Ci5 Fork Up Grade

[buzz9]

I HAVE JUST RECENTLEY AQUIRED A C15 BASKET CASE IAM NOW REBUILDING SLOWLY WHICH I INTEND TO USE FOR GREEN LANEING / TRIALS .

WHEN IT COMES TO MODIFICATIONS LIKE THIS I COME UNSTUCK ,

CAN ANY ONE ADVISE ME ON HOW TO CHANGE THE FORKS FROM THE ORIGINAL ONES TO A MORE MODERN REPLACEMENT SET. ?

ARE THERE FORKS I CAN REPLACE THAT ARE EASYER TO FIT THAN OTHERS ?

THE PROBLEM I CAN SEE IN CHANGEING THEM, IS THE BEARINGS ON THE REPLACEMENT YOKES I.E WOULD THEY NEED MACHINEING , MODERN FORKS SEEM TO RUN ON TAPER BEARINGS WHERE AS THE OLD C15 FORKS HAVE THE ROLLER BEARINGS .

HELP - CAN ANY ONE GIVE ME SOME GOOD PRACTICAL ADVISE WHICH WAY TO GO ?

Edited July 9, 2009 by BUZZ9

[walworth]

Hi, Buzz. I have done a couple of conversions like this, though not as radical as C15 to modern, usually drum brake t-shock converted to disc front end. Easiest way is to have a new "hybrid" steering stem machined by an engineering shop, and swap the yokes with the matching forks, you will retain the original C15 steering head bearings, with the modern adjusting nuts. You just need a mate who is an engineer , and who knows what you need of him. If you take it to the local engineers shop, and pay the going rates, a lot of general jobbing shops may not want the job, or quote you telephone numbers, because they will not take a job at a loss. I reckon on about 4 hours work usually does it. Steering stops may be an additional headache to consider, either engineered into the yokes, or welded to the frame, or a mix of each. As far as which forks will fit easier than others, sorry, no experience with C15s.

Remember that the purists will not let you enter P-65, you will have to run against the likes of T-shock Fantics and Hondas. To keep it P-65 you will need to fit an appropriate set of period forks, suitably "Funnied" if you want them to work better. That is a much more expensive job altogether. Good luck.

[woody]

First of all you need to decide what events you want to ride your bike in and with which clubs as this could have a bearing on which forks you fit.

If you are going to ride it at local modern bike events on the easy route where they may have a pre65 class, the chances are they won't be bothered about the spec of your bike as they won't know any better, therefore the forks you fit won't matter. Always the chance other riders may whinge though.

If it is to be ridden in classic club events then they may be a little more fussy over what you fit. Some classic clubs excercise a certain amount of tolerance with modifications, others go apoplectic if they find you have so much as a metric nut and bolt holding your chainguard on (slight exaggeration as I'm induldging myself...) Most clubs will accept forks doctored to have a period appearance (whatever that is as some that are allowed look anything but)

Most forks are 35mm stanchion diameter so you want yokes of that diameter. Montesa Cota 247 yokes (from 1973 - 76 bikes) are a good choice as they are basically the same as the billett yokes manufactured by several specialists and these are all accepted. Most yokes use bearings with an ID of 25mm and OD of around 47/48mm. The OD of the original C15 headstock bearings is obviously imperial measurement but works out at about 49mm so metric taper rollers won't go straight in. What I did was to make a sleeve to fit in each bearing housing in the headstock which sleeves it down to 43mm. Pyramid Parts do taper rollers with ID 25mm, OD 43mm and depth of 11mm (maybe 12, can't remember) This allowed me to use the Montesa yokes. You can then fit any 35mm forks. Ossa yokes are another 35mm option although they take bearings with an ID of 20mm, not 25mm.

Gives you a few options, but you really need to decide what events you are going to ride it in as to which direction you go in. Depends also on which part of the country you are in. If you aren't bothered about riding it in a pre65 class, then if the mods aren't acceptable to any given classic club, most have a specials class you can ride it in.

[buzz9]

thanks for your input , much appreciated .... and very good advise .

i do not really plan on useing it in pre65 so not too fussy ,.

i just want forks with better performance than the standard c15 forks .

i have dwelled on the problem a little more , in my opinion , i may be wrong ? the only thing stopping me fitting for example- x l -fork legs in the original c15 top yoke is the absence of the pinch bolts

most modern bikes have pinch bolts on the top and bottom yokes , the british c15 have pinch bolts only on the bottom yoke and the top yoke takes large nuts which screw in to the top of the fork legs and tighten ,to secure legs to top yoke .

i do not think it would be a problem to ream out the yokes to take larger diameter fork legs , the problem is the absence of pinch bolts on the top yoke ?

if there is a way round this i do not know .

