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Suspension advice for Scrambler build

EMM

CB750 Enthusiast
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I’m building my 1979 CB750L into a scrambler.
This is my first bike and have been doing mounds and mounds of research learning as much as I can and planning a roadmap for the build.
But by far, the most dizzying aspect of the build has been figuring out what to do for suspension.
I want to to increase the ride height by a couple inches as well as find a nice balance of on road/off road rideability.

I’ve seen things here and other places about doing a front end swap from a GL1000. I got confused upon doing research, because some threads say it’s pretty straight forward, and other threads have said it’s a nightmare and costs $$$$.

And as far as the rear suspension goes; I need advice on what size I should go with to achieve an added sturdy couple inches.
I am perfectly willing to fabricate and do the work I need to in order to make things happen, it’s just a matter of knowing WHAT to do and HOW to do it.
I have experience working on cars, but motorcycles are a whole new territory for me. It’s proven extremely difficult to nail down information about these old bikes.
ANY information, guidance, or knowledge you could share with me regarding how to tackle the suspension for this scrambler project is beyond appreciated.
 
In the FWIW department (as I haven't modded a 750L specifically); I think the GL1000 fork mod is primarily to gain the dual disc brake set-up. On a scrambler build that may not be desirable as it's a lot of extra weight. One solution would be to upgrade the existing forks with Racetech springs and emulators and then maybe go with a '75-'77 GL1000 spoked wheel (aluminum rim) for lighter weight and the scrambler look. You can run a single LH brake on that hub by just getting shorter bolts (that I have done).
https://racetech.com/ProductSearch/12/Honda/CB750K/1979
On the rear, a longer shock to get the desired stance, perhaps 14" eye-clevis, there are a number of choices. The longer length may increase swingarm angle and require a larger R sprocket and a pad atop the swingarm pivot area to handle occasional chain rub. A rear hub from a '77 750K laced to a '75-'77 rim from GL1000 to match the front - may work (not done it specifically).
https://www.randakks.com/progressiv...s-motorcycle=Honda/CB750 DOHC/CB750K-LTD/1979
 
How serious of a scrambler are we talking? Are you thinking gravel roads, or rocky singletrack? It's also important how much you weigh, because if your suspension isn't properly preloaded right now you can gain significant ride height just by changing to the right spring rates for you. These bikes have a reputation for being undersprung from the factory for anyone more than 150 pounds.

Adding significant ride height can do a couple unpleasant things to your handling on these bikes. I'm no suspension/chassis engineer but I foresee two problems.
The first is with adding significant rear ride height. It'll shorten your wheelbase, which will make your handling more twitchy. Can be an issue when trying to bomb gravel roads or trails, as any shift in the direction of the front wheel (say catching the side of a rock or rut) will turn the motorcycle more and that can make it harder to stay in control at speed. This is more common for track bikes than it is for offroad machines.
That being said, adding up to an inch of rear travel probably won't hurt, as it will have very little impact on your overall ride height. It will, however, make your bike look like it's leaning forward compared to stock, and put more weight on the front wheel.

The second is that these older forks are all pretty noodly. The longer you make the front end with vintage forks, the more flexy they become because of the increased leverage the terrain has on the front end. If you're tackling any serious obstacles like rocky trails at speed, you could end up bending the upper tubes. Swapping to a GL1000 front end gives you 37mm fork tubes, which is the same diameter as the CB750F from 81-82 but nowhere near as stiff as, say, upside-down forks from a much newer dirtbike or dual-sport. It'll also get you stiffer stock springs, but if they're not the right spring rates for you then you haven't actually tuned your suspension. If you're going to do a front-end swap for serious offroad duties it's probably better to go with something from a dirt machine but finding compatible triple trees can be very tricky.

I agree with jpdevol that going with proper Racetech springs and cartridge emulators, and lightening the front end, is a great avenue. If you pair that with a proper pair of rear shocks, like those from Ikon or Hagon or Progressive, you'll have some pretty great suspension.

Do you still have the stock exhaust? Swapping to a good aftermarket exhaust will drop some weight but can also increase your ground clearance!

Something that you may consider (which would be easier than raising the bike) is changing the exhaust, adding a custom-made bash plate, upgrading and properly tuning the stock-length suspension, and seeing how far you can go. You can probably ride a lot harder than you think with just over 5 inches of ground clearance.
 
