78' CB750 Front Swap

clem said:
Just to clarify, the offset is the distance between the centerline of the steering stem and the centerline of the forks. Changing the stem will not have any effect on this measurement. This one is important because it directly affects the trail. Newer bikes have much steeper rake or less in terms of degrees. I believe the cb falls in the 27 degree range where newer bikes are around 24 and maybe below that. Older bikes had more offset in them to make up for the relaxed rake and keep the trail in a desirable range usually well below 4 inches. News bikes have much stiffer frames and can handle the force of the reduced rake angles so the need less offset.
Either custom triple clamps or serious frame mods are required to make your front end work.

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Click to expand...

If there is 0 degrees of rake in the triple, the rake will not change, then it's only a function of the head tube. That is unless the forks are shorter which off hand I do not know but is very easy for him to determine. And if he goes to 17 inch rims there is no trail issue to the best of my knowledge. That triple has 35mm of offset, which works fine with a 17 inch rim as it was designed.

Again, that is assuming the length is the same. And honestly, if it's a little shorter it'll be fine. I've lowered the front end of a few sohc 4's by a little more than an inch and they ride extremely well regardless. Anything more than that and you may want to look at shorter rear shocks to offset the length diffence, just make sure you have clearance to do so.
 
Here is what I'm coming up with so far:

397aa1d42119f6591ff7d9092453106f.jpg


If anyone has the rake for the 78' CB750 I would appreciate it.


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Length and height are irrelevant. You need fork length. Knowing the trail on the R1 doesn't help you with the trail on the 750. And despite what anyone tells you, a GSXR or R1 swap on a CB750 is rarely, if ever, an improvement to stock handling. Lower the front end and welcome to the world of dive. There are other ways to beef up the front end, and for much cheaper, that will vastly improve road handling. Most of these fork swaps are for looks. They like the way gold inverted forks look. Not to mention, the CB750 doesn't need to be any lower. It just takes away from cornering capability.
 
To carry-on Deviants sentiment, you need to really decide what's important to you.

Is your main priority to have a flashy looking, but under performing bike? Basically the same thing that's been plastered all over the Internet and the magazines for a few years now? Lots of oooh's and aaah's at the local bike night but nearly useless on the ride there and back?

Or, do you want a bike that does everything a bit better than factory stock yet still has some curb appeal?

To make a modern USD truly work with these old bikes you basically have to redesign the frame from the ground up. There are some smarter folks than me that can explain the math behind it all, and I'm sure they will.

Also, as stated by Deviant, there are a plethora of options if you want to actually make your bike BETTER, not just better looking. Most of which will be way cheaper than all the needed surgery and custom parts the R1 forks require.

So, which is it? Go or show? Either way, it's your bike and folks here will help you make it a reality. But before you wear out your check writing hand building an art piece, you need to sit down and pick a direction.

Best of luck either way, and if there's anything I can help ya with don't hesitate to ask.
 
I have not done this swap myself but I have read up on them a bit. This is popular over on the SOHC/4 Honda forum, but to be effective, you need more offset. Going with stock CB750 offset (50mm for a later K model) that will help immensely vs the stock offset of the newer bike. Play with the numbers if you grafted the front end on as is (modern offset) vs using the CB offset and see where trail numbers get you. This also depends on what wheel you are using, stock 19, dropping to 18, or going with the stock 17 from the modern bike those forks came on?
 
I would. It was said you can shorten the forks, etc. But without new clamps you can't compensate for the lack of steering offset. There are a few excel spreadsheet trail calculators online. Go and play around with different numbers on one of those and see how everything works (Race Tech has a decent one). You should walk away from this experience with more knowledge than you walked in with correct?

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No worries! I really appreciate the help.

And yeah, I've already learned a ton so this is going well. It's a crash course for sure


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I know there's already two previous pages of discussion, but I'll throw my $.02 in as well.

to preface, I am a proponent of inverted front fork swaps....

but you need custom triples. 45mm of offset is more often than not a really good number to use for having custom trees cut. Cognito's are like $850 or so but they're quality and have some aesthetically pleasing design cues. There's a member on here making plane-jane custom trees for about $400 for a pair, but they're just flat plates of aluminum cut to shape and holes bored in them at your desired spacing and offset.

