Speedo Ratios

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
Locked
simonh
Member
Posts: 38
Joined: Thu Jul 15, 2021 7:12 am
Location: Cheshire

Speedo Ratios

Post by simonh »

Hello all, My model 8 restoration is complete and back on the road, however when riding, I felt that it was either unexpectedly fast or my speedo ratio was wrong. I have a new 2:1 Speedo gearbox, bought from AMC classic and a 100mph Smith's speedo SN3153/95 1600. Riding at an indicated 40mph felt like 30, so I rode past one of the 'your speed' radar detectors that are found in speed restricted areas. 40mph on my speedo is actually 28mph on the radar machine. The Speedo is the correct part number as far as I have checked and was NOS with zero miles on it when fitted by me. Any ideas?
OEW591
Posts: 158
Joined: Mon Jan 01, 1990 12:00 am
Location: CAMBRIDGESHIRE UK

Re: Speedo Ratios

Post by OEW591 »

Simon,

I have had similar problems with my G15CSR. Although a 2:1 ratio, is the speedo gearbox the correct one for a lightweight? The Magnetic (and from your clock code) lightweight clocks are calibrated at 1600 turns per mile (speed accordingly measured). The Chronometric use a different ratio.

Also, doe you bike have the correct sized wheels? My G15CSR has 19" rims where the standard models had 18" rims. The speedo will be inaccurate if the wheel and tyre size differ from the original. There is a calculation you can do by measuring the circumference of your wheel in inches and then dividing this into 63300 (no of inches in a mile) If the answer is 800, your speedo gearbox is providing an accurate reading. More or less and it will read fast or slow.

Some of the replacement gearboxes are of poor quality (nylon parts instead of steel / brass). I don't know what brand you have but I had 2 fail completely on my 1965 G80 within 50 miles.
simonh
Member
Posts: 38
Joined: Thu Jul 15, 2021 7:12 am
Location: Cheshire

Re: Speedo Ratios

Post by simonh »

Thanks for your reply. My wheels are 18" with 3.25" tyres both ends. I bought the speedo GB new from AMC classic, so hopefully it will be OK: The cable is also new. I originally thought that I might need a 1.25 GB, but I think that would make the cable turn faster and therefore show a higher speed.
39speedtwin
Member
Posts: 850
Joined: Thu Jan 01, 1998 12:00 am
Location: ESSEX UK

Re: Speedo Ratios

Post by 39speedtwin »

Your 18" wheel with 3.25" tire will turn 831 turns per mile, so with a 2:1 drive the cable turns 1,662 per mile. With a 19/10 drive this would reduce the turns per mile to 1,579. Therefore I believe the drive is wrong, a 1.25 drive would also be wrong.
However it may be that the speedometer is not calibrated correctly, at 1,600 revolutions per minute it should read 60 mph.
User avatar
clive
Member
Posts: 5657
Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Re: Speedo Ratios

Post by clive »

Your speedo is correct according to the 1960 smiths catalogue I supplied for uploading on the archives. But I fail to see how the ratio of 1600 can be correct for an 18 by 3.25 tyre and a 2:1 drivebox. Early postwar heavyweight speedos had ratios of 1600 which went with a 2:1 drivebox for a 19 by 3.25 tyre. When they rear tyre size increased to 19 by 3.50 the speedo ratio dropped to 1548 still with the 2:1 drive.
As Dudley points out the smaller 18 by 3.25 tyre you have fitted would turn more often per mile so would make the speedo read fast, but if Dudley's figures are correct (I have a list of ratios for different tyre sizes but its in the garage some 1 1/2 miles away and I have a lousy cold so I am not going to fetch it) then it should only be overreading by about 4% or just over one mile an hour at 30mph. Even if the 19:10 drivebox is the correct one the error would only be 1.3% and it would be under reading at less than a 1/2 mile an hour at 30mph.
The 1960 catalogue only advises 1:1 or 2:1 driveboxes being available anyway.
So the possible options are
1. despite your belief that 40 felt like 30 and the reading from radar machine both are wrong and you have a really gutsy lightweight.
2. more likely the NOS speedo was still reading 0 miles because it had been rejected due to not working correctly. The lightweight and all the later heavyweight magnetic speedos all work by a magnet spinning under an aluminium plate. This is attracted by the spinning magnet because induced current in it causes the aluminium to develop a magnetic field and the speed shown relies entirely on the correct tension in the hairspring attached to the aluminium plate. At least that is my understanding of it. So I think your speedo rather than drivebox is likely to be the problem.
clive
if it ain't broke don't fix
simonh
Member
Posts: 38
Joined: Thu Jul 15, 2021 7:12 am
Location: Cheshire

Re: Speedo Ratios

Post by simonh »

Thanks all for comments. The speedo could be the problem I suppose, it had zero miles on it and it and maybe it needs a service. When I get chance, I will try a heavyweight speedo attached with cable ties and see if it reads right. My tyre circumference is 83.4 inches, so would make 760 revs per mile. Much appreciated.
User avatar
clive
Member
Posts: 5657
Joined: Mon Jan 01, 1990 12:00 am
Location: LONDON UK

Re: Speedo Ratios

Post by clive »

simonh wrote: Sun Oct 10, 2021 7:28 am Thanks all for comments. The speedo could be the problem I suppose, it had zero miles on it and it and maybe it needs a service. When I get chance, I will try a heavyweight speedo attached with cable ties and see if it reads right. My tyre circumference is 83.4 inches, so would make 760 revs per mile. Much appreciated.
Well with that wheel circumference and a 1600 speedo working correctly your bike should be showing 31.5 mph when travelling at a true 30 mph. This is within the required standard of up to 10% over at 30 mph and would of course have made the bike when first sold seem a little quicker!
I would make the check you suggest with a chronometric speedo through the same radar sign before sending the speedo off for service but I am pretty sure it is the speedo that is at fault.
HOWEVER the cables for chronometric and magnetic speedos are different and a magnetic cable can damage a chronometric speedo as the tang is too long. So I suggest swopping the cable too or at the very least not screwing the magnetic cable fully home and only whilst you do the test. The difference in length of the tang is about 1/8 of an inch so perhaps two turns of the knurled nut.
clive
if it ain't broke don't fix
39speedtwin
Member
Posts: 850
Joined: Thu Jan 01, 1998 12:00 am
Location: ESSEX UK

Re: Speedo Ratios

Post by 39speedtwin »

Reference Clive's comment on cables, there are 2 sizes of the square, large 1175"-1205", small 1101"-1105", so if you have the small cable it will not drive a chronometric, also as Clive states the length going into the speedometer is different, magnetic 9/16" and chronometric 7/16", if pushed in too far the magnetic cable will damage the chronometric.
simonh
Member
Posts: 38
Joined: Thu Jul 15, 2021 7:12 am
Location: Cheshire

Re: Speedo Ratios

Post by simonh »

Thanks all, I will have a look at the cables also. I think I have an old chrono cable.

Much appreciated. Simon
Locked