650SS speed to revs

My mercury is being reassembled at present and my gearbox sprocket is a 21t. Perhaps thats the difference in ratio.

hello yes you are right the 650ss should be into the 60 miles an hour mark at 3000 rpm

Hi Paul. using my Speed v revs Calculator (SEE SEPERATE MESSAGE) your TRUE speed at 3000 revs and a 19T gearbox sprocket will be just over 51 MPH.

So at 7000 RPM, top speed should be 120 MPH...sounds about right to me.

Les

Previously wrote:

Hi Paul. using my Speed v revs Calculator (SEE SEPERATE MESSAGE) your TRUE speed at 3000 revs and a 19T gearbox sprocket will be just over 51 MPH.

So at 7000 RPM, top speed should be 120 MPH...sounds about right to me.

Les

Hi Les

On the basis of the above do you think my 21t gearbox sprocket is too large? Waht would the top speed be with a 21t on your calculator? What figure are you using as the rear wheel circumfenence?

Dominic

Hi Dominic. With a 21 tooth gearbox sprocket your overall top gear ratio is 43/21 x 42/21 = 4.095.

My formulawill give a theoretical maximum speeddepending on the revs chosen to give peak power output. So if we say the peak power on a Mercury is produced at 6000 RPM then the theoretical top speed will be 113.4 MPH.

In practice the top speed will only be achieved if you can produce enough engine power to match the wind resistance and tyre friction and losses in the transmission. If you over gear your bike the top speed is likely to be lower as the engine cannot produce enough power to match the power requirement for the theoretical top speed. I would say an unfaired large motorbike requires around 50 BHP to push 113 MPH so your Mercury is not going to get anywhere near thatspeed if the engine is standard.

However over gearing will mean lower cruising revs, less vibes, and less wear. The acceleration will not be as quick though.

This is why I suggested the calculators are useful for obtaining the best match for revs v top speed v actual desired style of riding.

The wheel circumference will generally match 19" x 3.50-4.10 tyres quite well. I'm not saying it is accurate to the nearest 1/10 of a MPH etc, but it will be quite close in practice.

Les H

Hi Les thanks for that. I went and counted teeth after reading your message. I have 21/42 and 21/42 both primary and final drives. The mercury in theory put out 47bhp. Be interesting to see what it will attain once I have it complete. Strangely the speedo fd is 19/10.

regards

Dominic

Hi Paul. I think you have forgotten your own question maybe?

Of course I worked it out for a standard set up!

You were suggesting that before you changed to a belt drive the speed was in excess of 50 mph and I have shown you were correct with your memory. In so doing I have proved your suspicion that the belt drive is giving you a different gearing. If you would like to take the time to count the teeth on the belt drive pulley cogs I will happily tell you what speed you should be getting now and tell you what new gearbox sprocket you require.

I have simply shown what you asked for, there is nothing more really to add, apart from counting the teeth of the new set up.

Les

PS.For a high power drive, it is beneficial to drive the clutch faster (have less primary reduction) This reduces the torque on the clutch and reduces the chance of clutch slip. One then has to compensate this speed loss with a larger gearbox sprocket. If the new primary ratio is known then a new secondary ratio can be calculated to compensate and get back to the old overall ration (near to). I would imagine this ishowthe belt drivehas been designed.

Les

CORRECTION: I said:

PS.For a high power drive, it is beneficial to drive the clutch faster (have less primary reduction) This reduces the torque on the clutch and reduces the chance of clutch slip. One then has to compensate this speed GAIN with aSMALLER gearbox sprocket.

So Paul, as your new speed is slower then the primary ratio is giving you a greater REDUCTION,ie: the clutch is turned more slowly, which is surprising and different to what I had imagined.

The speed difference (51mph down to 45mph) is 6% slower so you will need to increase the rear sprocket to give an extra 6% higher ratio.

So the nearest gearbox sprocket will be a 20 tooth one to replace the 19t which is standard.

This will get back you back to over 50mph at 3000 revs.

Les

Previously wrote:

Does anybody know the ratio of speed to revs for a 1966 650SS? In top, mine is doing 45mph at 3000rpm. After 22 years in the shed it's had a 2 year longrebuild and a Mick Hemming's(Norvil?) "Dominator"primary belt drive installed. I seem to recall that, whenI previously rodeit 24 years ago, 3000rpm was over 50mph. Is that my memory at fault or is something now amiss, such as the belt drive upsetting the ratios? The gearbox sprocket is 19T. Any advice will be gratefully received.

Hi Paul

Hopefully I can shed some light on this! I too have a 650SS with Norvil belt primary drive, this gears down the final gearing somewhat, I used a 22 tooth gear box sprocket which gave great cruising but relaxed acceleration, a 21 tooth is much better giving about 3000 revs= 60mph etc, this seems a good compromise between acceleration and cruising. hope that helps.

Regards

Nigel