Spacing a disc rotor so it sits farther away from hub than originally. 1

[gonzowerke]

Greetings,

A little background first...Honda made two different CB750/4 models between '76 and '78, the CB750K and the CB750F. The K had spoked wheels, single front disc, drum rear brake. The F had Aluminum mag wheels, twin front discs, and rear disc.

I have been riding a K for 20 years now, with the front forks, brakes, and rim from an F model.
I want to go to a spoked rim with dual discs up front, because I am tired of the mis-matched Frankenbike look, but I would like to retain the braking performance of twin discs.

My intention is to take the stock K front wheel, and an additional disc I bought, and make them work with the F forks and brakes. it is a straight up easy-as-pie bolt on conversion, as the axle and spacing are the same for both models, except for the brakes. The F rotors are dished in their centers to put the disc face closer to the fork stanchions, and further from the hub. Thankfully the disc thickness and diameter are the same as well. Too bad the F discs are 5 bolt, and the K ones are 6, or I wouldn't be having this problem. I want to move the K discs out so they meet up with the F calipers and Forks I already have.

My question is thus;
When fabricating a spacer to sit between the disc and the hub, would it be stronger(and safer) to make a single plate with the same diameter and bolt pattern as the K discs, just the thickness I need, or can I just shim each individual bolt with either washers or bushings? There are no lateral forces applied to the discs that I know of, just rotational, as they are not of a floating design, the calipers float on pins like an automobiles.


Thanks in advance for any help,

Chris

 

[kcj]

I have done something similar, but it was on a lightweight trials bike. Much smaller screws, very strong disc brakes, but the forces are a lot less. So I can't apply that to this question.

So the spoked wheel hub is already machined on the non disk side to just add a second disc? Huge plus....

My main concern would be the bending loads on the bolts due to the torsional (disc) force being applied xx out from the grip surfaces. How much offset are you talking about? 2 mm? 20 mm?

I think I would have an aluminum spacer machined up for a solid surface all aroun, then mill out slots between the bolts to reduce the weight. Make the area surrounding the bolts solid maybe 3/4 inch each way, then mill a large slot until the next bolt, leaving only about 1/8 inch at outer and inner as ribs just to keep the bolt pads aligned and straight.

I would not use bushings, unless the diameter was very large. I think they would introduce bending loads into the bolts as the disc tried to rotate. Flat washers maybe, if truly flat and all the same thicknesses, but any thickness difference would warp the disc very slightly.

Maybe a failure would just break bolts and still work ok on the other side, but it also might get into a bad spot and lock up the front wheel. Been there, done that, was very lucky.
Given the failure mode of losing the front brakes completely, or lockup of front wheel, I'd tend to go first class on the parts needed.

I envy your machined hub. I want to do a similar project on an RD350 (actually the DS7 model 250, before the series was called RD) and convert back to the original wire wheels, but with later dual disc braking. Granted they are very light, and the increase in unspring weight may hurt more than the increase in braking, but it would be a fun project.

kcj

 

[swall]

I don't think you will be able to make individual spacers or bushings all exactly the same thickness. With a one piece disc spacer you don't have to worry about this.

 

[gonzowerke]

KCJ,

Sorry, I forgot that critical bit of info. It will be 10mm each side for 20mm total.

I was thinking plate too, it's just I am doing this without anything like a lathe or mill. I have some 6061-T6 plate I can cut a spacer out of, and do all the mill work by hand(that's going to be so tedious), the center hole for the hub will be the b*tch!
The hub is symmetrical on both sides, so yeah, the disc just fits on there. I will need to put the new bolts on from the speedo drive side, as the nuts would hit the speedo drive housing otherwise.
1/8 of an inch is perfect, as there is a small shoulder on the axle side for the bearing housing.

Swall,

I was going to pop them out of plate with a hole saw, stack them on a bolt for alignment, and Tig weld them together at a friend's house. I wouldn't know if they were identical until I try this, and see the results. But, I'm going the plate spacer route now, so that is a non-issue at this point.


Thank you both for your replies!

Chris

 

[kcj]

Relate. I have to make do with hand tools and welding. Had an ancient but nice South Bend lathe but had to sell it during a move.

So, if the hub is machined both sides already, was there a dual disc version of that used? Maybe you can find the other disc, with offset correct, on ebay or some honda bulletin board group.
Or is the issue the mixing of wheel vs. forks, not being a standard?

Suggestion on 'hybrids': Use an engravier to put the year and model on the new disc, the calipers, and any other transplanted parts. Makes it much easier to remember the spare or repasir parts, and the future owners will appreciate it. I have friends with street rods (purchased) who learned the hard way they have no idea where the belt tensioner or steering pump originally came from.

Do you have any connections in motorcycling enthusiasts, or street rods, or anyone with access to a lathe? Taking your prints to a commercial shop may be too much $$, but an enthusiast who may be interested in the idea may do it for the cost of some new tooling or a case of beer.

It could be precision done by hand, but very tedious. Center with hole saw, drill locations very carefully, mill the slots with a dremel, then you could use feeler gauges, bluing, and hand file or lapping to get it flat. It will look hand made though, which may or may not show.
Depends on your time and patience available. I'd be searching for a lathe or mill and rotary table I think.

Good luck, sounds like overall it will be simple enough once you get through this hurdle.

 

[kcj]

forgot: you maybe could space the caliper out on the fork leg, but the 10 or 20 mm offset would introduce twist forces. I would avoid that. So, back to the disc spacer theory.

will there be this same spacer on both sides, one per disc? That moves the time vs. cost equations toward the lathe also.

 

[gonzowerke]

Forget all the measurements I gave earlier. That was a combination of bad eyeballing and comparing the two hubs that are completely different. Oh Well, live and learn.
I finally got the F forks back on the K bike, and swapped the K rim and F rim after taking measurements. The differences are that the K disc is 3mm flatter in offset than the F, and 9mm larger in dia. than the F. So, I either space the calipers inward 3mm, and machine the disc 9mm, or I make an adaptor bracket that moves the caliper in 3mm and up 9mm.
I have another stock K disc that I am using as the second rotor, I just need to get mounting bolts that are 10mm longer to use it. There's a chrome cover on the speedo drive side I will have to ditch, but it looks like it will fit. I am going to pull the disc tomorrow and swap it to the other side and take measurements to make sure it is the same and everything fits. I'll also get some pics I can link to so you can see what I'm doing.
Everyone else who does this conversion just goes and buys a GL1000 Goldwing front end complete and swaps it over, but that is expensive and complicated to source the parts. I have never heard of, nor can I find anyone on the net, trying to do what I am doing to accomplish this mod. Maybe there is a good reason for that and i will learn it after getting measurements tomorrow?

 

[gonzowerke]

Okay, I have done more investigation.The hub is not machined symmetrical. If I go the shim the rotor and move the caliper route, I will need a 2mm spacer for the left side and a 5mm one for the right. Bracketry to move the calipers will be a cake walk, it's keeping the speedo drive that will be a pain. I will have to machine the drive plate until it is flat, with the drive still attached, and then screw it to the hub, I will also need to turn down the heads of the bolts to clear the drive. I'm going to try everything to keep the cable drive so I can keep the gauges as is.

Here are some pictures of the hub and rotor.
[/url]http://s245.photobucket.com/albums/gg59/gonzowerke/Hybrid%20Suspension%20and%20Brakes/[/url]