In the FWIW department (as I haven't modded a 750L specifically); I think the GL1000 fork mod is primarily to gain the dual disc brake set-up. On a scrambler build that may not be desirable as it's a lot of extra weight. One solution would be to upgrade the existing forks with Racetech springs and emulators and then maybe go with a '75-'77 GL1000 spoked wheel (aluminum rim) for lighter weight and the scrambler look. You can run a single LH brake on that hub by just getting shorter bolts (that I have done).
https://racetech.com/ProductSearch/12/Honda/CB750K/1979
On the rear, a longer shock to get the desired stance, perhaps 14" eye-clevis, there are a number of choices. The longer length may increase swingarm angle and require a larger R sprocket and a pad atop the swingarm pivot area to handle occasional chain rub. A rear hub from a '77 750K laced to a '75-'77 rim from GL1000 to match the front - may work (not done it specifically).
https://www.randakks.com/progressiv...s-motorcycle=Honda/CB750 DOHC/CB750K-LTD/1979
Thank you so much! I had been looking around to see what my wheel options would be. I think you’re right. Spokes is definitely the right call. I’ll look into getting a set of spoked wheels off of a GL1000. As well as a rear hub. Replacing the rear hub would mean swapping out the rear braking system right?
 
How serious of a scrambler are we talking? Are you thinking gravel roads, or rocky singletrack? It's also important how much you weigh, because if your suspension isn't properly preloaded right now you can gain significant ride height just by changing to the right spring rates for you. These bikes have a reputation for being undersprung from the factory for anyone more than 150 pounds.

Adding significant ride height can do a couple unpleasant things to your handling on these bikes. I'm no suspension/chassis engineer but I foresee two problems.
The first is with adding significant rear ride height. It'll shorten your wheelbase, which will make your handling more twitchy. Can be an issue when trying to bomb gravel roads or trails, as any shift in the direction of the front wheel (say catching the side of a rock or rut) will turn the motorcycle more and that can make it harder to stay in control at speed. This is more common for track bikes than it is for offroad machines.
That being said, adding up to an inch of rear travel probably won't hurt, as it will have very little impact on your overall ride height. It will, however, make your bike look like it's leaning forward compared to stock, and put more weight on the front wheel.

The second is that these older forks are all pretty noodly. The longer you make the front end with vintage forks, the more flexy they become because of the increased leverage the terrain has on the front end. If you're tackling any serious obstacles like rocky trails at speed, you could end up bending the upper tubes. Swapping to a GL1000 front end gives you 37mm fork tubes, which is the same diameter as the CB750F from 81-82 but nowhere near as stiff as, say, upside-down forks from a much newer dirtbike or dual-sport. It'll also get you stiffer stock springs, but if they're not the right spring rates for you then you haven't actually tuned your suspension. If you're going to do a front-end swap for serious offroad duties it's probably better to go with something from a dirt machine but finding compatible triple trees can be very tricky.

I agree with jpdevol that going with proper Racetech springs and cartridge emulators, and lightening the front end, is a great avenue. If you pair that with a proper pair of rear shocks, like those from Ikon or Hagon or Progressive, you'll have some pretty great suspension.

Do you still have the stock exhaust? Swapping to a good aftermarket exhaust will drop some weight but can also increase your ground clearance!

Something that you may consider (which would be easier than raising the bike) is changing the exhaust, adding a custom-made bash plate, upgrading and properly tuning the stock-length suspension, and seeing how far you can go. You can probably ride a lot harder than you think with just over 5 inches of ground clearance.
I only weigh like 140lbs. So I’ll look into tuning and upgrading the already stock suspension in the front and see where that gets me. Honestly, I don’t need to lift the front very much, if not at all. The rear will be what I’ll focus on for now. However, I want to build this bike right. So I’m open to the idea of swapping out the front end still. I’ve seen the triple tree conversion kits on Cognitomoto that allows for different and more modern shocks. I just don’t know which front end would be best to switch in. Something close to stock, but upside down, and a bit thicker.
Replacing the front end means I’d need to have a completely different front wheel/brake system/speedo cable right?
 
Rear brakes on the Goldwing were disc and your 79 750L are drum, so yup, you'd need to change all that. That's a can of worms I've never opened personally but from what I've read it's a good amount of parts, fabrication, or both. It'd be much easier to keep the drum brake and get a rear wheel from a 79-81 CB750K as they were all spoked and came in either 17 inch or 18 inch sizes depending on your preference. Front spoked wheels from those same bikes should also be a direct swap and they're 19" wheels.

Front end replacement definitely means adapting everything that connects to it. So in most cases, yes, you'd need a different triple tree, a different front wheel, a different speedometer system, and you'd need to either change brake systems or adapt yours. There are a few swaps from similar period Hondas that may require fewer modifications but the further you get from 1979 and Honda the more research and custom work and effort is involved.

To upgrade the rear suspension properly you're gonna have to spend decent money, unfortunately, unless you're a careful mechanic and buy a few special tools. Good new rebuildable shocks from what I've seen are about $450 and up, with Ohlins being the top at $850. (Unfortunately Progressive 12-series shocks are not rebuildable as far as I can research.) Because I'm a brave and enterprising shadetree mechanic I've gotten expensive and very leaky aftermarket shocks for dirt cheap on Ebay and rebuilt them myself, but that's not feasible for everybody.