Edited July 10, 2009 by BUZZ9

[mcman56]

I am about to do a fork swap like this with Bultaco Alpina forks on a C15. I have a couple of questions on both methods described in this posting.

1. With the tapered Pyramid bearing method, the spacer/ reducing sleeve will be pretty thin. (49.5 OD, 43 ID)

How do you get it pressed together without damaging the sleeve? Is it more of a slip fit vs. a tight press? It seems that the sleeve would have so little mass that shrink fitting would not work because it would heat or cool too fast.

2. With the steering stem method, how does the stem come out of the fork clamps on a Bultaco?

Thanks,

Dan

[b40rt]

Why not get Montesa forks and link them up to a Montesa, should be a fairly/relatively easy solution.

Personally the appeal of a pre-65 bike is the crudity of it.

[woody]

I am about to do a fork swap like this with Bultaco Alpina forks on a C15. I have a couple of questions on both methods described in this posting.

1. With the tapered Pyramid bearing method, the spacer/ reducing sleeve will be pretty thin. (49.5 OD, 43 ID)

How do you get it pressed together without damaging the sleeve? Is it more of a slip fit vs. a tight press? It seems that the sleeve would have so little mass that shrink fitting would not work because it would heat or cool too fast.

2. With the steering stem method, how does the stem come out of the fork clamps on a Bultaco?

Thanks,

Dan

I'm not an engineer so I don't begin to understand the various fitting methods you mention in point 1. The sleeves are 3mm wall thickness which isn't thin to me, the previous sleeves I had were to take TYZ head bearings and they were only 1mm thick. All I did to fit the sleeve was to tap it in using a drift, same as I fit the taper roller outer cage. Once the sleeve was in I fitted the outer cage into the sleeve by the same method. Simple as that really.

To remove the stem from a Bultaco fork yoke you need to press it out, no other way that I know of.

[mcman56]

Thanks for the reply. Did you make the sleeve a slip fit or a real tight fit? What material did you use..steel...aluminium..or? Thanks for your help with this. Dan

[woody]

I made the sleeve out of aluminium (no idea what grade etc) The fit is such that it can't be pushed in by hand, it has to be tapped in with a soft mallet, same as you would a bearing.

[alan]

thanks for your input , much appreciated .... and very good advise .

i do not think it would be a problem to ream out the yokes to take larger diameter fork legs , the problem is the absence of pinch bolts on the top yoke ?

if there is a way round this i do not know .

You are not going to be able to hand ream the yokes out and keep the hole round!, it needs to be machined.

The best way to avoid the need pinch bolts on the top yoke is to machine the taper out but leave a 2mm step on the top side of the yoke. You can then used the fork top nut to pull the fork stantion against the step.........simples..

[mcman56]

Mr. Woody,

I followed your advice and it worked out well.

thanks,

Dan

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[woody]

Yes, that's how I did mine - glad the info was of assistance.

[mcman56]

Well....I tried to assemble the fork clamps to the frame but the stem is about .275 short. The nut under the top clamp has a tube on top that fits up into the top clamp. The top clamp has a pinch bolt that grabs this nut and then there is a simple flat nut on top. I have come up with a few ideas. Does anyone have a recommendation.

Need about 0.275

big nut under top clamp is .158

top clamp is .640 with .270 cross hole so wall is .185 above and below cross hole

lower clamp is .635, the bearing sits on a .210 step on the lower clamp

Option 1

press stem further up in bottom clamp. Engagement will go to .36 - not much

Option 2

Thread inside of steering stem and use bolt above top clamp instead of nut - maybe..is stem hard?..part of bearing will ride on threads

Option 3

use top nut under top clamp and make a nut similar to C15 nut for top - not easy to make nut

(Did Bultaco do this somewhere so I could buy the nut)

Option 4

shave .125 off of top of lower clamp, .06 from top and bottom of top clamp - a lot of work and weakens assembly

Option 5

Make new stem - a lot of work

[mcman56]

Well...I pressed out the stem and saw that it was not a press fit for the first 1/8" below the bearing. I shaved off 1/8 from the top surface of the bottom clamp and reinstalled the stem. I then shaved about 1/16" off of the top of the top clamp and everything fits nicely.

[mcman56]

Mr Woody,

I'm getting further along in my build and realize that I'll need fork stops...at least with the original tank. It looks like I could bolt on some type of stop plate on the lower fork clamp Did you need to add stops to your bike? If so, how?

Thanks,

Dan