USD Fork Lengths

Well I’m bored because I can’t ride my bike right now, so I got that bug thinking about retrofitting USD forks on again. Now I don’t know if it’s worth the hassle, and the basics of it I understand, but the roadblock I keep coming up against is length. Basically all USD forks are shorter than conventional “right-side-up” forks. Not necessarily by a lot, but enough that even if I found some that were otherwise compatible, it would effect the steering geometry. I also know that you can change the offset at the triple trees to get the trail measurement back to its original number, but I feel like that would most likely also change the handling. 

So long and short of it I guess is that I wonder why it’s so hard to find reliable measurement info for fork length. Like any bike you look at might list a diameter on the stanchions, and the stroke length, but rarely ever the overall length. It looks to me like maybe some sport touring bikes have longer forks than their crotch rocket cousins, but I can’t find info on those either. It seems like when we’re talking about only at most an inch difference, that somewhere out there has got to be a bike with forks that are a direct front-end swap for my bike (ZR550). 

I have seen the Australian company that makes extensions, and I think there’s a forum member here that also makes them. And sure, that seems like a good idea, but maybe someone makes entirely custom length tubes? Granted, I imagine that would be expensive. But it just seems to me like there’s this massive lack of readily available information. I remember back when I was making guitars you could get a catalog with basically every part you could possibly need and all the specs were there. Why can’t you get that with motorcycle suspension? 

I didn't use USD forks, I don't like the look of modern forks on old bikes so I used a set of 45mm showa forks off a 2001 Triumph speed triple, the clamps have a 40mm offset so I was able to make that work to get the geometry where I wanted it,  using an 18 inch front wheel, and by raising the rear of the bike a little. I had custom length tubes made up 100mm longer to get the ride height where I wanted it . Most USD style forks have less offset making getting the geometry right a bit harder. I found a shop on ebay that sold fork tubes in the model I wanted and asked them if they would make me a set to my desired length, they only charged me around 200 AUD {I'd have to go back through my ebay account to get the exact amount}, that was about 12 months ago....

Zx9R had one of the longest USD forks and could be a good starting point. Or just any if you find someone that makes you new tubes like Mike. Otherwise the ZR1100 Zephyr forks are a easy swap if you go for the entire front end as you can see on my green kwack thread.

/Anders
 

wrote:

…I don't like the look of modern forks on old bikes so I used…

I hear ya. It’s one of those things that I think can work well if it’s done right, but many bikes are best left unchanged in that regard. Which reminds me of the last time I was at my local dealership looking at all the new bikes, and it occurred to me that some of the forks are almost getting TOO big. Like I get that larger diameters offer greater rigidity, but there’s also more weight that has to be moved around, so at what point are we seeing diminishing returns? They start to look a little silly, with the “beefy” front end that’s overly large and out of proportion with the rest of the bike. 

Yup. When I did mine,the ZX9R were the longest Jap forks around. Dont know what the situation is now though and once you get into Euro bikes there could be a whole slew of possibilities.
I went with the ZX9R forks and to keep things (reasonably) simple, I went with yokes,wheel and brakes as well.The complete front end.Some people mix and match but then you're getting into a whole new world on engineering and expense.I was fitting onto a CSR1000 from '81ish. I just needed a new stem on the lower yoke(some people press in the old one) new bearings of course and I had to weld a couple of blobs either side of the steering stop to avoid hitting the tank when turning. Visually,I made a new front mudguard as the ZXR one looked out of place.
Once everything was together,it all worked great considering I made no calculations for rake and trail.Maybe I just got lucky but is handles and steers fine. Mind you,I drive like an old lady anyway.
 The clearance on the exhaust downpipes was a problem though.Other people have got away with it but maybe because I started with a cruiser,I had less height to begin with. I ended up having some fork extensions made up locally.I went with the max which was about 5cms,I think.( there are dimples in the ZXR forks and you can only extend so far before the lower yoke needs a sleeve.) this gives me enough clearance on the exhaust and some extension to spare.
Its all in the build thread.Somewhere!!
 Would I do it again??? Dont know!! I do like the Triumph forks previously mentioned.

P.S.I do remember that there is a chart out the somewhere with fork lengths and diameters.God knows where.It was a long time ago. Someone must have it.
P.P.S Aha! Just found this;
www.caferacer.net/threads/triple-tree-an...version-chart.14059/


 

Saw a guy in Japan that fit a ZXR250 front end on his Zephyr 400. I think it looks great. I asked if he could tell me about doing the swap and how the bike handles now, but haven’t heard back yet. 

Besides the ZXR250, I noticed the ZXR400 looks like it might have the same forks, so I’m trying to research both of those models. 

Just “bought” a ZXR250 lower triple/stem off Jauce.com. I’m not confident this was a smart move, I fully realize the possibility of getting ripped off, or just paying too much money for a part that isn’t worth it. But I felt like it was worth the gamble of (so far) about $40 to see if it works. 

By which I mean whether or not I can fit it to my bike. But again, as is the issue/question with this thread, I still can’t find good reliable info about the length of the ZXR250 forks I’m looking into. 

The ZXR is a lot lighter than the 550 so you'll end up having to change spring rates and have the suspension set up properly, why not look into racetech  gold valves for your forks, they make a massive improvement to stock suspensions... There's also another company in the US that sells something similar..

racetech.com/page/title/GVWHYGV
racetech.com/ProductSearch/12/Kawasaki/Z...20550%20(ZR550)/1995

 

wrote:

…why not look into racetech  gold valves for your forks, they make a massive improvement to stock suspensions...
 

Absolutely plan to upgrade my stock fork internals to RaceTech gold valves and springs if the ZXR250 forks don’t work out. But I do want to at least try and see if the ZXR250 forks will work. 

To update, I currently have ZXR250 forks, and top and bottom triples procured via Jauce.com/Yahoo auctions Japan, and am awaiting packaging of items and shipment to me in the USA. Hopefully I actually get them someday, and yeah, it might not work out and be a waste of money, but it’s something I really wanted to try, so I’m gonna be happy either way. 

I've come in a bit late on this thread but on my last completed project pictured i used a 93 ZR750 Zephyr frame ( or at least some of it ) to house a Z1 engine and i used 05 zx10r forks up front.
These are very short for use as stock on a normal KZ900/1000 frame and still short for the much lower headstock of the 750 zephyr.
I had to internally extend them 32mm to bring them up to the correct height for the 750 frame.
I would say that any of the recent 600 to 1000 usd's readily available in the US (rather than sourcing from Japan etc) would be very close to the correct length for the 550 (which is of the same frame design and stance as the 750 ) and on the 05 zx10r forks which are known for being a bit harsh all that was needed was a change of fork oil to a lighter weight and some tweaking of the damping and preload adjusters to get the correct fork action and ride.

Wow, that bike is looking fantastic! Are you adding any more fairings or lighting to it? Keep me posted, I wanna see it when it’s done. 

wrote:

Wow, that bike is looking fantastic! Are you adding any more fairings or lighting to it? Keep me posted, I wanna see it when it’s done. 



 

The pic IS of the finished machine.
Built for daylight use only and i've been running it since 2014.
Only has a brake light, horn and speedo to be street legal here in the UK.
Actually doesn't even need a brake light but fitted one for use in traffic to reduce the chance of being rear ended.
There is a detailed build thread on here showing all the fork mods etc etc but Photobucket blocked all the picture links and due to the way ths site works i'm unable to edit the thread and re instate the pics.