Could you elaborate on "build this bike right"? From the factory, these bikes were expected to do a bit of scrambling if you look at the tire treads available at the time! I guess it comes down to what kind of terrain you intend to tackle with it. That will determine how far you need to go in modifying this thing. Though if you care about a specific look, that has a lot to do with it as well.

In my opinion, the best thing you can do to make your build "right" is to attend to every maintenance need of your machine before you ever ride it. I have seen many older motorcycles with dangerous problems like ancient tires, original brake lines, crusty old brake fluid...you name it. I've met at least two people who crashed vintage bikes because maintenance that I'd consider vital simply wasn't done.
All four of the machines I've opened up the steering head bearings on still had their completely dried-up and useless factory grease, including a 1993 Suzuki and my own 1980 CB750K. Grease isn't meant to last 40 years!
Honda did a lot of fine engineering in making our machines and they're still pretty capable of many tasks with a few choice upgrades, but only if they're given the attention they deserve.
If you don't own a maintenance manual, buy one, and go over every single task in the maintenance schedule. Assume none of the items in the list have been done unless you have receipts to prove it or have already done it yourself. Make sure every rubber part on the bike is in good condition. Tires older than 6 years? Time for new ones. Brake lines still original? Time for new ones. Grease every bearing and pivot point, lube every cable...you name it.

If you return all the functional parts of your bike to factory fresh, you may be surprised at how much it can do. Even if you still want to mod it heavily after that, you'll at least have a perfectly safe machine to build on.
 
Rear brakes on the Goldwing were disc and your 79 750L are drum, so yup, you'd need to change all that. That's a can of worms I've never opened personally but from what I've read it's a good amount of parts, fabrication, or both. It'd be much easier to keep the drum brake and get a rear wheel from a 79-81 CB750K as they were all spoked and came in either 17 inch or 18 inch sizes depending on your preference. Front spoked wheels from those same bikes should also be a direct swap and they're 19" wheels.

Front end replacement definitely means adapting everything that connects to it. So in most cases, yes, you'd need a different triple tree, a different front wheel, a different speedometer system, and you'd need to either change brake systems or adapt yours. There are a few swaps from similar period Hondas that may require fewer modifications but the further you get from 1979 and Honda the more research and custom work and effort is involved.

To upgrade the rear suspension properly you're gonna have to spend decent money, unfortunately, unless you're a careful mechanic and buy a few special tools. Good new rebuildable shocks from what I've seen are about $450 and up, with Ohlins being the top at $850. (Unfortunately Progressive 12-series shocks are not rebuildable as far as I can research.) Because I'm a brave and enterprising shadetree mechanic I've gotten expensive and very leaky aftermarket shocks for dirt cheap on Ebay and rebuilt them myself, but that's not feasible for everybody.

Could you elaborate on "build this bike right"? From the factory, these bikes were expected to do a bit of scrambling if you look at the tire treads available at the time! I guess it comes down to what kind of terrain you intend to tackle with it. That will determine how far you need to go in modifying this thing. Though if you care about a specific look, that has a lot to do with it as well.

In my opinion, the best thing you can do to make your build "right" is to attend to every maintenance need of your machine before you ever ride it. I have seen many older motorcycles with dangerous problems like ancient tires, original brake lines, crusty old brake fluid...you name it. I've met at least two people who crashed vintage bikes because maintenance that I'd consider vital simply wasn't done.
All four of the machines I've opened up the steering head bearings on still had their completely dried-up and useless factory grease, including a 1993 Suzuki and my own 1980 CB750K. Grease isn't meant to last 40 years!
Honda did a lot of fine engineering in making our machines and they're still pretty capable of many tasks with a few choice upgrades, but only if they're given the attention they deserve.
If you don't own a maintenance manual, buy one, and go over every single task in the maintenance schedule. Assume none of the items in the list have been done unless you have receipts to prove it or have already done it yourself. Make sure every rubber part on the bike is in good condition. Tires older than 6 years? Time for new ones. Brake lines still original? Time for new ones. Grease every bearing and pivot point, lube every cable...you name it.

If you return all the functional parts of your bike to factory fresh, you may be surprised at how much it can do. Even if you still want to mod it heavily after that, you'll at least have a perfectly safe machine to build on.
I sincerely appreciate you taking the time to talk to me about all of this.

I'm doing hours and hours of research and reading as far as how to approach this. I bought this bike for the learning experience, so everything on this bike will be touched/serviced at some point. Safety is a big priority for me.
My goal as far as it's capability is 60% on-road and 40% off-road. I'd like to be able to rip through the Eastern Oregon woods of my hometown, ride on the old gravel logging trails, and drive through the small creeks with ease. That being said, I have respect for the fact this is a 500lb machine, and don't expect it to be able to do everything a dedicated off-road bike can.
I'd like to perform the front end conversion, especially for the experience. As soon as I find out exactly what I'll be swapping to, I'll do whatever I have to do to get it done. I just have to find the right front for the job.

