Bully MK5 offset

[huskibul]

Hi any of you bully blokes know if mk5 and mk6 pursangs have different frame geometry, i'am wanting to change my mk5 from tapered fork tubes to straight tubes with pinchbolt clamps but from what i can makeout the mk6 seems to have a smaller offset in clamps ?  i have other clamps(marzocchi)with more offset i could possibly adapt with a bit of mucking around  if mk6 aren't suitable ,any info would be apreciated,cheers :)p.s anybody had anything to do adjustable (concentric)offset bearings in steering stem?

[Mark Austin]

Hi,
I have a Mk 5 and a Mk 6 Pursang which both have the same front end. Ie alloy clamps with no taper.
I'm not sure the Mk 5 is original, but they both have the later triple clamps and they both handle exactly the same.
Cheers,
Mark

[JC]

Huski,

You are correct that Mk6 triples have less offset than Mk5. (They also have much better clamps for the handlebars). Mk6 & later triples have 34mm offset. I'm not sure what Mk5 & earlier have as I don't have a set handy to measure but I'd think they'd be around 50mm or more.

I don't know for certain what diff/similarity there was between the Mk5 & 6 frames, but I've long tho't they were probably built on the same frame jig (which incidently was probably the same for Mk 7 & 8 also). There doesn't seem to be any major diff like a diff in rake between them.

I have sometimes wondered if the Mk6 used a longer swingarm since Bultaco literature quotes same wheelbase for both models, but Don Newell once told me, when I asked, that he always tho't the swingarms were the same length but he'd never checked.

Perhaps Auscobul could measure the swingarms for you to check if they're same/diff

The Mk5 was known for the front end washing out when cornering, while the MK6 & later were known for sweet handling w better steering.  That is precisely the effect you'd expect from reduced offset, cos it brings the front end back effectively putting more weight on it & thereby getting better grip. (Of course it also produces more self-centreing effect so it may feel slightly heavier in the steering, but that can be compensated by using 1" longer shocks)

If I had a Mk5 I'd be using MK6 triples too. In fact thats exactly what I plan on a Mk4 project (which has the same issues as Mk5).  Obviously Auscobul has already done it & it works.

Hope this helps

[huskibul]

  Hi guys,yes the mk5 has around 55mm offset and if the mk6 has 34 it seems a lot of difference not to effect the handling ,is there any chance ausco that whoever put the later clamps on your mk5 also changed the swing arm or frame/swingarm? a little oversteer doesn't bother me but high speed stability could  get a bit hairy,maybe it's just the swingarm, would there be a chance one of you kind fella's(when u have a chance) measuring a Mk6 swingarm for me from centre of swing bolt to end of arm ,would be much apreciated ,  thanks AB &JC thumbsup 

[JC]

Huski,

I wouldn't be bothered about loss of hi speed stability. Even w the reduced offset clamps the wheelbase is still 1400mm/55.1". And thats what Mk7s & 8s were. I strongly suspect Mk6s were too - Bul probably just didn't get around to updating the wheelbase spec on their brochures I suspect. The Mk4&5s were a bit long in standard trim.

Also more trail (from reduced offset) generally gives more hi speed stability cos of the the increased self-centreing effect.

[huskibul]

Yeah no worries JC, I have a pair of 34 offset maybe I'll just throw them on when the time comes and start testing. Any chance of someone giving me that swingarm measurement at some time just for comparisons sake?it would be really interesting to know  if  bul kept the frame/swingarm the same and just chopped  20mm of offset, just seems a lot when most adjusments like that are done in small increments     Cheers P.S haven't ridden a Mk 5 since 1972/73 - can't wait!

[Mark Austin]

Hi,
Both frames are standard and unmodified, I'll measure both swingarms for you over the weekend and see if there's any difference.
Cheers,
Mark

[huskibul]

 Thanks mark i measured mine from centre of swingbolt to very end of arm on left side

[Mark Austin]

Hi,
Both of them measured 19.25" - from center of the swingarm bolt to the end of the swingarm. I have a feeling the Mk 5 swingarm may be from a later model, as it has a rear wheel from a Pursang Mk 8.
Cheers,
Mark

[huskibul]

Hi Mark, Thanks for that, it measured up to be the same as my Mk 5, Bul must've just docked 20mm of offset the get the handling of the Mk6 right ,wouldn't you think that they would have done that in the testing stage of Mk5 but i suppose thats the beauty of a lot of the old stuff thev'e all got their own characteristics,that works good for me, already have the clamps/forks,and don't have to think about swingarms,it will all become clearer as i start riding the bike i suppose ,Do you run original carb and ignition ?thanks again

[Mark Austin]

Hi Huski,
Both of them have new Mikuni carbies and electronic ignition. One from a YZ Yamaha and one special battery powered jobby.
Its not worth mucking around with old carbies and poor spark. I've spend too long pushing the buggers when they either won't go, or they just stop way over the far end of the race circuit!
If you need jetting specs for Mikunis, let me know.
Good luck with it, they're great fun - especially in dirt track.
Cheers,
Mark

[JC]

Mk8 swingarm is 19.25" also. No doubt Mk7 is too.

[huskibul]

  Yeah mark i will probably go the mikuni way, went that way with 72 sherpa a few years ago and it's been super reliable and really easy to start,(and i ride it heaps) do you use the std vm-36 on both your sangs ? would be great to get those jetting spec's off you, thanks for all your help.  thanks also JC 

[Mark Austin]

Hi Huski,
36mm on the 250 and 38mm on the 360.
I'll send you a PM on the jetting specs, I'll have to dig them out.
Cheers,
Mark

[huskibul]

  No worries mark , theres no hurry I'll have to chase a carb and adapter up, still got a bit to go with the resto, cheers