Conical rear hub offset.

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Conical rear hub offset.

Post by zuff » Mon Jul 01, 2019 3:58 pm

Afternoon all,
I have a query regarding the offset for the BSA/Triumph conical rear hub.
At present I have the standard B25 'crinkle' hub QD WM2 rear wheel. I plan to change this to the later conical non QD hub laced into a WM2 Borrani alloy rim. I need to tell the wheelbuilder the offset for the conical hub so that the rear sprocket is in correct alignment for the chain. I can take a measurement from the current sprocket to the edge of the rim. Will this be what the builder needs?
I have a 441 engine fitted but I'm assuming that the gearbox sprocket position is the same as the B25 engine.

Many thanks.

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Re: Conical rear hub offset.

Post by hhh » Mon Jul 01, 2019 4:22 pm

The offset only determines the position of the tire center in the frame. The chain alignment is set solely by the spacers on the axle and the swingarm positioning. Offset is normally measured from the hub surface (where the sprocket attaches) to the rim flange. There should be a standard figure available from the manual.

This thread has numbers for the stock B50/B25 which should be pretty close to what you're doing: viewtopic.php?f=5&t=6154&p=55951&hilit= ... set#p55951

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Re: Conical rear hub offset.

Post by beesa71 » Mon Jul 01, 2019 7:19 pm

Although the OP doesn't seem to tell us which swinging arm he is planning to fit the Conical hub too I'll assume it's a pre-OIF one.

From refurbishing Mead & Tomkinson B50's where they went the other way, ie fitted Crinkle hub in place of Conical, I'd suggest that the first thing to do is, most probably by swapping or machining brake plates, get the chain alignment perfect. Until you've done that your are wasting your time trying to measure offset.

From then it's simply a case of measuring from the hub edge (as suggested here and by Midge in the thread referred too) and getting that to your chosen wheel builder.

It's also been noted in a Forum thread that OIF machines do not have their rear wheels built central to the frame, as OEM anyway. M&T did and also had them offset differently depending on which way the track 'went'.

For OIF use I have a scrap rear section of frame and a swinging arm assembly, with appropriate spacers, that I give to my wheel builder instead of dimensions - he uses this as his truing jig.

Regards to all.

Paul.

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Re: Conical rear hub offset.

Post by zuff » Mon Jul 01, 2019 7:41 pm

Thanks for these very helpful replies. I will be using a Terry weedy swingarm. From midgies thread it looks like I should specify a 2 inch offset.

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Re: Conical rear hub offset.

Post by beesa71 » Mon Jul 01, 2019 10:42 pm

I don't necessarily concur with your thought; my view is that you need to set up the hub in the arm, then you can work out for yourself what your offset needs to be.

Paul.

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Re: Conical rear hub offset.

Post by zuff » Tue Jul 02, 2019 11:06 am

Thanks Paul. Yes, I was thinking about that. Get the new swing arm fitted, assemble the hub on an axle, line up the sprockets and then make a measurement. But without a wheel rim in situ what is my datum point? As I have a WM2 rim on the front I could take two strings back from the front wheel to give me the position of the rear wheel in alignment to the front wheel. Then measure from the sprocket to the nearest string to give a sprocket-rim distance? Forgive the rather untutored nature of the questions...I'm new to all this but enjoying the learning process.

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Re: Conical rear hub offset.

Post by kommando » Tue Jul 02, 2019 12:18 pm

You already have the spokes ?

Just use the spokes and make up the wheel without setting an offset but using the length of the supplied spokes to position the rim and get the rim trued.

Then mount on the bike, line up the sprockets and then move the rim to line up with the front rim by loosening the spoke nipples on one side and then tighten on the other, fluorescent tubes work well as guides if a bit fragile.

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Re: Conical rear hub offset.

Post by hhh » Tue Jul 02, 2019 6:08 pm

It may be easier to use the frame top tube center-line rather than the front wheel for alignment. It should be dead center in the frame (not the swing arm.)
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Re: Conical rear hub offset.

Post by zuff » Wed Jul 03, 2019 10:57 am

Kommando, I admire the thought that I might be building my own wheel but I'll be leaving that to the professionals....which is why I need to find the measurements up front.
hhh, I agree I need to get the wheel in the centre line of the frame. So if I assemble my hub on a bare spindle and get the sprockets lined up, will a measurement from the rear sprocket to the centre line allow a wheel builder to calculate the right offset?

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Re: Conical rear hub offset.