While the fork extenders are a nice thought, they're going to be $200+ and that's better spent towards the correct triple trees....and on their own won't make the geometry better by measurable margins.

in addition to the triples needing to be custom, running that front wheel is going to mean needing a wider rear wheel and tire to compensate for the wider wheel from the R1 up front. Even with the correct offset custom triple trees the bike is going to handle like dookie with a 120/90/18 rear and a 120/70/17 up front. Stock cb750 front tire was 100/90/19 which is 26.09" diameter, while the R1's 120/70/17 is 23.65" diameter....so you're already lowering the front end nearly 2.5" without accounting for the difference in fork length. as for tire width, 20% wider at the rear is a good working number give or take.....you'd need a 140 or 150 rear tire on the 18" wheel to get closer to that ratio of front-rear tire width. Not sure if that big a tire will be pinched too badly on the stock rim, might have to get a wider rear hoop and re-lace or maybe look into getting a 17" sportbike wheel to fit out back like maybe an fz6 rear wheel to match the R1 front wheel....and then you're into figuring out how to get bearings and axle to play nice, getting the sprockets to align (maybe modifying the cush drive or offset sprockets), and getting a disc brake system to function on a frame set up for drum rear brakes. But you may want a slightly longer swingarm at this point anyway if you're doing the R1 front end....which would mean also custom fitting or having one made.

it snowballs quickly, as you can see. lol.
 
focusinprogress said:
Stock cb750 front tire was 100/90/19 which is 26.09" diameter, while the R1's 120/70/17 is 23.65" diameter....so you're already lowering the front end nearly 2.5"
Click to expand...

I agreeing with much of what you're saying, but this is actually 1.22" or 31mm. It's the radius you're lowering by not the diameter.
 
focusinprogress said:
I know there's already two previous pages of discussion, but I'll throw my $.02 in as well.

to preface, I am a proponent of inverted front fork swaps....

but you need custom triples. 45mm of offset is more often than not a really good number to use for having custom trees cut. Cognito's are like $850 or so but they're quality and have some aesthetically pleasing design cues. There's a member on here making plane-jane custom trees for about $400 for a pair, but they're just flat plates of aluminum cut to shape and holes bored in them at your desired spacing and offset.

While the fork extenders are a nice thought, they're going to be $200+ and that's better spent towards the correct triple trees....and on their own won't make the geometry better by measurable margins.

in addition to the triples needing to be custom, running that front wheel is going to mean needing a wider rear wheel and tire to compensate for the wider wheel from the R1 up front. Even with the correct offset custom triple trees the bike is going to handle like dookie with a 120/90/18 rear and a 120/70/17 up front. Stock cb750 front tire was 100/90/19 which is 26.09" diameter, while the R1's 120/70/17 is 23.65" diameter....so you're already lowering the front end nearly 2.5" without accounting for the difference in fork length. as for tire width, 20% wider at the rear is a good working number give or take.....you'd need a 140 or 150 rear tire on the 18" wheel to get closer to that ratio of front-rear tire width. Not sure if that big a tire will be pinched too badly on the stock rim, might have to get a wider rear hoop and re-lace or maybe look into getting a 17" sportbike wheel to fit out back like maybe an fz6 rear wheel to match the R1 front wheel....and then you're into figuring out how to get bearings and axle to play nice, getting the sprockets to align (maybe modifying the cush drive or offset sprockets), and getting a disc brake system to function on a frame set up for drum rear brakes. But you may want a slightly longer swingarm at this point anyway if you're doing the R1 front end....which would mean also custom fitting or having one made.

it snowballs quickly, as you can see. lol.
Click to expand...

Forgive me, but you made that whole thing over complicated. Theres is no need for a different triple. None. Buy a custom hub from devin for the front, run 17's front and rear. Done.

By doing it this way, a narrower rim can be used up front, bye bye 120 front tire. As for the rear, run the lowest profile sidewall possible. As i previously stated, 1.3 inches can be taken out of the rear suspension as well if needed (providing the required travel is there, and i think it is).
 
My issue with the 750s, as I mentioned before, is they don't necessarily have the clearance. In stock configuration, the footpegs hit at the edge of the patch, which also happens to be the point at which the stator cover hits.
 
First off, special thanks to Jag767 for all of the help! The dude knows his stuff.

So, here are some of the numbers I've got for anyone else looking for a swap in the future. Feel free to double check in manuals but that's what I've come up with.

• 01' Yamaha R1 •
Fork length 29-¼"
Stem length 6-¾"
Rake 24.0 degrees
Trail 3.8"
Overall length 80.1"
Overall width 27.4"
Height 43.1"
Wheelbase 54.9"

• 78' Honda CB750K •
Fork length 29-⅝"
Stem length 7-½"
Rake 28.0 degrees
Trail 3.74"
Overall length 85.0"
Overall width 34.8"
Height 45.5"
Wheelbase 57.3"


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With what my numbers are looking like, I'll need to do a new stem and triple tree to get some decent offset. That and some new bearings and I'm off to a good start!


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