Gearing - How can I check this?

[nigeldtr]

I have a 1951 GF. In forth, at a very gentle cruising speed (around 1/4 throttle), the speedo tells me I am doing 45 mph and from the road speed, this seems about right. The only problem is that I keep looking for fifth gear - seems a bit slow, the engine his running nicely and not working - plenty of power and could easily be quicker. How can I check that the gearing is correct for solo use and not sidecar ratios?

Thanks

Nigel

[RichardL]

Nigel,

Take a look at the chart at: http://victorylibrary.com/graphics/A10-ratio-chart.jpg

At the bottom of the chart they give the arithmetic for calculating top gear, then, under that, they give the arithmentic for the intermediate gear ratios.

Richard L.

[RichardL]

P.S.

The formula won't change for plunger models, you just need to count the various teeth. Once you know the top gear ratio, you could calculate the intermediate ratios without knowledge of the what gear box innards you have. But that is a discussion for another time, if necessary.


Richard L

[chaterlea25]

Hi Nigel,
Quite a common observation!
I believe it comes from the nature of the engine, they are very torquey (?) and would seemingly take endless increases in gearing!
To check the gearing you will have to remove the primary case and count the sprocket tooth numbers and also the final drive sprocket numbers, then compare this to the handbook/manual
Another issue is the rear tyre size, a smaller than original size will make a marked difference!
Modern tyres are marked for example 90/90 this means the aspect ratio is 90% so the tyre is 90% of the width in height = 81mm
Old tyres were 100% aspect ie. the height was the same as the width
Remember traffic goes much faster now than in the 50's so to keep up with it an increase over the original gearing can also help,I have over geared my Super Rocket from standard, it now shows 3300rpm @ 60mph,
I am considering upping this again a further tooth on the engine sprocket
HTH
John

[Brian]

Being a plunger its most unlikely the primary has been altered. Standard gearing is a 18t gearbox sprocket and 42t rear. I run 19t front and 42t rear which I believe is ideal. Any higher gearing puts more load on the engine when going up hills or riding into head winds. With this gearing the bike will cruise very comfortably at 65mph and have enough go left for overtaking or going up hills etc.

[nigeldtr]

Thanks for all the information, I will do some checking. The main problem is to up the cruising speed a bit. At the moment I have loads of nut cases trying to get past me and then carving in afterwards to make a point! Most of the country roads here are 100 km/h (60 mph) which is too high for my comfort revs wise. The bike and I seem to like around 45 to 50mph at the moment!

Nigel

[A10Boy]

I'm going to raise the ratios on my A10.

I have a later DA10 with a 356 cam, 7.25:1 pistons and cast Iron head. It seems to go like a train, plenty of power but seems to need another gear.

I'm not sure whether to go from the 21 tooth engine sprocker fitted now to a 22t or 23t sprocket, anyone got any advice?

[chaterlea25]

Hi Andy,
Are we presuming the other sprockets are standard??
What rear tyre is fitted? read previous posts
My Super Rocket has a 23 tooth engine sprocket, but is in a higher state of tune than your A10
It still seems to want another gear
I have a 24 tooth which is an option for the RGS, I believe?
I am running a 100/90 rear tyre, the issue was worse when I had a 410/19 (I think, its a good while ago now)
HTH
John

[A10Boy]

Yes I think they are std.

Engine is 21t, - 4 spring clutch drum is std 43[?], -  gearbox is 19t - rear wheel is 42t. the rear tyre is a std Avon SM mk2, 3.50x19.

I think it would easily pull another gear. I'm tempted to fit a 23t Engine sprocket [additional 10% ?] maybe a 22t wouldn't make so much difference ?

[RichardL]

Dave,

By my quick calcs while I'm sitting on the train, assuming 13" rear axle to ground when you're on it: 60 mph @ 3512 rpm with 21 teeth; 3206 rpm with 23 teeth. Don't know if it's useful, but it's fun.

Richard L.

[nigeldtr]

A quick calc,

19:42 gives a ratio 2.21
23:42 gives a ration 1.83

which in percentage terms is 17% difference which could make quite a difference - not sure if I can use this line of thought ????

I will be replacing the timing main on my GF over winter so I can easily swap the sproket out at the same time please let me know how you get on. 17% could be too much for top, but as we never tend to go flat out, I don't think it will hurt ? can always drop down to 3rd if she starts working to hard.

Must be worth a try

Nigel

[nigeldtr]

PS at lower engine speeds, I think torque is important not max power so the state of tuning will not make that much diference - I may be wrong ????

Nigel

[A10Boy]

I think that 17% should be multiplied by the gearbox sprocket over the rear wheel sprocket. 19/42= 2.21

Unless I am very much mistaken.....???