Post by hhh » Wed Jul 03, 2019 3:29 pm

Zuff, yes, that along with the rim width (which he'll obviously be able to measure) will give the builder what he needs to set the offset. You'll want to measure from the mating surface of the sprocket where it meets the hub to the frame tube center-line.

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Re: Conical rear hub offset.

Post by zuff » Thu Jul 04, 2019 5:16 pm

Alternatively, since I am placing stock WM2 rim with a WM2 Borrani, how about this?
I measured from inner face of sprocket to the rim edge using a straight edge and vernier. 35mm
So if I tell wheel builder to make offset of new rim/inner face of sprocket on new hub 35mm that should give me correct sprocket position relative to the wheel when its all mounted up?
Attachments
offset on bike web.jpg
offset position.jpg
hub datum.jpg

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Re: Conical rear hub offset.

Post by beesa71 » Thu Jul 04, 2019 7:12 pm

But you don't then actually know whether the hub/brake plate assembly will fit in your arm (re' spindle dia, spacers/position etc).

My view remains to actually do all the work to fit the hub/brake plate (of your choice) to the arm with the sprocket In the right place for chain alignment then, and only then, measure the position of the sprocket mounting face (if that's your datum) to the centreline of the frame - that's then the detail you need to simply work out the offset against your chosen rim.

My experience with rims is that you can't simply say a WM2 (or whatever) is 'x' wide, they vary whether through being of different manufacturer or material. So, measure yours too and include it in your calculation.

Regards.

Paul.

PS; I assume you've noted the different diameters of spindle/swining arm slot/adjusters etc with the hubs you are comparing.

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Re: Conical rear hub offset.

Post by minetymenace » Fri Jul 05, 2019 8:17 am

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Re: Conical rear hub offset.

Post by zuff » Fri Jul 05, 2019 12:06 pm

Thanks Beeza71
I have ordered complete conical rear hub fittings (spindle, spacers, bearings etc) from Draganfly and a new swingarm from Terry Weedy (for purely cosmetic reasons) so nothing will be kept over from the current setup on the bike. This is a standard B25 crinkle hub assembly. So, yes I understand your point that my datum should be centre line of bike to inner sprocket face when sprocket is correctly aligned on a dry fit of hub assembly.
If anyone is interested, I'll be selling all the standard parts coming off the bike including later type ceriani style front forks and TLS front brake.

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Re: Conical rear hub offset.

Post by minetymenace » Fri Jul 05, 2019 12:16 pm

Do you realise that they are completely different Zuff? At the interface between your oif and pre-oif bits you will need to do some fettling, your mod is not just a matter of changing parts especially if you are changing the swing arm, you will need to get the grinder out of the box, fire up the lathe and/or make sure you have gas in your welder.
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Re: Conical rear hub offset.

Post by beesa71 » Fri Jul 05, 2019 1:04 pm

and I've a use for the whole rear wheel/brake etc assembly so if yo'd let me know your asking price that would be good.

Paul.

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Re: Conical rear hub offset.

Post by zuff » Fri Jul 05, 2019 3:35 pm

Thanks Minetymenace. I understand they are completely different and am prepared for fettling. I've got an angle grinder (just like any tealeaf) but I aint got no lathe or welding gear....I'm not a real welder (as the saying goes!) But I know some blokes who are! So thats why I have committed to buying all new parts to go with my fetish for the conical rear hub. I somehow think that the front end is going to give me even more grief as I'm swapping out the forks for brand new Ceriani GP35 replicas hand made in Hungary. The email discussions have been hilarious. I think I've given him the right info but when the engineer refers to the steering stem as the axle you can be sure there is plenty of opportunity for misunderstanding!

But if it was easy, everyone would do it, right?!!

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Re: Conical rear hub offset.

Post by minetymenace » Fri Jul 05, 2019 10:03 pm

zuff wrote:I'm not a real welder
:laugh :ban :laugh

That saying is as old as your bike!!
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Re: Conical rear hub offset.

Post by zuff » Mon Jul 08, 2019 11:49 am

OK,
So I set up a centre line on the frame.
I measured from the inner face of the current sprocket to the centre line to give me A
I measured the width of my Borrani rim to give me B
So the offset for the rebuilt wheel with this new Borrani rim will be A minus half B.
I can give this dimension to the wheel builder. It was 3 mm different to the Offset measurements I made with the original Jones rim in situ and tallies with difference in width of the Jones and Borrani rims.
Offset.JPG
Attachments
Offset calcs.pdf
(175.92 KiB) Downloaded 12 times

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