Issue centering this comstar wheel

[chickenStripCharlie]

As far as i can tell, i installed the bearings and inner spacer properly. Everything was seated as it should be.
I measured the spacers needed for my GSXR forks as follows:
- bearing to bearing outer surface (at inner ring): 82.5mm
- gsxr fork to fork: 170mm

(170-82.5)/2 = 43.75mm per side

After i got the spacers i noticed my wheel wasn't exactly centered! The only explanation i can come up with is that the bearing centers are not the same as the hub center, and that the bearings are "offset" to one side a little bit. Is this possible? Has anyone run into this before?

Since the axle goes through the bearings (A, B) it is not in line with my rotor surfaces R1,R2, higher up so i cannot line it up to center the hub, so i measured at the bearings, only to run into this issue.

Does this make sense to anyone? Have you seen this before, where the bearing centers are offset a little compared to the hub centers ??
It makes me think something is wrong here, because i can't think of a reason why the bearings would be offset, but i'm certain i seated the bearings properly, using the proper length distancer in between.

This comstar wheel does have the speedo boss on one side and bearing retainer ring on the other, so it *MIGHT* be possible that it has some funky bearing offset on it??

This is the comstar wheel i'm using:

 

[ridesolo]

According to DTT senior statesman @beachcomber, whom I would trust implicitly when it comes to about anything mechanical and especially anything motorcycle and automotive related, an EXACT side to side centering isn't necessary. He's told me that some models of BMW actually leave the factory w/ a certain amount of offset between the front and rear wheel. Based on that I'd say that as long as you are spaced right to have the calipers and discs all matched up correctly you'll have nothing to worry about.

 

[Tim]

100% possible that the bearings are not centered under the tire.

What are the lengths of the original Honda spacers for stock forks? That should tell you the story - if you have them. Then the difference between GSXR and stock fork widths would be split and added to each of those original spacer lengths, assuming they are different.

Naturally you don't want the wheel shoved to one side to line up the rotor with the caliper, if that means a massive offset. A few mm isn't an issue. But I'd work to center the wheel if for nothing other than aesthetic purposes and then work to space the rotor and caliper to line them up.

 

[chickenStripCharlie]

According to DTT senior statesman @beachcomber, whom I would trust implicitly when it comes to about anything mechanical and especially anything motorcycle and automotive related, an EXACT side to side centering isn't necessary. He's told me that some models of BMW actually leave the factory w/ a certain amount of offset between the front and rear wheel. Based on that I'd say that as long as you are spaced right to have the calipers and discs all matched up correctly you'll have nothing to worry about.

how can you have the discs spaced correctly, if the hub they are on is not spaced correctly?
Currently, my bearings are spaced right, and the hub is off center. I will have to get the spacers modified a bit, but then the hub (and discs!) will be properly centered, but the bearings will be a little bit offset.

I agree, though, i'd rather have the wheel & discs centered than the bearings, but it doesn't sit right with me that the bearings are offset, without knowing why

 

[chickenStripCharlie]

100% possible that the bearings are not centered under the tire.

What are the lengths of the original Honda spacers for stock forks? That should tell you the story - if you have them. Then the difference between GSXR and stock fork widths would be split and added to each of those original spacer lengths, assuming they are different.

Naturally you don't want the wheel shoved to one side to line up the rotor with the caliper, if that means a massive offset. A few mm isn't an issue. But I'd work to center the wheel if for nothing other than aesthetic purposes and then work to space the rotor and caliper to line them up.

Thank you so much for the quick confirmation!!

The original spacers are in a box of parts somewhere, i will dig them out tonight and see.
I will definitely want to center the wheel, so i will rework the spacers a bit, but thank you for confirming it is possible they might be offset at the bearings!

 

[chickenStripCharlie]

It is coming together ... slowly but surely

 

[beachcomber]

Confusing two separate issues here I think.

Hub, discs, etc - important. The position of the RIM relevant to the assembly / bike centre is UNIMPORTANT. As stated, BMW [ and other ] OEM machines leave the factory with front / rear wheel offset .... makes absolutely NO difference whatsoever. Alignment front to back is of course essential.

 

[chickenStripCharlie]

Confusing two separate issues here I think.

Hub, discs, etc - important. The position of the RIM relevant to the assembly / bike centre is UNIMPORTANT. As stated, BMW [ and other ] OEM machines leave the factory with front / rear wheel offset .... makes absolutely NO difference whatsoever. Alignment front to back is of course essential.

Correct.

I had, incorrectly, assumed that if the bearings were centered, the rim/hub/discs would be as well.
As i learned in this thread that may not always be the case.

Thanks everyone.

