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...

http://i153.photobucket.com/albums/s216/squirrel_hunters_photos/IMAGE_00209.jpg

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.

[BILLY]

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]

*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...

http://i153.photobucket.com/albums/s216/squirrel_hunters_photos/Geometry.jpg

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 (http://www.maxtonsuspension.co.uk/) will strip them down for me but depending on what needs changing and what needs making I could be looking at £300+

ABE (http://www.allbikeengineering.co.uk/) should be able to do it by making longer stanchions and a few other bits for £150 a leg.

Race Lab (http://www.racelab.co.uk/) 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 (http://www.tesperformance.co.uk/Catpage2.htm) 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.

[21QUEST]

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...

http://i153.photobucket.com/albums/s216/squirrel_hunters_photos/Geometry.jpg



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

Sorry Bud, sort off forgot about this. From the above figures shortfall in ride height is about 31.5mm
3mm difference in offset(less offset in ZXR) will translate to 3mm more trail with same ride height. So in effect you'll want the front ride height a bit lower than standard ride height using CBR yokes.

You can work that out with a program to give an idea of starting point but need the circumference of the tyres for both CBR and ZXR wheels(mounted). Also need the steering head angle for the CBR.

Alternatively you can just get the front about 10mm to start off with and adjust to your sweet spot.
Come to think of it, 25mm reduction in ride height would reduce steering head angle by about degree and about 6mm reduction in offset(in general). if that is so then you will have only lost about 3mmm in trail which can depending on how you look at it be a good thing....but then ground clearance and radiator clearance might be an issue.

See if you can dig up numbers for the above. Program for calculations are on Tony Foale's web site by the way.



Ben

[21QUEST]

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

Those two companies appear to be you best option with regards to value for money. The only other option would be stepped yoke but to be honest with 30mmm they'll look a bit cack and you'll loose clamping height for clip ons.

Ben

Ps: forgot to check on those forks I mentioned earlier. Should know by Tuesday/Wenesday.

[squirrel_hunter]

It appears that theres a little more to think about then I first thought...

The CBR has a rake of 24.3 degrees and a trail of 91mm (manufactures claims) using the Castor program from Tony Foale coupled with the 23mm offset from the yoke gives me a wheel radius of 257mm.

Now accepting the radius (as I calculated it at 259mm using a tape measure and under-inflated tyre) assume keeping the rake the same due to maintaining the ride height, the ZXR yokes have an offset of 20mm which gives me a trail of 94mm.

So if I wish to maintain the geometry of the bike I need to alter the rake to 23.7 degrees. So a difference of 0.6 degrees, taking a 25mm drop to a degree I need to lose 15mm in ride height at the front. Thus the difference in fork length is only 15mm.

I suppose I could then just drop the forks by the remaining 15mm, however would that give me enough room between the wheel and radiator/ fairings and ground clearance? There is only one way to find out I need to refit the forks and do some measurements...

I know that standard forks have been dropped by up to 20mm, 30mm being to low, however if I have everything correct then I am only dropping them by 15mm which should quicken the steering slightly without adverse effects. The only things to remember is that the measurements I have taken have some error margin, hopefully not much! And all fit ups have been with the CBR wheel as I'm yet to source a ZXR wheel but will try and find some measurements from somewhere (also considering using the CBR wheel at this point).

Again your thoughts please cause my head hurts :geek:

[squirrel_hunter]

Well I've spent the day in the garage...

Stuck the ZXR's back in to get the ride height with the forks set as they should sit in the yokes. And although the measurements aren't the most accurate it looks like I'm only loosing 5 - 10 mm from under the engine.

The bars may be a little low but that shouldn't be much of a problem, they shouldn't hit the frame or anything else so the MOT should be fine and I could always get them bent up a bit.

So off the yokes have gone to have the stems changed. The ZXR stem can be pressed out but the CBR will have to be cut and then modified to fit the ZXR yoke. I'm expecting them back at the end of the month, can't wait.

The bike is now resting with a set of 916 forks and the CBR's wheel held in with the ZXR axle and I'm off to eBay to look for a ZXR wheel and brakes...

