and maintaining the 3" rear because I know chain clearance wont be an issue. I know of someone running a 4.25" rear here in town, just not sure how much work it's worth in the rear.

Overall i'm looking for a wheel appearance like below so it's going to be some work for sure. to get the similar tire sizes while maintaining the chain angle/frame clearance.

^That bike has a 19" front and an 18" rear.
I would assume that by building a "cafe racer" that you would be interested in practical upgrades that will make the bike handle better through turns and go faster.
Based on that, I would be inclined to keep the larger diameter wheels for clearance, perhaps drop to 18' front. With 17's you will not only loose ground clearance but wider tires require more lean angle as well, which has already been compromised by the smaller tires and lower front end. You aint getting big HP out of this motor so a fat tire makes no sense. You will find the limits of the chassis before the limits of a decent tire at a stock size. Also the extra mass that a wider wheel and tire will add will make acceleration slower.
If you are going USD forks it would be smart to brace the frame as the new forks are much stiffer than the stock ones and all that force is now translated into the frame instead of the stockers flexing a bit. There is a thread here showing ideal places for bracing. Personally I would upgrade the swing-arm as well. The stock swing arm on these bikes is fairly spindly. Good shocks and a stiffer swing-arm will give you more results than the front end alone. If you only upgrade the front end and nothing else it really isn't much of a suspension upgrade, everything needs to be ramped up equally. If you are really serious about performance buy Race Tech's "suspension bible" and tune your forks and shocks.

I would highly recommend running your projected geometry numbers before you commit to anything.
RB racing has loads of helpful calculators

rbracing-rsr.com/tirediamcalc.html
rbracing-rsr.com/advchoppercalc.html

^agreed, the forks/trees/brajes etc are the easy part. But having a modern front and a stock rear isn't going to do you any favors.

If you're lacing wheels to go with your front end you are going to need to have a conversion axle made up or luck out and have the ability to just to a bearing swap on the hub, then you are going to need a pair of adapter plates and/or spacers made up to locate the matching discs. A lot of work to have a great brake setup in the front, a wider tire and stability, you should definitely look into bracing the frame, and running tires the width of the donor bike (front and rear) which, yes means an offset sprocket, shaving the sprocket carrier, new brake rods and or spacing the rotor and sprocket out. You're also going to lose ground clearance with the forks (longest available USD forks are half an inch or more shorter than the kz offerings) then you are losing AGAIN with the smaller tires, which either means bumping your header EVERYWHERE, or making a tight fitting exhaust with a flat collector to stop it from hitting, or sucking it up and lacing a pair of 18" rims front and rear.

Thanks DoctoRot for the input. I have been wondering about the 17' on both ends, largely I knew i'd want to get more out of the cornering performance so I'm game for looking into anything that will get me that while lowering the weight of the bike overall. I have the option through Cognito moto to run a fully GSX-R converted front hub with a newly laced wheel between 17 and 19 inches. I guess I was going the direction of standardizing to the size of the donor fork rather than standardizing the new technology to closer-to-stock dimensions I.e. 18 or 19" wheels for the ground clearance. Also I will definitely be changing the rear suspension, just hoping to not re-do the swing arm in entirety so I'm looking at between 13.5" rear from progressive, to a 14.30" from fox to a full 15" from burly. Also, thanks for the frame bracing suggestions, I'll start digging on finding that!

DoctoRot wrote: Based on that, I would be inclined to keep the larger diameter wheels for clearance, perhaps drop to 18' front.

If you don't mind me asking what is on your KZ1000 in terms of wheels and tires, that's a really solid looking setup.

my 1000 has 18 x 2.50 front 18 x 3.50 rear rims running BT45's 110/80/18 front and 140/70/18 rear

Since you are loosing quite a bit of height with the USD fork i would be inclined to go with a 19" front wheel. I would figure out what your rake and trail numbers are with the new front end before you buy shocks, that way you can tweak your numbers by going longer or shorter in the shocks. I lost fork offset with my ZRX swap so i got 2" longer shocks in the back to get the trail down.

heres the frame bracing thread.
kzrider.com/forum/5-chassis/294594-frame-bracing

if you don't want to swap the swingarm you can brace it. easy cheap way to stiffen the rear up considerably. heres mine

DoctoRot wrote: I would figure out what your rake and trail numbers are with the new front end before you buy shocks, that way you can tweak your numbers by going longer or shorter in the shocks. I lost fork offset with my ZRX swap so i got 2" longer shocks in the back to get the trail down.

That was exactly my thinking once I knew what trail looked like once the new fork is on. Am I correct that stock trail is 4.2"? I have a feeling I'll end up with a 19" front just to get the most ground clearance I can. Also thanks for the link to the frame bracing, the visuals are really helpful.