1980 cb750 front end conversion questions

[surfish95747]

All right all you gurus! I have in my mind to do a Yamaha R1 front end conversion on my machine. I am curious if bring the front wheel size from stock 19" down to the modern sport bike size of 17" front wheels, means I would have to change the rear wheel size? Right now I have a 17" rear wheel. I do see that most modern sport bike wheels are both the same diameter front and back.

[shinyribs]

You don't HAVE to change your rear rim to match the front rim. But if they are both 17's then that's really lucky. A R1 front rim is probably wider than your 17'' rear,though.

[surfish95747]

So its okay if I have the same size rear wheel (17") as the front wheel (17")? Eventually I would like to get a R1 rear wheel and fit it to my bike, but that wouldn't be at the same time I do the front end conversion.

[shinyribs]

Matching rim sizes won't affect anything. The overall diameter of the tire had no idea what size rim it's on. The reason most bikes have a larger diameter front tire ( and I think this is what you've got on your mind) is it simply rolls over things easier. That's why you see dirt bikes running really good sized 21" fronts. Some of the older models even had 23" and 25" rims.

Here's where I would be careful with mixing and matching. Whatever tires you buy make sure they are the same model. This is not very important on most rim sizes, but 17" tires are a little different. There are many different profiles ( the actual curvature of the tread when viewed from the edge, not the side) and mixing profiles can make a bike handle really funky.

See if you can get some solid specs on the front end your looking at. Length form axle centerline to top of the fork tube, fork spacing and tree offset ( center line of the fork tubes relative to center line of the steering stem).

Gonna your brief because phone. Too little offset can cause weird, lazy trail numbers. Also brings the tire closer to the bike= header clearance, tho not a big deal with 17's. And ofc the obvious ground clearance issues if the legs are real short vs stock.

Glad to help with any specific question. I've been done this road with success and mistakes. I'm no pro, but I know what to look out for and how to save $ and headaches along the way.

[shinyribs]

Here's info on 09-11 R1 fork lengths. Axle centerline to top of fork tube is 733mm

Link to fork info- click me


Here's info on R1 yoke offsets. No solid numbers, but I've asked around and 25mm seems to be the consensus. 25mm is barely over and inch, so not much there.

Tree offset info.


733mm is ~ 28 3/4''. CBR600 F2,F3 forks are 30''. I reference these because that's what's on my '80, and with a 120/60-17, that fork length retains stock ride height. With R1 forks you will be looking at about 1.25 - 1.5 inches lower. Definitely not horrible.

And just for fun a rake & trail helpful thingy.

[surfish95747]

Thanks for the info! I am definitely going to go with the R1 front end. Those numbers help. Any idea what I would do with the stem? I know I have to put the All Balls Racing conversion bearing on, but will I need a shorter or longer stem? I know Cognito Moto sells conversion stems, but I had never heard of these until I was on their website. Thanks!

[shinyribs]

The easiest, cheapest and best thing to do with the stem is to press the stock stem out of the Cb bottom yoke and put it in the R1 yoke. It'll bolt on just like stock.

Now, chances are the CB stem is going to be mush smaller in diameter since it's steel and the R1 is most likely going to be aluminum. What I have done in the past is build up the CB stem with weld and then turn it down to fit the yoke I want to use. Much easier than building a new stem from scratch. And you don't need to buy bearings unless yours are worn out since you'll be using all stock stuff at this point. If you run in to a snag and can find help local, give me a shout. I've done a bunch of these for guys on another forum.

[surfish95747]

Shiny, will a 3.5" wide front wheel with a 2.5" rear wheel (both 17") work together at least for a little while? I plan on doing like you said and getting a cbr600 f3 rear wheel which has I think a 4.5 or 5" wide rear wheel to even out those width differences. Thanks for putting up with all my questions!

[shinyribs]

As long as they are matched tires it should be fine.

I don't know of you've ever seen it out not, but lots of 17" tires come in some pretty aggressive profiles. You tend to imagine a tire as half-round, but some more aggressive profiles look almost like a triangle when viewed from the end.

[shinyribs]

Oh, and yeah, the 600 f3 rear rim is 5". Stock, they had a 160, but you can squeeze on a 170.

Something to remember, your gonna have to run a smaller rear sprocket than stock due to availability of pays, so you'll probably end up buying a larger primary sprocket to compensate. I think 18 is all you can fit up front with a 530 chain. Also, you'll need to space out the primary sprocket to align with the new rear wheel. Simple piece, really.  I can make one for you if you get to that point.

All this info should be in my thread for my '80.

[shinyribs]

For some reason my phone won't allow me to edit. Anyway, chain clearance is bad with a 170. Best to stay with a 160.  Honestly, I couldn't tell them apart visually. They're both nice and fat.

[shinyribs]

Hopefully this can help.

http://honda-cb750-s.456789.n3.nabble.com/How-do-swap-any-front-end-on-to-any-bike-video-td4058328.html

[surfish95747]

Wow, thank you so much Shiny! I really appreciate you taking the time to make us all that video! It was extremely helpful! I will be getting to this conversion very very soon. I just need some clip ons and I'll be good to go! Thanks again!