Converting a standard C15 to off -oad spec.

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I've just picked up a running C15 recently for not very much and it's a bit of a bitsa, engine '59, frame '62. This engine was recently rebuilt with a roller big-end and is in a soft state of tune but the cycle parts could do with quite a bit of work. This'll be my third ceefer (all were in road-going spec), so I'm generally familiar with the platform and the engines. I've always fancied a C15 in a more off-road spec (even considered converting my old SS80 into scrambler) and because I live in rural Shropshire, 2 miles up a single track road it seems this one would be a good starting point - I currently have no intention of using it for competition at this stage in life (but you never know!), I just want something a bit more suitable, C15T-like, for these rough roads. I got rid of my cafe racers years ago!
The bike has standard road forks but the C15T fork components seem very hard to come by so I may have to keep the standard forks for now and change later if the opportunity arises. I note that the 20" front wheel originally specified would be hard to get tyres for now so I will settle for re-lacing the standard full-with hub with either 19"or 21" WM1 rims. I've been struggling to find dimensions etc. for the standard C15T forks, hence this post. I plan to go to the Telford Classic Off-road show this February to see what info I can glean.

• My first query is whether the standard road forks and yokes could accommodate the additional 2" radius over the current 17" wheels? Did the C15T, for example, have longer stanchions to allow the travel of the larger 20" front wheel and would they be needed for a 21" ?
• My second query is that I'm considering keeping the 17" rear wheel and fitting a dual-sport knobbly. Would it be better for geometry to re-lace it too to 18"? The current shocks are 11.9" items.

I'm happy to spend what's necessary make it credible for handling a bit of the rough stuff but not interested in throwing money at it until I achieve a best-of-breed competition machine!
Any observations about my plan most welcome.

[Oldgoat54]

Hi
  Back in the day I had a C15T (B40 engine - C15ss cams), and the best improvements were from shedding weight.  I ended up with MP front forks, Bultaco wheels, and an alloy tank from a Cheney Victor.  Unfortunately life got in the way and I put it back to standard to sell it along with the other bits to fund house purchase.  Enjoy your green laning!

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So I guess, alloy rims on the wheels would be a start? Early C15Ts had the full width hub according to the books I have so I'll keep that. Messing around with forks and yokes etc. might be more than I want to indulge right now. That said, I've had other bikes that started with a simple change and that was just the start!

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Well, my continued research, meanwhile, has found this page: https://www.bsaotter.com/bsa_c15_t_replica.html
This gives me at least one answer I was looking for: Mike Waller converted a road C15 to C15T spec and said, "The front wheel fitted had to have a 20" rim for the wheel not to touch the frame on braking.
The 65 degree head angle was 2 degrees steeper than the road bike, this was not a different frame but just jacked up at the back slightly with longer rear shocks."
So, if not a 20" rim because of poor tyre availability then I must consider a 19" rim.
By the way, my newly acquired bike already has the advantage of having a high level exhaust tucked inside the rear subframe tube, one of the things that attracted me to it as a possible lower cost off-road project.

[limeyrob]

I like off road projects, you can get rid of a lot of weight, save having to prep and paint a lot of tin ware, make some bits and bobs and get a really nice looking special.  You can get some very nifty rubber rear plates with a small rear light and fit a removable 4 or 5 inch headlight.  I did this on my Matchless and wired it so all the light switch and dip were both on the headlight with only the horn and brake swich on the bike.  Then the plug to the headlight was simple.

[scifi]

Living on a Bridleway in Shropshire..?  You might be someone I know..  Hint..  Smelly Husky 250.  (Went to Oulton Park on it, to see a practice day, last month.)

I did have a C15 years ago, and used it for Trials, Moto-x, and Grass-Track.   The Trials set up used Wide ratio gears.   The 20 inch front wheel with MX tyre  fouled a mudgaurd bolt on full bump.   I lowered the compression to about 6:1 by machining 1/16 inch off the top of the standard piston.  The carb was left as the standard 7/8 inch Amal.   I think I used a flat BSA Bantam silencer, on the high level exhaust pipe.

I shortened the solid foot-pegs, OK in 1970, but ACU Regs now require folding footrests.

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I'll look out for a smelly Husky. The only ones round here have mad starey eyes! 

Useful tip about the 20" front wheel - I'd ruled it out because of poor tyre availability - and 21" definitely will cause problems unless I get longer forks (BSA apparently commissioned Dunlop to release special 20" tyres to support their 'parts bin' design).

I have since found out that the standard fork stanchions are

• 20 3/8" (517.5mm) long overall,
• 1 1/4" (31.75mm) dia in
• 6 1/4" centre yokes, using an internal spring

and the competition/trials/scrambles (or whatever) stanchions are

• 21 13/16" (554mm) long overall
• 34mm dia in
• 6 1/8" centre yokes (so called narrow forks) using an external spring.

Ideally I'll convert to the longer, narrow forks but I suspect that a 21" front will be still too big.

