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Author Topic: Got a free '78 GS 750! Gonna attempt a hardtail bobber (sort of..)  (Read 3546 times)

Offline villageidiott

  • Posts: 13
Greetings.

First post, first time using a 'bulletin board' site, first build thread and first build. Please be patient.

Managed to convince my little brother to part with my dad's old GS750 (thinking it would be a quick way to get cheap wheels on the road)... then I discovered this site. Now I am sure this will be more expensive than I planned, more difficult and a ton more interesting!

The bike was unbeknownst (is that a word?) to me.. in pieces and in boxes with a rolling chassis. Last time I had seen it it was complete and needed a headgasket. Times change. Little bro thought about a rebuild but then bought a '72 Karmen Ghia and relegated the GS to the yard shed. The bike has been sitting since '93 and has 36K original kilos.

Here are some pics (if I can get this all to work for me)...

Hoping to hardtail it and make a bobber-like bike out of it. Something fun and original. It will be a slow build as I have bar exams to study for and other boring law stuff to worry about and I have less money at hand than would be ideal.

Thanks for looking!


Offline Big Rich

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  • Heaven is so far away.
Oh my........you've got your work cut out for you. Where are you from - Canada?

If you haven't already, check out the GSResources - they know every little detail about that bike.
Taking a break for a bit. So direct all pm's elsewhere please.

Offline villageidiott

  • Posts: 13
I wanted to show a picture of the bike as I remembered it. It is a 78 GS 750 (not the E model), so it has one front disc rather than two.

The more I look at it, I think i paid too much for it! ha.

The other issue is that because I didn't tear it apart, I am going to have a great time putting it back together. I love learning but this curve is going to be steep!

The first pic is the original bike (right down to the colour) and the second pic is sort of the look I am aiming at.

Please let me know what you think. Any comments are welcome.




Offline villageidiott

  • Posts: 13
Yes I am from the Kingston area (couple hours east of Toronto).

I chose this project because I really want to know how bikes are built, including engines. I have been around bikes all my life but haven't ever rebuilt one (besides tops of 2 stroke dirtbikes which a monkey could do).

The good news is all the major parts are there in some form or another. The bad news is I have no idea what half the parts are! The exhaust and front fender are missing which is fine as if my memory serves me they were swiss cheese anyway.

My first task will be to get the front and rear wheels off and clean the entire frame. Get rid of the rubbish tires and polish the rims etc. The wheels were so badly covered in crap I thought they were aluminum. I am also going to grind down the centre stand mounts as that will never be back on the bike and I will have to cut a chunk of the back end off just to get it in my SUV (sort of cafe style cut of back end).

More pictures soon.

I have a lot of work to do.

Offline hillsy

  • Posts: 2888
What's the registration proceedure in Canada like? Is it the same as the US where you need a title? And if so, do you have that?
 
You might want to investigate the costs involved before you get too carried away - because there's no such thing as a "free bike".
 

Offline Big Rich

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I might want to add - hard tailing a bike is a LOT of work, and can be extremely dangerous if not done properly. Have you looked into maybe making a brat style bike?

And if you decide to run mag wheels, let me know. I've been looking for a spoked rear wheel like yours.
Taking a break for a bit. So direct all pm's elsewhere please.

Offline villageidiott

  • Posts: 13
Ya I have the title so no worries there. Alternatively, I could just keep the bike in dad's name (he is still the legal owner, though he has signed the ownership over to me).

As for costs, I am not worried, as there is no rush to get the project finished. The engine will need at worst (famous last words) a top end rebuild. I can do that with a little help from my father (an excellent mechanic... planes, cars, backhoes... you name it). I have looked over the parts and everything is there apart from the aforementioned exhaust and front fender. I assume some of it will be unusable and trash. I also assume that I will have to put in a massive amount of elbow grease. I have an excellent workshop at my disposal so that will help a little. I wish I had a machine to do powder coating!

I was sort of thinking that 2000 over a year and a half should get me most of the way there but I would like to spend less. I expect I will spend more!

By the time this project is done, I will hopefully know the basics of how to tear apart and rebuild an inline four DOHC japanese bike. That is whole point of the project.

Please be patient if I sound like a newbie. I am. Few months from now I may wonder how differently this topic introduction would have been written.
I am just smart enough to know that there is a ton I don't know and there will be lots of things I hadn't planned. 

Offline villageidiott

  • Posts: 13
I really need a little help on tire sizing.

Is it possible to fit a 130/90-18 on the back of this bike and still have chain clearance? The stock tire is a 4.00H-18 4PR. I don't know what the 4PR means but I would like to put the widest tire possible back on the rear wheel.

Does anyone have any experience with this?

Thanks for you help!

Offline VonYinzer

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First things first... Hardtailing that frame will be really quite difficult for the novice builder. The "hardtailed" bike in your one pic is exactly how NOT to go about it. To do it right you'll have to remove a good bit of that frame and build the replacement section from scratch. So... You'll need a tubing bender, a proper welder (and more importantly the skill to use it), a frame jig, axle plates made, and so on...

As Rich said, a "bratstyle" bike may be a more viable option. At least for now. Another option is possibly finding a chopper frame made for the cb750 and simply replacing the motor mounts. There have been dozens of company's making chop frames for the cb since the early 70s.

Don't mean to sound like a downer, but the reality is that what you build can end up killing you if done poorly. You said time isn't an issue so take your time and do it right.

Good luck and keep us posted.
That shit's bananas.

Offline VonYinzer

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Read through here if you haven't also...

http://www.dotheton.com/forum/index.php?topic=12664.0
That shit's bananas.

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: 2888
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

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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.
That shit's bananas.

Offline hillsy

  • Posts: 2888
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.