 

[beachcomber]

I spent much of my youth [ 50's and 60's ] watching a guy called Phil Bishop building alloy wheels for the good and the great of our hobby ..... he also built many for me. I could never get the hang of the 3D jigsaw puzzle to try it myself, even though he did try his best to school me a few times. He also built many wire wheels for cars where the spokes where positioned deliberately off centre to achieve the rim offset that was required. The bottom line is that in a two wheel tandem tracked vehicle there is absolutely NO necessity for the front and back to be on the same centre. Neither is there any requirement for that to be so in construction and use regs. As long as your hub / discs are correctly positioned within the frame - the relative position of the rim is unimportant. Don't lose any sleep !!!!!

 

[chickenStripCharlie]

With all due respect, Do you have any science to back that up with?
Because everything I've ever read has always stated that the front and rear wheels MUST track inline, exactly.

 

[beachcomber]

Just read the BMW OEM workshop manual ... don't take my word for it. I've only been a design development engineer for 55 years - what do I know, with all due respect.

Now then, less flippantly - show us the "what you've always read" to back your statement up.

 

[pidjones]

It is however pretty important to have both in the same vertical plane.

 

[beachcomber]

With all due respect, Do you have any science to back that up with?
Because everything I've ever read has always stated that the front and rear wheels MUST track inline, exactly.

I've shown you mine - now you show me yours.................

Here's the relevant "proof" from the BMW OEM workshop manual - you will see from the diagrams and accompanying text that BMW give acceptable offsets of front to rear wheel centres.

Not "he said, she said, but it might have been her brother's auntie" - but as specified by BMW and of course their teams of qualified engineers and designers. BMW have been practicing this "off-center" design for years - starting with the Airheads back in the late 50's. BMW are not the only manufacturers to do this. Oh yes, there hasn't been one BMW that has failed a design or roadworthy test because of that.

And just to add to the debate ..... many sidecar outfits run the same off-centre specs. My own 60's race outfits as an example with the aforementioned Phil Bishop building in the offset via the spoke arrangement..

As to other written proof there's excellent books by Tony Foale and many other respected motorcycle designers if you really want to understand the science behind it.

Science ? Yes I could bore your fuckin rigid with pages and pages - but start with the BMW info. and work your way through the relevant design books .... as I did when I was learning my trade.

 

[chickenStripCharlie]

I've shown you mine - now you show me yours.................

Here's the relevant "proof" from the BMW OEM workshop manual - you will see from the diagrams and accompanying text that BMW give acceptable offsets of front to rear wheel centres.

Not "he said, she said, but it might have been her brother's auntie" - but as specified by BMW and of course their teams of qualified engineers and designers. BMW have been practicing this "off-center" design for years - starting with the Airheads back in the late 50's. BMW are not the only manufacturers to do this. Oh yes, there hasn't been one BMW that has failed a design or roadworthy test because of that.

And just to add to the debate ..... many sidecar outfits run the same off-centre specs. My own 60's race outfits as an example with the aforementioned Phil Bishop building in the offset via the spoke arrangement..

As to other written proof there's excellent books by Tony Foale and many other respected motorcycle designers if you really want to understand the science behind it.

Science ? Yes I could bore your fuckin rigid with pages and pages - but start with the BMW info. and work your way through the relevant design books .... as I did when I was learning my trade.

That is very interesting. Thanks for sharing that. Any idea for the reasoning behind that? It seems the offset is only 1.5mm (in this case) but it is obviously intentional, yet the reason is unclear.

Very cool find. Thanks!!

 

[beachcomber]

That is very interesting. Thanks for sharing that. Any idea for the reasoning behind that? It seems the offset is only 1.5mm (in this case) but it is obviously intentional, yet the reason is unclear.

Very cool find. Thanks!!

The reason is very simple ........... BMW in their design philosophy [ or the bean counters ] use the same frame / engine combo with a variety of uses in mind from simple touring - to sports. A good example of form following function. In my 55+ years of automotive / motorcycle design and development [ and manufacturing ] career I have penned many designs where the front / rear track is not centred and although the dynamics are different - the same applies to cars / 4 wheel vehicles.

In this example [ BMW ] that means that everything from the gearbox / swingarm pivot forward is in the same relative position [ exactly ]. However, that same design accommodates rear wheels from 3.5" to 5.5" [ Kay series ]. Given that the rear drive [ shaft ] cannot be moved [ dictated by the gearbox output ] and the front wheel / forks are in a fixed position the ONLY solution is to run the rear wheel track off centre. The measurement of 1.2mm is relevant to the stock 3.5" - 4" rear wheels ..... now increase that by another 1" or 1.5" [ 5" and 5.5" wheels ] and that differential grows substantially - ie 10mm and 12mm respec. My bikes ram 15mm offset for the past 10 years. One "expert" suggested that one side of the tyre would wear more than the other .... whatever.

