Topic: 1975 CB750, 19" front and 19" rear?

[R1gg5]

I've pored through the forum looking for input and advice on a wheel size question - but have come up dry.

I picked up a solid 75 CB750K - and am planning a cafe build with an eye on performance and hopefully some nice aesthetics to go with it. I've worked out most of the plan for the bike, but need to get some opinions and insight from some of you seasoned Honda experts out there.

For the wheels, I'm planning on starting out with all new Sun rims and spokes from Buchanan. The hubs will be stock from the bike, but polished up or possibly having a little color PCed on. I really like the clean look of matching rim sizes that I've seen around online, but want honest input on whether or not it's a good idea.

I'm looking at doing 19"s front and back. Since I'm starting fresh, I can really go with anything. I've read a few posts talking about 18" wheels front and back, but that would obviously change things on the front end and may be more hassle than it's worth. That's what has brought me to the 19s.

So if I do 19 inch wheels, are there performance concerns I should weigh?

What tire sizes would work out best?

Any disadvantage going with the same width rims front and back? I'm thinking 2.75" width rims.

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I'd really appreciate any advice and thanks for reading!

[trueblue]

Just remember that if you raise the back you are reducing both rake and trail, you may be able to get around this with a lower profile tyre, but I'm not sure on what options you will have regarding tyres.  You could overcome this by lowering your rear shocks but then your swingarm angle will be off.  It can be quite difficult making modifications to motorbikes without affecting the way they handle and most of the time it isn't for the better.  There is almost always a trade-off.

[SOHC4 Cafe Racer Fan]

You are probably better off going with 18" front and rear -- greater selection of tires.  Or you can go with a stock set up of 19" front 18" rear, thereby keeping the stock geometry.

[greenjeans]

You should be able to find some tires for 19s - didn't alot of those Nortons roll 19" F & R ?

You should have a decent amount of room in the swingarm - you just can't go to wide or it starts messing with the chain line.   Of course there are
offset sprockets etc.   

I'll be watching this one

[R1gg5]

From what I've learned on the forum, the nearest match to the original stock profile for tires is this:

Front: 100-90-19
Rear: 110-90-18

Is that correct? I'm working out what tire options I'd have, but I need a point of reference to work from.

[R1gg5]

If I were to put 100-90-19 tires on both front and back, it would hardly affect the original sizing, from what I can tell.

http://www.tacomaworld.com/forum/tirecalc.php?tires=110-90r18-100-90r19

Looks like the biggest changes would be to the width of the tire (from roughly 4.33" down to 3.94") and then to the sidewall height (from 3.9 to 3.54). But the sidewall height is the aesthetic goal anyway.

That tire has several options you can find:

http://bit.ly/VTeaP0 (motorcycle-superstore.com results page), and I've been leaning toward purchasing the Avon Roadrider tire (open to suggestions).

Given that this configuration is nearly identical to the original specs, is there still a concern about rake and trail on the bike? Any other performance-oriented issues that would arise? Thanks so much for weighing in guys. I'm excited to move past this challenge and get the bike torn down and on its way.

[R1gg5]

Another question:

Wheel width, I have three choices for the 19":

2.75, 3.0, 3.5

The 3.0 and 3.5 are deep drop center rims. If I'm going to have a single deep drop, then I'd prefer that they both are. Advice?

[trueblue]

If you are going to go with a 19" rear, you will want the 100/90-19 tyre to keep it roughly the same height as stock.  The trade off is you will have a smaller contact patch on the rear which equates to less grip.  I also don't know if this will adversely affect handling.  In regard to rim width, if you are using the same width tyre as the stock front on the rear, you want a rim the same as the stock front .

[Retro Rocket]

R1gg5, either stay stock or go 18's, these bikes handle great with 18 front and rear, most racers use these sizes because there are lots more tires to choose from and the bike will still handle well , Good luck finding decent 19 inch rear tires. My 3 old Honda's will all wear 18's front and back and i can assure you they will handle....  Not being rude but it sounds like you don't know much about setting up these old bikes to handle well, putting a 19 on the rear is going backwards....

[Terry in Australia]

+1, this is my K2 with 18 inch rims front and rear, and it handles better than any CB750 I've ever ridden.

[R1gg5]

@Retro Rocket (and the rest of you), thanks much for the input. This is exactly why I'm asking you all - and I appreciate any input, no matter how blunt. My goal is to avoid wasting money and making mistakes, particularly in this area as it's pretty expensive to build the wheels new like this.

Last call - anyone out there have positive comments on the 19" wheel set? I'm not trying to push the issue, just looking for the most complete set of input and then I'll make the call on this.

Truly, thanks to each of you who have commented so far! I'm eager to get my bike build on the forum. You guys are great here.

[R1gg5]

@Terry in Australia -

If I go 18" wheels front and back, what modifications do you recommend to get my bike to "handle better than any CB750 you've ever ridden"? I'm guessing there's a little more to do to get her dialed in.

[jgmmgood]

Many Nortons ran 19" front and rear.  I just replaced the old tires on my 69 fastback with the roadrunners you mentioned.  They are sweet in my opinion. They are actually interchangeable, identical sizes, etc.
  To reiterate what others have said, 18" gives you much more selection for tires out there.  Just finding 19" rear for that Norton took my options WAY down. 

[Terry in Australia]

@Terry in Australia -

If I go 18" wheels front and back, what modifications do you recommend to get my bike to "handle better than any CB750 you've ever ridden"? I'm guessing there's a little more to do to get her dialed in.

Throw away the standard rear shocks and replace with Ikons, ditch the OEM forks and install some modern(ish) forks, the 18 inch wheel set, better brakes, tapered roller steering head bearings, and bronze swingarm bushes.

As JG says, the 19's work fine on his Norton (one of the best handling British bikes of all time) but that doesn't mean they'll work on your Honda, and as he's said, tire choices are virtually non existant. Keep asking mate, but I'll guarantee there's not too many (if any) one here who'd consider installing a 19 inch rear wheel anything more than a retrograde step. Cheers, Terry.

[R1gg5]

Thanks all - 18" front and rear it is. Your input was great!