Rear Brake drum

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Re: Rear Brake drum

Postby bitza » Fri Jul 05, 2019 5:18 pm

If the slot on the brake is worn a spot of weld and a file should fix that , check that the locating lug does'nt have a groove worn into it caused by the chattering of the brake plate over the years.

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Re: Rear Brake drum

Postby bob121 » Sat Jul 06, 2019 7:19 am

I think it's both, but I'm going to deal with it on the plate. I'll run a couple of welds on the back of the plate and shape accordingly. Can't face the ball ache of getting the chain guard off and back on. I'll try to get started on it next week. Got a friends bsa to get started on first.

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Re: Rear Brake drum

Postby Group Leader » Sat Jul 06, 2019 6:27 pm

Winkie wrote:I am not a technical expert so cannot explain the mechanics behind it, but with a chronometric speedo the needle will stop at the speed it is showing when the cable breaks. Someone more knowledgeable than me will come along and explain why, but that is what chronometric speedometer do.

Me neither, but I do know that the chronometric samples the speed at intervals of approximately 0.7 seconds and registers the speed at which point the needle is effecively clamped in that position. When the next sample occurs the needle will either: advance to the new higher speed attained during that period, drop back to the new slower speed or stay where it is if the speed is constant. The needle is only released at certaiin points in the cycle (by some tiny little cams and finger springs). If that point in the cycle never comes because, for example, the cable has broken the speedo needle will not be released and it will preserve the speed at which the cable failed.

Fantastic and fascinating bit of kit the chronometric speedo and this is an excellent article on the beast.

http://www.wdbsa.nl/Jaeger-Chronometric-Overhaul.pdf

Alan (still looking for a reasonably priced, functional but not cosemetically brillant chronometric speedo..... :) )
1953 AJS 16MS, 1939 BSA 250 and a 1/3 scale Sopwith Triplane but that's another story ..... :lol:

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Re: Rear Brake drum

Postby bob121 » Sun Jul 07, 2019 1:24 pm

Cheers. I've downloaded the file. I'll have a read of it later. Hoping the weather is ok in the morning. If so, I'll take the aj and dump the oils and get it ready for the repairs.

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Re: Rear Brake drum

Postby bob121 » Sun Jul 07, 2019 6:03 pm

That was interesting reading. I find it intriguing that someone chose to run with a design that essentially freeze frames a reading every 3/4 over a design that gives a constant. I would have assumed that the latter would be easier or more cost effective.

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Re: Rear Brake drum

Postby Group Leader » Sun Jul 07, 2019 9:44 pm

bob121 wrote:That was interesting reading. I find it intriguing that someone chose to run with a design that essentially freeze frames a reading every 3/4 over a design that gives a constant. I would have assumed that the latter would be easier or more cost effective.

Which is what the later magnet / ali drum / Eddy current speedometer design is.

The Eddy currents created in the aluminium drum by the continuously rotating magnetic field from the magnet rotating in close proximtity to the drum create a torque ~proportional to rotation speed. This torque is countered by the increasing torque from a hairspring as it is tensioned as the drum/needle rotate. When the two torques balance the needle settles and shows the current speed.

Such a design has a number of benefits for a manufacturer; they are very simple, they have few parts and as a consequence are far cheaper to make than the clock mechanism that is the Chronometric. Hence, as you point out, they are far more cost effective. They do have the disadvantage that they are generally less accurate than a Chrono and are not particularly linear however their accuracy is "adequate" for the humble road vehicle.

The thing that amazes me about the Chronometric speedo is just how robust these apparently delicate, precision clock movements are given the environment they work in on our motorcycles, highlighted by the fact that they have done so for decades. They appear to be extremely accurate too - I like to check the calibration as I pass each radar speed sign of which there are many these days :rofl:

Alan
1953 AJS 16MS, 1939 BSA 250 and a 1/3 scale Sopwith Triplane but that's another story ..... :lol:

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Re: Rear Brake drum

Postby MalcW » Mon Jul 08, 2019 8:06 am

Group Leader wrote:
bob121 wrote:The thing that amazes me about the Chronometric speedo is just how robust these apparently delicate, precision clock movements are given the environment they work in on our motorcycles, highlighted by the fact that they have done so for decades. They appear to be extremely accurate too - I like to check the calibration as I pass each radar speed sign of which there are many these days :rofl:

Alan


I once checked mine using a handheld GPS. Over a range of speeds it was always within 1mph of the GPS speed.

Malc

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Re: Rear Brake drum

Postby bob121 » Mon Jul 08, 2019 5:58 pm

Group Leader wrote:
The thing that amazes me about the Chronometric speedo is just how robust these apparently delicate, precision clock movements are given the environment they work in on our motorcycles, highlighted by the fact that they have done so for decades. They appear to be extremely accurate too - I like to check the calibration as I pass each radar speed sign of which there are many these days :rofl:

Alan


Mine is on its way to 60 years old. I did send it of to be checked over before the bike was put on the road, requesting the original reading to be kept.

Started pulling it down today. Some jobs to do. Looks like the chain guard will be coming off after all, as I've had to take off the transmission cases. So I'll clean up the stop on the swing arm as well.
Is there anyway of reducing the amount of oil loss through the casing behind the clutch. I know it can't be sealed, but far too much is getting thrown over the gearbox and chain which means its getting thrown everywhere. To the point I had to see if it was the gearbox seal failing.

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Re: Rear Brake drum

Postby dave16mct » Mon Jul 08, 2019 6:15 pm

The usual cause is too much oil in the primary chaincase. You only need enough to barely touch the very bottom of the primary chain. Also don't leave it on the sidestand overnight or you may get some leakage from the engine topping up the chaincase.
Dave.

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Re: Rear Brake drum

Postby bob121 » Mon Jul 08, 2019 10:07 pm

Cheers. Spends most of the time on the centre stand. Unless I'm out and have to park up. I think I had the oil almost to the top of the chain. So I'll be more attentive to where I fill it to.
I think a canister filter is a necessary future modification after seeing the state of the oil after a thousand miles since the last change. I'm running an air filter which is reducing the contamination to some extent.

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