This debate has raged for years in the BMW Kay [ K1000 series ] road and race forums and the above indisputable facts have been now accepted - reluctantly by some of the "experts".

However, proof of the pudding in the real world as to whether it makes any difference [ adverse ] - I have used the 5.5" rear wheel / offset combo for 10 years [ and supplied dozens of conversions ] and Ben Kingham has won the B.E.A.R.S. Championship OUTRIGHT for 3 years with a 5" rear wheel / offset in his Kay. His team [ all Kays ] is also very successful in Endurance racing. Ben has used and developed parts that I originated on my my Kay - RAMSHOX being the most commercially successful.

Rather than clog up these pages with endless science behind the design, if you want to learn more I can recommend Tony Foale / Vic Willoughby "Motorcycle Design - the Theory and Practice" or Tony Foale's latest tome "Motorcycle Handling and Chassis Design - the Art and Science". The Foale / Willoughby book was my "bible" in my early years and I'm not so egotistical that I don't still refer to it today.

In innovative design [ rather than following the herd ] it is sometimes necessary to push the envelope, but if someone has done / proved something previously - why try to reinvent the wheel ? [ almost pun intended ]. I'm happy to continue this debate - just keep an open mind - that's what let's the ideas come in and keeps us out of the "let's all be different together" club.

 

[50gary]

This is my Comstar wheel, the front as pictured is built from three different Comstars. The rim is a rear 2.15" x 18" with a CB900 hub machined to accept larger bearings for the modern larger axle and the plate spokes are original pattern Comstar from a 1.85" x 18" I used real aircraft hardware and it trued up very well. The Rotors (GSX-R) required some adaptors for correct offset. Worked out very well, the bike is cafe CX500 that I did back in 2010.
Cheers, 50gary

 

[50gary]

how can you have the discs spaced correctly, if the hub they are on is not spaced correctly?
Currently, my bearings are spaced right, and the hub is off center. I will have to get the spacers modified a bit, but then the hub (and discs!) will be properly centered, but the bearings will be a little bit offset.

I agree, though, i'd rather have the wheel & discs centered than the bearings, but it doesn't sit right with me that the bearings are offset, without knowing why

In your drawing (first post) Bearing A is not completely pressed into the hub resulting in the misalignment. Pure guess is that the bearings are somewhat wider (ht.) than the OEM bearings and you pressed bearing B in first then put the internal center spacer in then pressed in bearing A and that bearing A is not seated in the hub. When I did my GSX-R to ComStar conversion I made all new spacers center included so I could use aluminum for lighter weight. I measured everything on a layout table to make sure it was on the money. Front to rear track alignment should ideally be zero but .020"/.5m is fine.
Cheers, 50gary

 

[chickenStripCharlie]

In your drawing (first post) Bearing A is not completely pressed into the hub resulting in the misalignment. Pure guess is that the bearings are somewhat wider (ht.) than the OEM bearings and you pressed bearing B in first then put the internal center spacer in then pressed in bearing A and that bearing A is not seated in the hub. When I did my GSX-R to ComStar conversion I made all new spacers center included so I could use aluminum for lighter weight. I measured everything on a layout table to make sure it was on the money. Front to rear track alignment should ideally be zero but .020"/.5m is fine.
Cheers, 50gary

The rims we are talking about are different. Yours is an earlier comstar. Mine is a later year. Also known as "reverse comstar".

The bearings used are actually thinner. 9mm vs stock 13mm. (42x25x9mm vs 42x15x13mm).

The wheel is off by about 4mm, if I remember correctly. It's not possible to be off that much due to bearings not being seated properly.

 

[50gary]

Okay, if your bearings are good and your rotor adaptors are also good (equal both sides) then center the wheel assembly in the center of the forks by making two different length axle spacers. Yeah, my Comstars have steel "plate spokes" not aluminum as the reverse do. Also the reverse spokes are slightly shorter even on the same dia. wheel. My front is 2.15" X 18"
Cheers, 50gary

 

[chickenStripCharlie]

Okay, if your bearings are good and your rotor adaptors are also good (equal both sides) then center the wheel assembly in the center of the forks by making two different length axle spacers. Yeah, my Comstars have steel "plate spokes" not aluminum as the reverse do. Also the reverse spokes are slightly shorter even on the same dia. wheel. My front is 2.15" X 18"
Cheers, 50gary

it's a very tough wheel to measure. The bearings are recessed but due to the way the speedo boss sits it has a pulled out "hub" which makes it impossible to measure the bearing seats against the rotor seat surface properly. Hence i never noticed as i only measured at the rotor surfaces.

Your wheel looks pretty sweet. What fasteners did yo use? That might be the first one i've seen disassembled and successfully put back together.