Lengthening USD Forks

[squirrel_hunter]

Right firstly this isn't for the SV but I thought the more techie people would see this...

I'm currently working on a CBR400RR NC23 and am attempting a USD fork conversion. Bit of searching pointed me in the direction of the ZXR400L to use, it will need a little work to the lockstops and the stem (more to come on that later) but my biggest problem is the length of the forks.

The ZXR fork is approximately 75cm from spindle center to fork top and the CBR 78cm. This 3cm difference is giving me problems relating not only to clearance but to geometry also. To solve this problem I would like to lengthen the forks. So how would I go about doing this?

My thoughts so far after a little googling revolve around fabricating a top yoke that could support the lower fork legs. Lengthening the fork leg at the top somehow (essentially unscrewing the top sticking something 3cm longer in the top and then doing it all back up). Or currently my favorite lengthening the damping rod, this would make the stanchions protrude but should not effect strength as its not an extreme difference. However I'm not sure how I would go about this and what other things would need changing spring etc let alone how I would make or get a longer damping rod.

So your thoughts please?

To get you in the mood here is a pic of the bike with the ZXR forks slotted in...



Big thanks to Strechie for driving me to Biggleswade today to pick them up and to helping out in the garage as well as taking that photo that has made me continue with the project.

Not to put a damper on your mod but wouldn't the RVF400 forks be a better and easyer conversion? you could always drop the rear of the bike to get your geometry something like!

[northwind]

Racetech do a lengthening kit for some forks, not sure if this would be one. Or you could do stepped-down yokes, but that's a pretty ugly solution.

[squirrel_hunter]

From what I've read the RVF forks don't have the clearance needed, something to do with the mudguard hitting the radiator. Though now that you mention it these could be modified in some way to stop that (I hope!). I got the ZXR idea as they fit the NC29 (although with some modification to the stem etc) so decided they would be the best route to go.

I've also ridden the RVF and found it a very firm ride, to hard for road use in my opinion. Further a set of RVF forks are hard to find and outside of my budget!

As for dropping the rear, I like your thinking but the CBR is already a low ride height bike and I have had ground clearance issues with the standard forks due to the race exhaust I'm currently running on it.

[squirrel_hunter]

Had a look over at Racetech but they don't list the ZXR400, will drop them an email, but is there anyone in the UK like them?

Also what are stepped-down yokes, never heard of that before?

[21QUEST]

*Are the CBR forks flush with the top yoke as standard?/ how you would normally have them.
*Also have to check how high the adjuster nuts/locking nuts are as measured from the dust cover
*Also check the distance as measured from the bottom part of the stem hole to the top of the top yoke surface(assuming yoke not stepped)

All that would give you the true difference in ride height between both sets of forks.

Best would have been to measure from the middle of the spindle to somewhere on the frame(underneath head stock maybe) before removing the CBR forks. This is obviously with the front raised(topped out).

Regards lengtening possible making new caps(extended) but unless doing it your self will cost you about £90 or so. Stepped top yoke but may mean your clip ons sit very low. Third option is indeed internal which againg should be possible but ned someone(suspension expert) who knows how to I guess.

Anyhoo first thing I'll say is to find out exactly how much ride height you'll be looking to make up. Do you know what the "yoke offset" is for the two different yokes ie CBR and ZXR


Ben

[squirrel_hunter]

*As standard the CBR's forks are flush with the top yoke.
*Not sure which adjuster nuts/locking nuts and dust cover you mean?
*Will get this measurement later tonight

Will also measure from the spindle to the bottom of the headstock with both the CBR and ZXR forks. I don't know what the yoke offset is between the forks, how would I find this out?

So if after I do these measurements and I do need some modification to the forks I would prefer to go either the internal route or the extended caps. But any suggestions on who would be able to carry this work out if I find that its not something I can make?

[21QUEST]

Originally Posted by squirrel_hunter

*As standard the CBR's forks are flush with the top yoke.
*Not sure which adjuster nuts/locking nuts and dust cover you mean?
*Will get this measurement later tonight

Will also measure from the spindle to the bottom of the headstock with both the CBR and ZXR forks. I don't know what the yoke offset is between the forks, how would I find this out?

So if after I do these measurements and I do need some modification to the forks I would prefer to go either the internal route or the extended caps. But any suggestions on who would be able to carry this work out if I find that its not something I can make?

Ignore the highlighted bit since it's no hassle for you to put both forks on with the require yokes(everything mounted as should be). That'll sort out the difference properly and also give you a known baseline to work from.

I've got a couple of people that could do either. Funnyly enough I might have a pair of bent ZXR400 forks somewhere so if you decided to go for that and I can find them I'll let you have the top caps if required. Sometimes the original caps need modding for the extended.

Lets start with the measurements. Regards offsets in the diagram(link below) the offset is 34.5mm

http://forums.sv650.org/showthread.php?t=84335


Ben

[squirrel_hunter]

Been in the garage tonight to make some measurements. They are not perfect but give a good indication of the problem, oh and I've only got Paint to play with...



I couldn't mount the ZXR yokes as I was only using a jack from a Fiesta and it wasn't that stable. I also have to get the stem swapped to be able use them properly.

So from the measurements I took, the head stock takes 21cm and then the yoke on top (forgot to measure the ZXR so presume the same) another 2.5cm. The CBR forks extend 52.4cm from the bottom of the head stock a total of 75.9cm. Presuming this would need to be the same for the ZXR which only measure 73cm I'm left with a shortfall of 2.9cm.

I've also included the offset. Presumably this would also have some effect on the length required, the further out the longer the forks would need to be to compensate.

So what do you think? Does this 3cm difference look right and is what I'm proposing possible?

[squirrel_hunter]

Right the story so far...

Been speaking to a few companies to see what this is going to cost me:

Maxton will strip them down for me but depending on what needs changing and what needs making I could be looking at £300+

ABE should be able to do it by making longer stanchions and a few other bits for £150 a leg.

Race Lab will be getting back to me. The guy I spoke to was incredibly knowledgeable and took the time to explain everything to me that may need doing. At the moment he believes that by shimming the cartridge in the fork and lengthening the damper rod providing the stanchions would not extend below the oil seal this would be the best option. However if the stanchions are to short then new ones would need to be made. He's looking into this for me and will let me know what they can do and an estimate.

Finally TES Performance can do fork extenders for £105. They can give me an extra 50mm which will allow me some adjustment. They make the extenders themselves and would need the top nut/ fork for a 5 day turn around.

Still need to email RaceTech but its currently between TES Performance and Race Lab, though as always please suggest alternatives.