I think it's badass you're a shadetree mechanic. To be like that is one of the goals for me in my long journey of learning to twist a wrench.
I've been forced to work like that thus far in my experience with cars because I come from a family without a lot of money. But God is good and I've worked hard to get to a better place financially. So I have some basic tools and a roof over my head to tackle this build.
I'll also be purchasing a welder and getting some experience doing that, which I've always wanted to learn.

"Eating an elephant". Those are the words I have running through my head the entire time I'm doing all of this. I'm breaking everything down into chunks and taking my time.

If you're ok with it. I'd love to stay in touch with you, and as the build progresses I'll share pictures here on the forum.
 
You're welcome to PM me. I'll definitely be following your progress here.

A big front end swap is a pretty large undertaking for your first motorcycle, though. Is your current front end in good shape? The parts needed to rebuild and service these old damper rod forks, getting them back to factory condition, are usually really cheap. Less than $50.

How long have you owned this bike? You might want to consider getting the bike actually riding well before you start doing serious mods to it. I'm saying this because with little to no motorcycle experience before, you may not know what "correct" or "good" handling and chassis characteristics feel like, and could accidentally end up making a change or forgetting something that you don't know is incorrect or annoying or possibly dangerous.

Most folks with little to no experience who attempt a build like yours get in way over their heads and end up selling it before it's finished, from what I've seen. They pop up every so often on my local classifieds.

I'm not saying you're going to end up that way. You just need to be aware of how much effort it's going to take, and the potential risks, and the amount of unknowns you'll be wading into. It's a lot easier to get a bike back to stock, where you have a shop manual or two with clear directions, than it is to do extensive modifications. Especially so when you're new to wrenching and to motorcycles in general. I think it's better to give yourself some easier goals before you tackle something as big as a front end swap -- like getting your bike on the road and riding well with stock parts. This is in no way a waste because you'll learn on the way there. You'll figure out what tools you need, how the bike is supposed to feel, and you can always resell your reconditioned stock parts when you no longer need them.

I probably sound like a broken record.
 
I think that's really wise advice. My first step is absolutely going to be getting the bike running and riding it around for a bit.
I've been studying my brain to mush in preparation for rebuilding the carburetors, which is what I'm pretty sure is the only thing that keeps the bike from running right now. After I get the engine running smoothly, I'll definitely need to service/replace the stock front end. I'm sure it rides, but I can tell just from looking that the front end needs some love. I'm all for getting the stock parts to spec, just like you said, it'll be better to sell them that way if I decide to move forward with anything crazy. However, I am weary of cost though, I don't want to have to sink $400 into getting the oem front end working, when I could've spent a couple extra $$$ to upgrade the front end entirely. If I do some research and find that it's not too expensive to have my current forks serviced, then I'll just do that.

I'll keep you and everyone else posted here. Hopefully, along the way, this build will inspire other people to look at their CB750s differently and know that if I can do it, anybody can.
 
Luckily you should not need to sink significant money into rebuilding the stock front end unless the fork tubes are too rusty or bent to use or something is damaged. With your weight, you probably don't even need to change out the stock springs.
If the chrome upper tubes are in good shape and nothing is bent and you can get the damper rod bolts off of the underside without a hitch, all you should need are fork seals and dust boots and oil. Maybe damper rod piston rings if you're feeling extra fancy. All of those together shouldn't cost you more than 55 bucks, shipped to your door. You certainly don't need to add costly cartridge emulators; they're just nice to have because they modernize how the forks behave, pulling them closer to modern sportbike forks.
The OEM service manual actually specifies ATF for fork oil if you want to be extra frugal, and both forks combined take less than a quart.

Carburetors, despite what you've probably seen elsewhere, are not tiny nightmare boxes full of dark magicks. As long as you're careful and organized you can successfully clean them out. I would definitely suggest finding a video tutorial or a manual with lots of pictures if you can find those.
One piece of advice: take them apart one at a time, and don't remove them from the brackets keeping them all together as one rack of four. You don't need to. You can pop the top off the slide and the bowl off the bottom of each carb and get all the parts you need to out that way, plus the odd accelerator pump and cutoff diaphragm here and there.
 
I haven’t been able to find an in depth video for these carbs on YouTube. But I do have the amazing guide written by Sean Grier. Read that a couple times. I’ll probably ramble my way through making my own video and post it.
And I didn’t know that keeping them on the rail was an option! I’m definitely gonna try that.

Some sweat, effort, and 55 bucks is well worth it for those front forks. I’ll check the FSM and start reading up on how to do it.
 
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