Front wheel spoke gauge - twin disc build

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mdt-son said:
It depends on the configuration. If the wheel was designed for one brake disc and caliper only, it's more than likely that adding a second disc and caliper (which exerts the same braking force as the first one) will cause an issue when braking hard. A braking force acting tangent to disc at the center of the braking pad creates a braking torque (Mt) which is transmitted to the rim and tire.

Consider this simplified equation: Assume all spokes have the same angle (alpha) in the central plane towards the rim tangent line. Disregard the out of plane angle.
Let T be a component of the spoke tensile force acting tangent to the rim bead of radius R. Then,
Mt = n * T * R
Substituting the spoke tensile force (S), we derive at
Mt = n * S * R / cos(alpha)

Now, if Mt is doubled, tensile force in the spokes will also double. A manufacturer would have to size for the worst case, meaning thicker spokes are needed. Assuming the same spoke material is used, and safety factors remaining constant, the new thickness d2 = d1 * sqrt(2) = 1.4 * d1 (rounded).

Adding a disc in a responsible way inevitably makes the wheel heavier! Namely, by 40% of the initial spoke weight. In addition nipples will be a little heavier. We neglect the weight loss at the hub and rim by drilling larges bores for spokes and nipples.

- Knut
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I'd never thought about it years ago
We just added another disc and calliper to most Japanese bikes and triumph tridents , Bonnie's etc
I can't remember anyone going up in spoke size when adding an extra disc
I know a tyre will break it's hold on the road at exactly the same speed regardless of whether it's a drum,disc,or dual disc
So I assume the torque through the spokes (at maximum) braking would be the same?
Maybe it's under this threshold there's a problem
I've just never seen it
 
baz said:
I'd never thought about it years ago
We just added another disc and calliper to most Japanese bikes and triumph tridents , Bonnie's etc
I can't remember anyone going up in spoke size when adding an extra disc
I know a tyre will break it's hold on the road at exactly the same speed regardless of whether it's a drum,disc,or dual disc
So I assume the torque through the spokes (at maximum) braking would be the same?
Maybe it's under this threshold there's a problem
I've just never seen it
Click to expand...
The braking force (and torque) is limited by the friction between tyre and tarmac. I haven't taken friction into account above.
If a bike fitted with one disc makes the tyre exceed the limiting friction force towards the tarmac, then adding a second disc will not change the braking force - it's limited by the laws of physics (friction * weight loading at the front wheel while braking - note that CoG moves forward during braking). However, if you fitted a stickier tyre at the front, the braking force and torque will be higher under ideal conditions, and a double disc will then be able to exert a higher braking force than a single disc can.
Apart from this, the advantage of a double disc is equal reaction forces at the sliders and stanchions, i.e. no twisting of the fork, provided friction forces at the brake pads are identical.

-Knut
 
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mdt-son said:
Hi

I am building a new front wheel for my Mk3 featuring almost straight spokes. I will be fitting twin calipers to custom made sliders.
Spokes will be single butted and my initial choice is 7/9 gauge (or nearest metric size). Is this a sensible selection when using twin stoppers?

-Knut
Click to expand...
Doesn't sound like (or starting with) an original hub or sliders, dunno what calipers, maybe the rest original?

Sounds complicated along with ltd details, why not just swap in a 35mm tubes twin stopper spoked wheel front end? Like from a Yam xs650? Prob Brit equivalents (t140) out there also.

Does 'custom made sliders' involve/mean alum welding?
 
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baz said:
Is it a known thing that fitting an extra disc causes an issue with the spokes?
Click to expand...
I've never thought so but a Norton disc front it weird in that the two sides are very different. I have no answer for the question at hand or yours, but my guess is that you would want to use the heavier spokes usually used on the disc side of a Norton on both sides. But then, what about the offset? Maybe a mixture of MKIII and pre-MKIII spokes, drill the hub to accept the heavier spokes, re-bend half of the spokes, and then fiddle with the alignment?

I would probably use a T140V front end or at least wheel - simple to bolt on the 2nd disc and the wheel is already strong enough.

I bet this keeps me from sleeping tonight :(
 
marshg246 said:
I've never thought so but a Norton disc front it weird in that the two sides are very different. I have no answer for the question at hand or yours, but my guess is that you would want to use the heavier spokes usually used on the disc side of a Norton on both sides.
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Thank you Greg. I have not been aware of the different spoke gauges used on the Norton single disc brake front wheel. I will revise my spec and select a 7/8 gauge spokes (or nearest metric size). The question about offset alleviates itself by selecting a different hub. I am using a hub off a BMW F800GS. It's symmetrical and provides straight (or almost straight) spokes. The genuine spokes are very heavy - 4.5 mm or 6.5g - but then, the Beemer is an adventurer and my bike is a roadster, so I think it's safe to decrease the load requirements.

I will reveal choice of sliders later but they will be Norton-like. Calipers are Brembo, like the ones offered by CnW.

- Knut
 
marshg246 said:
I would probably use a T140V front end or at least wheel - simple to bolt on the 2nd disc and the wheel is already strong enough.
Click to expand...

Spokes are 10g - fairly thin, but there are 40 pcs. Maybe tailored at 70's feeble brakes? I wouldn't use this gauge if I were to fit a 2nd disc and/or updated calipers.

-Knut
 
mdt-son said:
Spokes are 10g - fairly thin, but there are 40 pcs. Maybe tailored at 70's feeble brakes? I wouldn't use this gauge if I were to fit a 2nd disc and/or updated calipers.
Click to expand...

