Gearbox sprocket changed - speedo checked

[Greybeard]

I recently changed the gearbox sprocket on my Plunger A10 from 18 to 20 teeth. I've just been out for a short test drive to ascertain the inaccuracy of the speedo since changing the gearing and to get a general feel for the ratios. I bungied my tablet device to the top of the petrol tank so I could use a GPS speedo app to check the speed. I haven't yet been able to reach a very high speed; traffic limited me to about 60mph, (100kph). The speedo is under reading by about 10%. The new ratios are very pleasant I was changing into fourth at about 60mph, (real) so 70 should be fairly easy to achieve. This means that dual carriageway cruising, or even motorway, (if I'm desperate) is possible. Previously the bike felt like it needed a fifth gear. The machine didn't appear to have any trouble pulling the longer legged ratio but there were no steep hills to ride up and the bike only has a single seat so carrying a pillion isn't possible.

[duTch]

 Good to know GB, readings maybe no good to me, as I have the wrong speedo (1620)  anyway and already figure a 10% deficit with stock gearing (also have a Garmin bicycle GPS speedo mounted ), I guess with a 20T, I could go a few more %.

 But good to know it pulls well. My main gripe is the gap between 2nd & 3rd, 2nd needs to be a bit taller, but I don't think easily possible

[Greybeard]

It's a lovely sunny day here today so I slipped away for a burn-up on me bike, (https://www.youtube.com/watch?v=l3TVEH1DPbg). Didn't manage the Ton but a cruising speed of 70mph was obtained. This is since fitting a 20 tooth gearbox sprocket; was 18 teeth and when I bought the machine a 49 tooth rear sprocket. I changed the rear wheel sprocket years ago for a 48 tooth one.

I was correct, the speedo is now reading about 10% under real speed so I need to be careful. Can chrono speedo mechanics adjust for such things as changed gearing?

[duTch]

 

Can chrono speedo mechanics adjust for such things as changed gearing?

 I believe so, but they (or you) could maybe get or make a differently calibrated dial and fix it yourself. Not sure how the needle is attached though, getting over that could be fun. Maybe a 'stick on' one?

[chaterlea25]

Hi All,
A decent repairer of Chrono  speedos will be able to fix the calibration
The speedo reading is relatively easily done by altering the weight on the balance wheel
The milometer is more complicated as the ratchet wheel teeth number will need altering
A swop to the correct milometer barrel is easier for the repairer
The dials are more or less all the same

John

[Greybeard]

 

Can chrono speedo mechanics adjust for such things as changed gearing?

 I believe so, but they (or you) could maybe get or make a differently calibrated dial and fix it yourself. Not sure how the needle is attached though, getting over that could be fun. Maybe a 'stick on' one?

I was thinking about a couple of little red arrows on transparent labels; if there is such a thing.

Thinks: I wonder if Letraset make waterproof characters. Must research that idea.

[Greybeard]

Hi All,
A decent repairer of Chrono  speedos will be able to fix the calibration
The speedo reading is relatively easily done by altering the weight on the balance wheel
The milometer is more complicated as the ratchet wheel teeth number will need altering
A swop to the correct milometer barrel is easier for the repairer
The dials are more or less all the same

John

The mileage doesn't really matter too much.

If I told the speedo guy the ratios do you think he would know how to set the unit up, or would I just tell him the percentage of innacuracy?

[trevinoz]

GB, if you still have the 48 tooth rear sprocket, you are still under geared with a final ratio of 2.4:1 compared with the standard Flash gearing of 19 to 42 which gives you 2.21:1.
Does your speedo have the revs per mile printed on the face?
By the way, with your gearing, your overall ratio is 4.8:1 compared to the standard plunger 4.42:1.
Swinging arm is 4.56:1.

[bsa-bill]

Back in the day (I've been told that it's a naff thing to say these days) people would have a sidecar for the family runs and take it off now and then for meets and stuff, don't know if they ever bothered about the speedo.
If you came to the notice of a member of the constabulary in a corker when navigate through a 30 you could probably outrun him anyway

[Greybeard]

GB, if you still have the 48 tooth rear sprocket, you are still under geared with a final ratio of 2.4:1 compared with the standard Flash gearing of 19 to 42 which gives you 2.21:1.

OK.

Does your speedo have the revs per mile printed on the face?

No

By the way, with your gearing, your overall ratio is 4.8:1 compared to the standard plunger 4.42:1.
Swinging arm is 4.56:1.

Interesting. Thanks for that I formation; it's very useful.

[duTch]

 Hey GB, for some reason this came to mind and I had to dig it out.
 Yours must've been revving it's guts out with a 18/49?

 For some reason I'd thought you'd changed the rear to 42T a while ago, but I got it wrong.
I had originally used a 48T? rear sprocket that I found later was off a A65 (bolt-on sprocket, good idea at the time), and with the close ratio box was ok, but when I changed to a STD box, first gear was really-really low.

 All I'm saying is that when you change your rear sprocket to a 42, hold on tight- you'll feel like you're in a TT

[worntorn]

I have an incorrect speed on my Super Rocket. Like yours it reads low. One day it would be nice to fit the correct ratio speedo on there, but for now, I have removed the needle and repositioned it so that it is perfectly synched with the GPS reading at 65 MPH. This means that it is within 1 or 2 MPH of true speed between 50 and 75 MPH.
It also means that the needle sits at about ten MPH when the bike is at rest. It's not a perfect solution, however it's preferable to  having to remember to always add some number to indicated speed to avoid speeding tickets.

Glen

[Greybeard]

Hey GB...
Yours must've been revving it's guts out with a 18/49?

Indeed, hence my mocked up image of a caravan hitched up to my plunger.
In the 1970's when I first got the bike with the 18/49 ratio and no sidecar it wasn't hard to lift the front wheel if I was a bit enthusiastic on pulling away!   
I changed the 49 tooth sidecar rear sprocket to 42 teeth soon after I got the bike. Actually it appears to possibly be from a 'B' series bike as it is narrow so probably for 1/4" chain.
Changing the gearbox sprocket to 20 teeth has made quite a difference. I'm going to change to the correct rear sprocket soon.