Anybody ever substitute a KZ400 early 40-spoke rear drum for KZ650-750-900 drum?

Hi - I'm thinking I'd like to use a 40-spoke early type KZ400 rear drum on a 750 build.

Any thoughts on pitfalls? I'm mainly concerned with the downsized bearings in bumping the axle size from 17mm to 20mm, whether this would affect bearing life, load capacity & speed rating etc. Should ask ALL BALLS RACING 'cause they're the ball-bearing experts.....

I suppose there are always later-era swing-arms where the axle has been scaled down from 20mm to 17mm or even 16mm, while the 20mm axle has been bumped up front where it always belonged! But I don't want to ruin the PERIOD-CORRECT thing, the TIME TRAVEL factor.

So yeah - if anybody has TRIED this? In HONDA circles, it's quite common for the CB500/CB550 rear drum to be used on up to 836cc CB750K's and they race 'em with even smaller drums. Lucky for the KZ crowd, the KZ400 drum's actually got a proper CUSH-DRIVE on it with a 3rd wheel bearing installed. PERSONALLY speaking, the CB500/CB550 is something I'd never consider using due to the cush-drive design alone. The KZ400 hub SEEMS promising. S'pose I should look at the bearing size. It's surprising how many more modern bikes where the front axle size has been bumped up from 15mm to 20mm, use a bearing with the same diameter of outer race. Thing is, those front bearings are often placed further apart so surely that's gotta have a big effect on bearing life vs lateral forces etc.

But I'm sure there are OTHER considerations as well. IMHO the shoes being 30mmx180mm vs 40mmx180mm in the 650/750/900 drums ISN'T a set-back. Neither is the fact that the drum lining is substantially thinner, as I'd only ever wind up using my rear brake in the occasional PANIC stop. My front brakes are substantially beefed up already. So it seems to follow that despite being a retro-fried build with all early '70s running-gear components, the modern RIDING STYLE is what relates most directly to the rear brake - the lighter use of which and less frequently at that, is what supports this idea imho. And as for any question of the drum overheating, it'll be ventilated quite a bit so I expect it to cool far far better than the stock configuration.

Nevertheless - I'm curious to hear what all YA'LL have got to say on the subject. I'm gonna lace it to a 3.50x18" Super-Akront rim which was originally laced to a ZED rear drum. From what I gather from other wheels where I've employed used pre-drilled & RE-drilled rims, (like my "KZ440LOL" project with the Suzuki 4LS in 3.0x16" Borrani rims with 110/70-16 & 140/70-16 Maxi-Scooter tires & all-NOS belt-drive, etc etc) it would SEEM like there should be zero issue with pairing this pre-drilled rim with this hub, given that the spoke-flange is only slightly smaller diameter and width than the original hub fitment. The spoke holes might even require a reaming but I doubt it, as the 4LS in the "KZ440LOL" took the heavy-duty Harley spec spoke gauge - which I think is a swaged 7-6 gauge - (They're like Navy-Cut cigarettes, they're so damn short!) The bent heads were a bit of a fidget but they squeezed through. The 40-spoke KZ400 drum should be just fine. After all, I'm using the same heavy-duty spokes on the 36-spoke 160mm drum for the KZ440LOL rear wheel, which I'm FINALLY getting around to building.....

The bigger bike will be the 'featherweight' and it's for my daughter to use on the highway, single-seat-only, really tricked out with beefed up yet lighter-weight front brakes, just a whole bunch of cool stuff has been set aside for this project. I'd feel like it were incomplete if I didn't ditch a substantial chunk of un-sprung mass from that rear wheel, to go along with the top-shelf lightweight RIM it'll be laced to - And such a low mass hard charging JOCKEY in the saddle, I should think this 750 will be a serious challenge for by big-bore rebuilt 900.....

Surely all of these OTHER considerations only bear out my argument that this drum SHOULD be adequate. If I absolutely have to, I suppose I'll bump the rear axle diameter down to 17mm, even if I've gotta use sleeves on the ends for a 20mm-spec swing-arm. I'm sure THAT'S been done before as well ... ???



-Sigh.

I've laced a 2.15 rim to a KZ400 hub.
No big deal. You will need a jack shaft
or a spacer for the rear sprocket as the
chain offset is different.


.

IIRC the bigger diameter axle has more to do with the width it is spanning. Most vintage bikes are spanning 9-10 inches at the axles while most modern bikes are spanning 12+ inches. this requires a larger diameter bolt to retain rigidity, also larger diameter bolts allow the axle to be hollow allowing for weight savings.

I put a '78 kz650 rear drum (17mm axle) on my '80 kz750twin (20mm). I have a modified GS1100E swingarm and because it is much wider I kept the 20mm axle. I found appropriate sized bearings for the hub to retain the 20mm axle. Then had the brake plate, and sprocket carrier spacer to bored out to 20mm. I also made a new internal spacer for the hub. Worked great.

I'm curious why you don't use a 650 hub. Just for looks of the smaller hub?

The only time I think the smaller brakes will be a major issue is if you live in a mountainous area.