I agree that it's normally worth going up at least one gauge size when changing to stainless although my T160 with double-disc conversion has standard 10 SWG spokes only in stainless (that would have the same single disc brake and wheel assembly originally, as the T140) and those spokes have been no problem on what is a significantly heavier (even heavier than the 850 Mk3 :)) bike.
 
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74ok said:
Doesn't sound like (or starting with) an original hub or sliders, dunno what calipers, maybe the rest original?

Sounds complicated along with ltd details, why not just swap in a 35mm tubes twin stopper spoked wheel front end? Like from a Yam xs650? Prob Brit equivalents (t140) out there also.

Does 'custom made sliders' involve/mean alum welding?
Click to expand...
1) No, no, Brembo, yes
2a) You now NVT changed steering geometry for the 850s. For simplicity and cost reasons I am retaining the fork crown/lug ("triple tree") and stanchions.
2b) XS650 and T140V employed old school cotton reel hubs. I am aiming at a better design featuring straight spokes.
3) No, it's nearly impossible to weld a slider without experiencing distortion. Maybe laser welding would work. Details to be provided later.

- Knut
 
mdt-son said:
Spokes are 10g - fairly thin, but there are 40 pcs. Maybe tailored at 70's feeble brakes? I wouldn't use this gauge if I were to fit a 2nd disc and/or updated calipers.

-Knut
Click to expand...
I know a guy with dual discs on a T140 with Brembo calipers and I know the wheel is strong enough because when I took the bike out for a test ride after working on it I accidentally did a stoppie. Any wheel than can survive a stoppie with me on it is very strong! No, I did not like the experience and was way more careful after that.
 
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might be it

Front wheel spoke gauge - twin disc build
Front wheel spoke gauge - twin disc build
 
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L.A.B. said:
I agree that it's normally worth going up at least one gauge size when changing to stainless although my T160 with double-disc conversion has standard 10 SWG spokes only in stainless (that would have the same single disc brake and wheel assembly originally, as the T140) and those spokes have been no problem on what is a significantly heavier (even heavier than the 850 Mk3 :)) bike.
Click to expand...
Les,
A modern breaking system featuring 4 piston calipers (times 2) and floating large diameter lightweight discs is in different league compared to a Triumph of the 70's, even if it's been uprated with AP calipers. Disc(s) are still comparatively small and fixed - at least those provided by LP Williams are. Calipers have 2 pistons only and I believe they are fixed, as were the original calipers. There may be higher performing kits out there. The T160 is no doubt heavier than a 850 Mk3, but how well does it retard? That's the decisive question, i.e., the maximum braking torque. Will a wheel equipped with 10 SWG spokes sustain say 100 squealing stoppies? I for one don't trust this combination.

- Knut
 
74ok said:
might be it
Click to expand...
Yes, you are right. Good picture. Anyone wanting to explore this route - don't use the BMW/KTM part - it's made of a low grade aluminum alloy and corrodes like mad. Woody's Wheel Works makes a much better replacement hub made of AL 6061 T6. (I am speaking of technical merits here - no endorsement implied).

- Knut
 
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mdt-son said:
A modern breaking system featuring 4 piston calipers (times 2) and floating large diameter lightweight discs is in different league compared to a Triumph of the 70's, even if it's been uprated with AP calipers. Disc(s) are still comparatively small and fixed - at least those provided by LP Williams are.
Click to expand...

Agreed. My point was that the 10 SWG stainless spokes seem to cope well enough with the weight and the extra disc.

mdt-son said:
There may be higher performing kits out there. The T160 is no doubt heavier than a 850 Mk3, but how well does it retard?
Click to expand...

The standard twin Lockheed (RH is modified LH) or Grimeca (below) racing calipers with 4.10 tyre can certainly lock the wheel at normal(?) speeds.

Front wheel spoke gauge - twin disc build
 
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L.A.B. said:
Agreed. My point was that the 10 SWG stainless spokes seem to cope well enough with the weight and the extra disc.
The standard twin Lockheed (RH modified) or Grimeca racing calipers with 4.10 tyre can certainly lock the wheel at normal(?) speeds.
Click to expand...
Thank you Les. I didn't know the twin disc conversion brakes that well. I will stick to my 7/8g stainless single butted spokes, as I have 36 spokes only vs. your wheel which has 40 spokes.
The ability to lock the wheel depends of how much weight rests on the front end, and of course the friction between tyre rubber and tarmac. I suspect the Triumphs are a little light at the front, i.e. rear heavy. I don't know how well the Norton 850 fares wrt. weight distribution.
Modern bike design go a long way in moving mass forward, thus increasing breaking performance.
 
L.A.B. said:
Agreed. My point was that the 10 SWG stainless spokes seem to cope well enough with the weight and the extra disc.



The standard twin Lockheed (RH is modified LH) or Grimeca (below) racing calipers with 4.10 tyre can certainly lock the wheel at normal(?) speeds.

Front wheel spoke gauge - twin disc build
Click to expand...
I have the same setup on my t160 but I have semi floating 300mm discs
I've never worried about the front wheel spokes until now
I shall keep a check on their tension
 
mdt-son said:
Thanks, yes I have detected it thanks to Greg's mentioning.
Click to expand...
He called it a 1/2 & 1/2 mix with a disc setup.

The available parts suggest that a whole wheel is likely done in either or, lighter or heavier gauge.
 
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