Topic: Question: 19 Front and 16 rear? what gives? (cb 750)74

[Jaydem]

I went to look at a potential cb 750 for myself. I know that the 74 750's should come with 19" front wheels and 18" rears. This bike had 19" front and 16"....kinda threw me off.

Was this an option from the factory?

Thanks

[MCRider]

Not a factory option. But a common accessory item in the day and still popular now. Depending on the tire, it lowers it a bit. And the 16s would get better mileage than the 18s that were available back then. I think with modern tires that is not an issue now.

We sold a lot of them in shops I worked in back then. And as said they are still popular for the looks as well, tho they didn't do much for me.

[Jaydem]

ok..thank!

So i guess all it would take is finding a 18" rim and throwing it on there...right?

[mycb750k6]

Some people called them Harley rims and they were a must-have when I was a kid. You still see a lot of them of ebay CB750s today. A lot of folks paid big bucks for them a long time ago. All the CB750 choppers had the 16 inch rim as well.

You'll need spokes as well as a rim and then lacing and truing and a tire, tube and rim strip. You might be better off just buying the entire CB750 rear wheel. There's always a few on ebay cheap.

[KingCustomCycles.com]

"its a chopper thing, you wouldn't understand" 

[MCRider]

ok..thank!

So i guess all it would take is finding a 18" rim and throwing it on there...right?

Yes it was a rim and spoke conversion. So you'll need a set of spokes too, generally available for about $30 I think.

If you want to upgrade the whole situation, you can get a wider alloy 18" rim suitable for a wider tire, Maybe $150+ and stainless steell heavy duty spokes suitable to make your pants wet, for $90 or so. And lacing/truing you can do yourself, lots of tutorials here how, or hire it done for about $100 probably.

[Stev-o]

Just buy an entire 18" wheel and sell your 16" to some short guy!

[HondaMan]

Chop rider like the low, wide look of those rims. And, under heavily-laden touring bikes of the 1970s (mostly the 750) it helped handle some of the weight. They don't corner well, though, as the rear profile drops off later than the front when the bike heels over, causing the bike to feel like it "falls" into a deep corner, making it harder to pick it back up at speed. But for Interstate riding, they are still popular.

[BrianAdair]

Chop rider like the low, wide look of those rims. And, under heavily-laden touring bikes of the 1970s (mostly the 750) it helped handle some of the weight. They don't corner well, though, as the rear profile drops off later than the front when the bike heels over, causing the bike to feel like it "falls" into a deep corner, making it harder to pick it back up at speed. But for Interstate riding, they are still popular.

Well, darn .....
I just bought a new 16" rim with stainless spoke kit for my build project, but now I'm thinking I wasted my money...
I bought it for the look, I do like the look better, but i sure don't want to lose the cornering ability, or make it worse, I want it to corner good, my last bike (2007 Suzuki Gsx-R) I would scape pegs and didnt' have a chicken stripe.
I like corners..... 



I read this also by HondaMan, and I'm confused on this also...

You're right about the equal tire sizes and cornering, IF the rear tire can support the weight as well as the front. On tight tracks, we often ran 3.50 rear tires on the 750 with 3.50 or 3.25 front to stop the oversteer. It only took about 500 racing miles to take the sidewall off the 3.50 rear, though, because of the combined accel/decel that tire suffers. Honda did not make the front forks far enough apart to allow for a 3.50 on the early "K" bikes, so it takes some judicious mods to make it work, and the tire pressures were limited to about 38 PSI or it got too wide and rubbed something in hard deflection situations. One solution was to use the TT100/K81 profile tires, which were called a "4.10" even though the horizontal width was exactly the 3.25 distance. The matching TT100 rear for this setup is the "5.10", which works out to a 4.00, or a "4.50", which is like a 3.50 .

When the bike rolls over, the contact patch rolls to the sidewall of the tire. If the cross-sectional radius of the front and rear tire are identical (and we ignore flex in the frame for the moment), the caster on the front wheel remains the same as the frame heels over. If the rear radius is wider or larger, the rear raises up a little, making the front caster drop off a little, and the bike oversteers deeper into the corner than you would expect. On bikes like the old BMW R75/5 series, the equal-size (and both 18") tires dramatically demonstrated the "grooved cornering" scenario by going exactly where you pointed it in corners. On bikes like the CL series twins from Honda (same frame as CB in most cases, but 19" front wheel and fender added), the bikes tended to understeer. These bikes were much more frequently on their sides (based on my mental observation of bikes coming in for repairs: CL bikes got much more case rash than CB bikes), and even my own CL350, though sporty, steered poorly at speed and made a lousy track bike.

The hidden flaw in all this profile-matching business: rear tires on most bikes are subject to heavier loads than front tires. So, the designers are left to cope with a guess at the rider's weight and seating position as the bike enters the corner, and take a stab at how much deflection the rear tire will actually see. I fully suspect that the 750 started life with an 18" front wheel and a 3.50 tire, and track testing proved, too late for Production changes, that the 18" tire caused significant oversteer. The squeezed-in 3.25x19" wheel probably was a "fix", along with the raised-looking front fender U-bracket to lift the fender about 3/4", and the triple trees were changed at the last minute to adjust the trail, which would have then been over 4" and provide a heavy steering feel, to provide the odd 3.75" trail of these frames (nearly every other bike of 500cc or more has 4" to 4.25" of trail). The 3.25x19 tire was unique to the 750 when it came out, and harder than hen's teeth to get until about August of 1970, as I remember.

If you watch high-speed riders on track bikes, you will often see them riding the pegs, not the seat. They are trying to match the tire deflection front and rear by moving their weight to neutralize the bike's weight on those patches (hence the forward riding position of the roadracer, prone and over the front). On modern superbikes with super horsepower, the rear tires must be larger to cope with all that torque, and this whole phenomenon is even more pronounced. To help cope, the designers of the monster bikes we see now often have same-size wheels front and rear to reduce this disparity.

Isn't it a wonder we can keep them upright?   

[Kouros]

My CB750K 19+16

[BrianAdair]

It looks good man! I like the look, how does it ride? what size tires are you running?

[Kouros]

Complete rookie here Brian. Taking my time. Nothing connected and not a running bike to report the ride with the wheels setup. Just showing what it could look like with 16+19. Thanks tho.

[Really?]

The 19/16 setup is what I was going for on Momma's ride.  I think it worked out well.  It was purpose built.