[Robw#70]

the forks are do-able without massive cost, but id need to have a look inside to see the options.
using slugs in the top of the forks is horrible and only fit for badly made chops imho.

personally id fit the forks as is, undo the tops and let the bike drop right down on its stops to check wheel clearance.
if its all ok lob it back together and ride it- spending hours on calculations and mods to get it right on paper will never tell you what it rides like good or bad.

ive got a 23 sat in the garage with an rgv front end all fitted, dont know whether to sell or finish it atm

[squirrel_hunter]

the forks are do-able without massive cost, but id need to have a look inside to see the options.
using slugs in the top of the forks is horrible and only fit for badly made chops imho.

personally id fit the forks as is, undo the tops and let the bike drop right down on its stops to check wheel clearance.
if its all ok lob it back together and ride it- spending hours on calculations and mods to get it right on paper will never tell you what it rides like good or bad.

ive got a 23 sat in the garage with an rgv front end all fitted, dont know whether to sell or finish it atm

Yeah I'm going to stick it in and see if it works, though it might be a little while before I get to test how it rides. Plus still haven't got the yokes back yet.

Post up a pick of your 23, would be interested to see it. From what I found out the RGV and ZXR forks are the same length but the RGV has a stepped yoke. How does it ride and did you have to do any other mods to get it in there?

[Robw#70]

I'll sort some pics out, the bike was left with me to do some work on about 4 years ago, the owner moved to Ireland and now doesn't want it, he crashed it 6 years previous and got someone else to fit the front end so I dont know the ins and outs of how it fitted, but it does have some rather large lock stops welded on the bottom yoke leaving it with almost no lock.

[squirrel_hunter]

Cool thanks. The lock stops will also be a problem for the ZXR swap, but instead of welding some more on the intention is to drill and tap the yokes lock stops and use a bolt so I can set it up once its on the bike.

I know of someone who is fitting a 929 front end to a 23 or 29 (can't remember now) and is having problems with the wheel rubbing the fairing. I had a quick look at mine the other night when the race fairings turned up and it looks close. But thats with the 23 wheel between a set of 916 forks so it may be different with the ZXR forks on. But is this an issue for yours?

[Robw#70]

Her you go, not the prettiest im afraid
http://muchos.co.uk/members/robw/08042007249 (Small) (Custom).jpg
http://muchos.co.uk/members/robw/08042007248 (Small) (Custom).jpg
http://muchos.co.uk/members/robw/08042007250 (Small).jpg

[squirrel_hunter]

Thanks for the photos, that looks very interesting.

I thought the RGV forks were the same length as the ZXR's but they look much longer in the photos and I don't have a pic at the same angle to compare. I know this is a big ask but would you be able to do me a couple of quick measurements of the length etc?

Also I know you said that you didn't do it but any chance you know what bars are on that? I think this my be an issue for me as they appear to clear the tank much better then the ZXR ones will but I think thats because they are mounted above the yoke, which gives me an idea!

Also do you know if thats a standard NC23 wheel as it looks as if it is? I think I'm going to have to source a ZXR front for mine as I just picked up a set of brakes and disc's and the disc won't fit the 23's wheel, but am still waiting for the yokes to come back to be sure.

Finally as you said it might not be the prettiest, but it looks good to me, a proper work in progress. Apart from the fact that radiator looks new, so if you ever decide to part it out...

[Robw#70]

Right had a quick measure and they're approc 72cm from top of fork leg/cap (not incl adjusters etc) to the centre of the spindle.
It looks like its had a new stem as its been welded in to the yoke, not sure if its the cbr one or a made job, but it doesn't fit very well around the top yoke as the spacers/nuts dont meet the top yoke, which will need sorting as well as the lockstops

[squirrel_hunter]

Cheers for that. Their about the same as the ZXR's under the weight of the the bike, yours just look longer.

If the stem is the CBR's I'm surprised it doesn't fit well as it would have to have been cut out of the CBR's and then welded onto the RGV's yoke. And yes those are some mighty big lock stops, but nothing that can't be sorted with a Dremmel!

Let me know what you decide to do with it as I would be very interested as it shows a lot of potential.

Thanks again.