BTW, pls correct me if my information above is faulty: I've driven myself half-blind poring over websites (on the Internet, it must be true!) and various books!

[limeyrob]

Spent a very enjoyable day at the Mike Kemp Classic Trial over at Aldershot (as a spectator..). Big turnout, anyway point is there were quite a few nicely prepared C15s doing very well. Remembering this post I had a look a the forks, only one I believe used the BSA forks, most had later non-BSA forks and front wheels, usually with ally yokes and longer travel plus a matching 6" or 7" single sided hub.
But just to bring us back to reality someone was campaigning a rigid B31 with girder forks - respect!  My favourite was a C11 special, rigid rear end, a home made ally barrel, Aerial ally head, titanium exhaust and flywheel mag ignition!

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That C11 certainly demands respect too: mechanical marvel indeed!

[scifi]

If you go for Profile forks and a 21 inch front wheel, you could find that the steering angle is too flat, and the trail will not be suitable for tight sections.  Any 20 inch tyres are very rare these days,  So I think you are stuck with what you have.

To do some organised classic trials, then maybe a Yamaha TY175 is more suitable, and lighter.  Also it has all the right gearing, and is easy to start (40 times in a trial.)

If you just want to be Off-Road capable, then just change the tyres to knobblies, and that will get you around most forests and green lanes.  You might find a rear 4.00 trials tyre will be a tight fit, and maybe a smaller MX tyre will be needed.

[limeyrob]

It was interesting to compare the bikes, all were "classic" so the most modern were the Hondas and Yamaha twin shocks with a good smattering of Bultacos.  Then the Brits - C15, Tiger cub, Triumph twins, a Goldie or 2, some Aerial singles the odd Norton and a good number of Matchless /AJS, Greeves / Villiers and Bantam specials.  Surprising number of rigids.
There were 3 options for each section and while the really old bikes with girders did the "easy" (less impossible) option many of the C15s and Matchless G3s did not and went for the intermediate or hard. Only the lightweight bikes could clean the tough sections, the big difference seemed to be the weight on the front wheel and the overall weight. The only rider to clear the toughest section hopped the bike around the tight turn, I defy anyone to do that on a Matchless or C15.

[scifi]

Vee Rubber do a 3.50x17 Trials tyre that should fit your original wheel.  Most 400 section tyres are run at 4psi in trials, but as your wheel does not have security bolts, the 350 Vee Tyre could be run at 12psi, without slipping round the rim.   Ebay link..

https://www.ebay.co.uk/itm/312366354956?_ul=GB&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5339019732&toolid=10001&customid=eb%3Ag%3Avms%3Aeb%3Ap%3A312366354956%3B{GCLID}&adtype=pla&loc_interest_ms=&loc_physical_ms=9046482&gclid=Cj0KCQiAyP3KBhD9ARIsAAJLnnYCByFeh-v21DkKoFCwtrrNkbvozyLiUfRgWNb-iwrRGD72yjhQGqAaAgjuEALw_wcB

I have not yet looked for 275 or 300 x 17 front tyres, but suspect they are readily available, as junior trials bikes run that size wheel.

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I have not yet looked for 275 or 300 x 17 front tyres, but suspect they are readily available, as junior trials bikes run that size wheel.

Thanks for that info, scifi.
I have decided to keep the rear as a 17in for now (using the existing hub and rim, may go up to 18in eventually) and plan to run a 3.50 x 17 rear - I probably won't run them much lower than 15 psi anyway, this is just for some of the rougher bye-ways open to all traffic [BOATs] round here (there's one right next-door to me), not full competition courses, I'm not that hard core!
I'll put a single seat on it, and cut back the lugs that hold the pillion rests to allow the swing-arm to sit lower so I can put longer shocks on (they're currently 11.9" shrouded).
As for the front, while I source all the bits for the heavy-weight forks that I want to fit, I'll run standard road forks for now with a 19" wheel shod with 2.75 x 19 off road tyre (the wheel's currently with CWC). The forks need only a little work as do the steering head bearings.

While the engine, as I say, is pretty good (it actually started with just an experimental prod), the rest of the cycle parts turn out to be less than one might hope for, with a horrible assortment of Metric, SAE and Imperial fasteners - my A1 bugbear - sawn off tank lugs, missing wheel nuts and spacers, bits of Bantam and WHY? and so on.

I was hoping to make it to Newark tomorrow (Sat) to get more bits and info/tips, but we've only just cleared a path through 30cm of drifting snow so looking unlikely now.

[V500]

with a horrible assortment of Metric, SAE and Imperial fasteners - my A1 bugbear
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I'm with you on that. Why would anyone do that?

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I'm with you on that. Why would anyone do that?

As a get-you-home measure, sure, I've done that, we all have I'm sure, but assembling so you need to use three different of spanners to remove four fasteners seems like an act of wilful self-harm.

That said, having worked on a late Triumph Daytona (72-73), I found out how much the factory itself indulged in pick and mix fastener codes! I can't speak for late Oily Frame Beezers from experience, but I'd imagine they are just the same.