1988 851/750 sport bastard case

customhead

A risky proposal I know... but anyway Ill give it a shot.
I am going to make up a bit of a custom using an 851 frame but fit up a 750 sport engine.
I am tempted to modify the frame and use an OVER RACING f1 type swingarm as the pivot is a smaller diameter than the 851 and the sport engine has the same pivot size as the F1.
Measuring my 1988 851 swingarm and the OVER swingarm I find there is about 20 mm difference from pivot center to swingarm axle adjustment 'center'; the F1 type being the shorter.
I heard that the Suzuki TL1000S's (supposed) handling problems were due to a short swingarm and was wondering if anyone had an idea if I could be heading down this somewhat dodgy path.

thanks and yes I can handle the potential abuse,

Sean 😄

Pompey

Just curious, what do you like about the 750 engine?

FlyingG

Greetings,

I did a similar job with my 851 when I was racing a 750SS in lightweight twins/superbike. The little SS was a chatter machine. After living life on the edge for a few years, I decided to stick the engine in my 851, which had the engine out anyway. Had to cut out the X-bracing between the cylinders. I don't know if the '88 models have that bracing. I added a bolt-in brace.

It all worked incredibly well & it cornered like a tabbycat on carpet. Still, it was considerably heavier than the SS chassis (since there's more of it, I guess), & I really did not want to trash the tank & front/rear subframes, so I eventually pulled the 750 motor & had the X-brace welded back in. I finally got some good chassis help & fixed the SS's chattering problem with a much longer than stock Penske rear damper.

I did not have any issues with the fitment of the swingarm, as I recall; but you're doing something different there.

Cheers.

FlyingG

Regarding the swingarm length, longer is better, all things being equal - more forward weight bias & better rear grip. All Ducati swingarms are relatively short because of the engine config, but that hasn't kept us from winning a few zillion races, right? Still, longer swingarms have been part of the race kits since the 916, as I understand it.

I think the TL's biggest problem was the rotary rear damper, which would heat-soak, then the damping would pack up & head home. My chassis tuner (Kent at GMD Computrack Atlanta) had a big box of stock TL dampers under the workbench. He even gave me one to take home!😁 The TL racers were replacing them with a standard-style damper kit from Penske.

I like your project - a super-trick 750 Sport. Cheers.

brad black

no abuse, tho i'd be inclined to point and laugh.

if you recase the motor in later 750 cases you can use the 851 swingarm.

do you have a chassis without motor? i could almost use one of them.

customhead

Ill swap you a frame for some cases and a swingarm Brad 😄

In answer to the previous question I had a 750 sport while living in Japan (amongst about 20 other bikes) and was amazed how the engine performed compared to my friends santamonica F1. we both had 39 fcrs fitted and I was using japanese made short manifolds. His spun up so quickly but went nowhere while mine was a torgue monster. Sprocket size etc may be factors but I have always loved the engine. I felt I could ride it to the edge whereas with the 900ss and 916 i also used to ride i had to hold back a lot more and I really loved to ride it hard. My setup was pretty crap in those days so I loved fighting it for control and now that I have ridden the 851 frame I want to combine the two and see where it gets me.

I am interested to see if I have to cut the x-bracing as well seeing as I have two seperate FCRs and short manifolds.

btw the frame and tank arrived today! beautiful!

Sean

sagot

If you building a bike like this, may I suggest that you make your own frame and save the 851 frame for an 851.

customhead

Now thats a cool idea, but my expertise has not risen to that elite level yet. Its a bit of do what you can with what youve got and after living in Japan the land of customising I dont feel the need to keep things original especially if i am starting with bits as opposed to fixing up an existing machine.

besides, Im not too sure if i could make something better than that frame from scratch design wise anyway...what if it rode like crap? and dont forget the expense...gotta live within your means...

Sean

Pompey

Building your own frame?!!! Blimey, that does seem excessive... If you like the 750 engine then why not I say. I enjoy my DB2 for the same reason, 900ss engine you can ring its neck everywhere and feel great about it. Can you not use a standard 851 swing arm, they are not to expensive second hand and come up fairly often on ebay?

customhead

The 851 swingarm has a small diameter pivot axle which would fit your 900ss engine fine but the 750 sport engine has the same pivot diameter as the F1 which is why Brad suggested using later 750 cases. Actually I do have a spare 6 speed 400ss engine from 1996. I reckon those cases should be the same as the 750...Brad?

Pompey

How about machining some shims either side?

brad black

can't you fit some sort of pivot bearings into the engine cases to take the 851 swing arm pivot? should be able to find something or fit an insert as required.

400 cases are the same as 600, cylinder holes are smaller than 750 pre 98. plus they're not as strong as 750 pre 98.

customhead

Now that sounds like a good idea. Ill have to drop in and talk about it soon. Mmmm pity about the 400 engine. oh well I can still use plenty of stuff off it.

FlyingG

Regarding the bracing, I used separate FCR's on short manifolds, too.

Maybe the problem wasn't carb clearance (though it could be that), but just getting the engine in the frame at all. I don't know if you could do it with the heads removed w/o cutting anything. Seems like I would have gone that route if at all possible. Been too long ago to remember, I'm afraid.

Also, I removed the lower diagonal brace, as well. Sounds horrible, I know, but Kent at Computrack measured the geometry & said it was the straightest factory bike he'd ever seen.

I'm on the path to setting it all back aright, now. Cheers.