single down tube frame question

xstim · Jan 30, 2010

Here is an odd ball frame I picked up. I have plans for the swing arm, but I am thinking of using the rest for brat style some thing or other. The head tube is not xs so is a little hurdle.
The question is will my engine cases be ripped apart in no time? The downtube is cut off below the front mount. I have seen Mule's bikes set up this way But I think he is a miracle worker. This was a pretty rough sand racer or hill climber I think. I only have the frame and swing arm, but lots of stock XS's in the shed.
I don't want to be courting death but it would be different. Whatcha think?
Cheers
Tim

XS1961 · Jan 30, 2010

Ok off-road but a bit too flexible for road use maybe ... :shrug:

inxs · Jan 30, 2010

- looks like a trimmed down std frame...447 type...on the whole i tend to agree with the previous comment...id think twice before serious road use

xstim · Jan 30, 2010

I thought off road bashing would be about equal to road use, but maybe not?
Serious road, like a Cafe I have a Tx frame, but I was hope full this might be adequate for a bobber. I get the feeling your saying not even this?
Tim

inxs · Jan 30, 2010

- those first generation frames are susceptible to flexing unless you want to spend time and effort welding in gussets...off-roading is stressful, sure, though youre not likely to be doing the critical high speeds as on roads, in traffic...as with every XS frame, steering head bearings and swingarm bushes-bearings are a must

xstim · Jan 31, 2010

Somewhere I saw some info about adding a extra cross member near the swing arm piviot. I don't know just what was involved. Looking at it, I am thinking under the swing arm connected to the down tube and side gussets near the foot pegs. Does this make sence? Can anyone help with any frame improvements after swing arm bushings and head tube bearings?
Tim

jay760 · Jan 31, 2010

Looks like the engine has been angled forward a tiny bit but might be an optical illusion, I've often wondered how the frame handles with the bottom tubes taken out, I've seen it done several times before, somebody must know.

inxs · Jan 31, 2010

- i think the motor angle is an optical illusion...the rear and top mounts look original...if you tilt from the original rear mounts you have clearance problems with the case-rear against the downtube

- the frame tends to crack near the rear motor mount..many add gussets where the frame cracks - a patch rather than a cure ... weld a peice of pipe between the frame's side plates just below and parallel with the swingarm...tie it to the spine just below the motor mounts

shaas · Jan 31, 2010

I'm no engineer but I pretend to be one in my shop when no one is looking...

The XS wasn't designed to use the engine as a stressed member and with the removal of the lower frame section forces it to become stressed. That said, off road riding beats the snot out of a bike.

XS1961 · Jan 31, 2010

Off-road frames are pretty simple, flexible & light, most of the work is done/absorbed by the long-travel suspension , all the twisting is controlled with wide bars and a lot of legs out/feet down plus the motors are lighter than road-going engines.. On road bikes too much twist/flex makes them unstable especially at high speeds so a heavier more complex frames needed..
years ago I had a Triumph T140 that had the frame cut in an almost identical way to xstims example - it handled like a bloody jelly !

pie plate · Jun 6, 2015

Excuse me, I know this is an old discussion, and this is a bit off topic - If a single carb as was sometimes fitted to Enflields and Triumphs, ie mounted on a symmetrical "Y" cast aluminum manifold (as I recall), anyway if so, then the large center tube would interfere, unless it were moved, modified. Question: Has anybody done that on his XS ?

Thanks! Great old discussion, planin' my next XS...

fredintoon · Jun 6, 2015

pie plate said:
Excuse me, I know this is an old discussion, and this is a bit off topic - If a single carb as was sometimes fitted to Enflields and Triumphs, ie mounted on a symmetrical "Y" cast aluminum manifold (as I recall), anyway if so, then the large center tube would interfere, unless it were moved, modified. Question: Has anybody done that on his XS ?
Thanks! Great old discussion, plannin' my next XS...

Hi pie plate,
there's several single carb XS650 manifolds that you can buy and they all get past the single fat seat tube by deking off sideways to make non-symmetrical intake tracts.
Betcha the site's search button would find them for you.
What I'd do would be to weld an eye into the seat tube just like on my plunger frame BSA Gold Flash.
Note that the Flash had it's carb in front of the seat tube and it's eye let a rubber adapter through to connect with the air cleaner.
My XS650 mod would have a symmetrical manifold on the engine with a rubber hose going through the eye and the carb behind it with one of those Britbike frying pan air cleaners to keep the dust out.
The stock battery box would have to go, but nobody leaves it there anyway, right?

single down tube frame question

xstim

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