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Author Topic: Got a free '78 GS 750! Gonna attempt a hardtail bobber (sort of..)  (Read 8325 times)

Offline villageidiott

  • Posts: 13
Hello VonYinzer
Thanks for the advice... I will have to take a bunch of time to consider what to do. Luckily I am not in a hurry.

I do have a question. If you properly weld a swingarm in place or in the case of the picture above (though not with that flimsy looking piece of metal) place metal tubing (read as weld and not bolt on) from the tank area to the swingarm, why would that be less structurally usable than welding a new rear section in? I was toying with the idea of cutting the metal where the secondary pegs are located, heating it and bending it under the swingarm and welding it (well them) as support. I thought perhaps I could also weld the swingarm where it pivots.

Perhaps its a crazy idea.

I appreciate the help. I am a novice but grew up on a farm and have done a lot of different mechanical builds and fixes. I am no wiz in the welding department to say the least and I haven't rebuilt a multi-cylinder engine before. Again, this project is about learning so thanks again for your help!

Offline hillsy

  • Posts: 4061
I was toying with the idea of cutting the metal where the secondary pegs are located, heating it and bending it under the swingarm and welding it (well them) as support. I thought perhaps I could also weld the swingarm where it pivots.



If you mean like this it's all kinds of wrong:







Proper hard tails do not use the swing arms off soft-tail bikes. They are not designed to flex, whereas a properly designed hard tail IS meant to flex. There's more science to the shape of a hard tail than just the lines and the look.


If you do go with using the original swing arm I would advise you weld it up solid at the pivot point.

Offline villageidiott

  • Posts: 13
So am I to assume that cornering on an inflexible hardtail will be dangerous at speed if you hit a rough patch? Hmmm... thanks a lot for the info guys.

Problem is... I really like the look of that bike in the photo above. I love the seat and the clean lines at the back of the bike. That said I would rather not end up in a ditch!


Offline VonYinzer

  • DTT SUPPORTER
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  • Posts: 18337
Well... Here's the deal...

The swinger is meant to pivot at the frame and be sprung at the other end (obviously). It is not designed to he a rigid section in the frame. Not only that but a stock bike sits way higher than a chop should. To handle properly the bike should be lower and longer than stock. Not a lot, but a few inches. The only way to correctly do this is with a properly built hardtail section. It's really way more complicated than that, but trying to type it out on my phone is too much of a pain to dive into it. Haha. Go through that link I sent you. Lots of info about proper frame building in there.
Like a river that don't know where it's flowin'
I took a wrong turn and just kept goin'

Offline hillsy

  • Posts: 4061
OK - rough edit time....
 

 
The lines in red are where the frame rails should be. Then cut out the swingarm and subframe support rail. Of course a better option still is to lengthen the wheelbase by a few inches (4" is the accepted norm / max).
 
The way the new frame works at the back is that it flexes and pushes the bumps and other forces into the main frame(s) of the bike. The extra length helps the flex and complements the "suspension" effect of the frame flex.
 
The current set-up concentrates all the forces into the middle of the rear subframe support rail (not good). Add in the fact that the original "not designed to flex" swingarm is in there and you have a stupid harsh ride as well as a ticking time bomb for the frame to crack.

Offline villageidiott

  • Posts: 13
VonYinzer and Hilsy... thank you so much for the technical lesson. I am so glad I didn't chop the frame past the shock mounts when I was putting it in the back of the SUV!!

So I have been doing tons of research on other options. Brat-style, Cafe... they are both interesting but I think a Cafe style might be a bit more doable for my first build. I can still mess about and do some modding and playing around but can also keep from ending up in a ditch somewhere!

Something like these examples?


Offline hillsy

  • Posts: 4061
I think you should be first concentrating on getting your motor back together and just getting the bike functioning / running / on the road before you think about a style or other dress-ups.
 
That motor may well need a crank re-build if it's been sitting the best part of 20 years with the cylinders off, so that is going to empty your wallet real quick. Whilst the Suzuki roller bearing cranks are bulletproof in operation, they can get rust in the rollers a lot easier than plain bearings and this is their downfall if they are left sitting without oil - and you have to pretty much do ALL the bearings when you re-build these cranks (I spent over $3K re-building an 1100EF motor some years ago so I know how easily the costs can spiral).

Offline villageidiott

  • Posts: 13
So I am thinking about ordering some Bridgestone tires... but am I safe to order 130/90-18s for this bike?

As an aside, after some serious elbow grease... the rims have come out VERY clean. I imagined pitting etc. and there is virtually none.

Check out these pictures... ( i haven't polished the spokes yet)...


Offline Big Rich

  • DTT SUPPORTER
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  • Posts: 9970
  • Heaven is so far away.
You need to find the rim width - it's normally stamped near the valve stem hole. Most likely, yours will say "2.15*18".........than a 120/90-18 is the largest tire you could fit.

Offline villageidiott

  • Posts: 13
Well I hope not. The bike has only had the head off for about 2 years and I am hoping it doesn't need a bottom end rebuild. It was sitting in a dry shed and I will be opening it up with dad to take a look. Lord help me if the crank is in trouble.

I have the bike apart now which is why I am thinking about change. I will have to deal with the engine at some point regardless of what I do with the frame. I don't want to put it together just to take it apart again.

I first saw the GS when I was in my late teens. I absolutely hated the length of the bike. I was on a Yam RZ 350 at the time and couldn't believe my dad was driving this "limo" style bike. I have mellowed since then (now in my mid-30s) and have come to appreciate the older vintage bikes but i will never leave the bike as it was. It is an easy decision now esp as I have already chopped the frame just behind the shock mounts.

Today I used the grinder to remove the center stand mounts. The center stand was SO heavy. I also placed the tank and side covers on it to see if any inspiration came to me. Nothing yet!