Knackered Forks on 3CV

[vduk]

Despite a rebuild with new oil, the forks on my 3CV bottom out very easily on lumpy British roads. I'm guessing that the springs are tired and that the damping is well past it's best.

I've failed to find any replacements on flea-bay, and even if I did, the chances are they'd be much the same.

I was wondering what other forks might fit? I did wonder if the XJR 1300 forks would be a simple replacement? I don't really want to change the front wheel, though I would happily fit blue spot calipers, if a set came along at the right price. Times are more than a bit hard at the moment, being self employed, but I really would like to sort the front end, so any ideas would be much appreciated.

Thanx

Ralph

[markmartin]

A straight rate spring in the .95-1.05 kg/mm range ought to fix your problem.
http://www.amazon.com/Race-Tech-Fork-Springs-S3234095/dp/B000GV2DGW

If you can afford a few more $$, a set of cartridge emulators will make those (any) springs more compliant.
http://www.racetech.com/HTML_FILES/EMULATORGV.HTML

There are some new emulators coming out at this site that sound promising also.  I think he may be carrying springs also. Prices, shipping and service are excellent here.
http://www.rpmracingca.com/products.asp?cat=39

[FJ111200]

XJR forks are 43mm compared to 41mm for the FJ, but the yokes are a direct swap into the headstock, so a complete front end from the XJR is no problem.
The only problem being is the £££'s, bloody stupid money for owt XJR. I blame the blingers driving up prices.
I've put some Hagon progressive springs in one of my FJ front ends, and it really improves things at a fair enough price.
What's up with the XJR front wheel anyway?  I'm running an FJ front end with an XJR front wheel. I had the fork bottoms drilled to take the wider XJR spindle, but this is on one of my XJR projects. I'll post a picture.
Brand new EBC Prolite discs aswell.

[racerrad8]

There are some new emulators coming out at this site that sound promising also.  I think he may be carrying springs also. Prices, shipping and service are excellent here.
http://www.rpmracingca.com/products.asp?cat=39

The fork valves should be here next week. One note for those who are thinking about buying springs. These fork valves, a true shock valve, will require a lighter spring.

We are working on a chart based on wight for the proper spring for each individual application. I can say I like my .85 I have in my forks the best. I weigh 200 pounds and do not really carry any cargo.

More to come in the next week or so.

Randy - RPM

[ELIMINATOR]

ralph,
         probably stupid comment from me, but have you played with the adjusment?

[vduk]

Thanks for the advice.

Much as I like the idea of the more modern forks, it looks as though it could be quite expensive; I would probably have to machine some modification bushes for the original wheel spindle, then either struggle fitting the old mudguard or dig deep and buy an XJR one ... .

Although I do abuse the Old Girl by riding her exclusively in the winter and not cleaning her and giving her only essential maintenance, I don't want to fit too many non original bits as I have a feeling that one day I will give her the serious restoration she deserves.

I think I'll try some Hagon fork springs and perhaps some nice thick fork oil  I suspect she had thicker fork oil previously as the forks got noticeable worse after an oil change. What grade do you guys use?

[andyb]

While you're changing things, if you otherwise like the quality of the ride, you can simply increase the amount of fluid in the forks, and that will help to reduce bottoming.  Specifically, you're reducing the amount of free air in there, which acts as a secondary spring of sorts when things are nearing the limit of compressive travel.  If you underfilled the forks a significant amount, you'll find that the measured sag is adequate (assuming the springs are appropriate for your weight, early FJ's are frequently only good for those of elfin stature), but will lack any real resistance to bottoming out.

According to my Clymers manual, the original fork oil should be straight 10 weight, or 10w30.  A 3CV is the 88-90 model if my memory is correct, and they had ~10% stiffer springs than the earlier models. 

I think my standard advise would serve you well here (but that's what I usually think, and that's why I so frequently give the same out!):

Measure first and find out what you've got going on.  You'll want to know your static sag numbers (explained in good detail in a pdf file here, and explained in a zillionty places online that can be found via google or similar.  Ideally, you'd also know what the current springs actually are.  Someplace, this has been answered, but I'm unable to find it with the current attention span that I have:  Original 88+ springs were dual rate if memory serves, and it's fairly easy to see that when you're holding one of them (the coil spacing has two distinct stages).  If a prior owner went through the aftermarket, it could have fully progressive springs (the coil spacing should be continously changing through the length), or possibly straight rate springs (which just look like they came from a great big biro). 

Once you've got an idea of what your sag numbers are, you'll be able to see which route you should go, as far as how stiff a spring to buy, or if it's okay, to a grade or two stiffer oil, as well as the amount of oil used.  A starting point could be 15wt oil filled to proper spec, and then add enough to raise the level of the oil within the fork (compressed, spring out) another 1/2" or so.  If your sag numbers are too big, the bike is sitting too low in the stroke, and you'd need massively thick oil (because the proper answer is a stiffer spring at that point). 

On my 90, I had acceptable sag numbers with the springs that were in it (which appeared OEM, though they did measure as underspec by length), so going up one step thicker oil, and using a half-inch more of it gave a nice plush ride over the rather horribly frost-heaved pavement that much of my riding was done on.  Had I lived someplace with roads that actually turned, a stiffer spring would likely have been quite helpful (and I definitely could have used quite a bit more spring rate when I was riding with a pillion!).

[simi_ed]

Anothr thought is the busings in the fork.  They do wear out and will icrease stiction when worn.  Since you're adverse to changing forks entirely, a set of cartridge emulators would be a huge inprovement over the OEM damper rods.  Just putting in thicker oil is not going to help anything.  As others have suggested, the fork springs really need to be replaced as well.  Straight rate are the way to go.

Good luck.