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Dominator Speed/RPM Calculator

Norton Models (not Commando or P11)

Dominator Speed/RPM Calculator

Postby Connie73 » Tue Apr 18, 2017 7:05 pm

Gday all,

Wondering if anyone had a link to a calculator to work out the above. I know the sprockets and gear ratio's just not how to convert them all.

Cheers

Connie73
Posts: 37
Joined: Tue Jan 27, 2015 4:05 pm
Location: Sydney, Australia

Re: Dominator Speed/RPM Calculator

Postby Connie73 » Tue Apr 18, 2017 7:06 pm

Sorry,

It's for a 55 dominator.

Cheers

Connie73
Posts: 37
Joined: Tue Jan 27, 2015 4:05 pm
Location: Sydney, Australia

Re: Dominator Speed/RPM Calculator

Postby Rohan » Tue Apr 18, 2017 10:35 pm

Not sure about a link, hopefully someone will oblige.

I have a little chart, drawn on the back of an envelope !
Before we delve any further, what you need to do is draw a chalk line across your back tyre at ground level,
and sitting on your bike wheel it along the driveway until the chalk mark is exactly one revolution advanced.
Mark both points on the ground, and measure between them the distance apart they are.
I did it in feet and inches, although mm is a heck of a lot simpler well a bit anyway.

I think your 55 dommie (featherbed ?) is mechanically identical to my 51 dommie gardengate,
although you may have an alloy head and an extra half horsepower. 2 extra hp if its a 99... ?

You can also work backwards from roughly what speeds it will do in each gear,
although you need to know the gearbox ratios to give meaningful rpm numbers.
It also helps if you know how many teeth on all your sprockets (in case they are not standard),
but there is a way of figuring this out without tearing your bike apart.
Cheers.

Some of this is aimed at future readers, since you have your sprockets and gear ratios.

Rohan
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Joined: Thu Aug 26, 2010 5:30 pm

Re: Dominator Speed/RPM Calculator

Postby Rohan » Tue Apr 18, 2017 11:18 pm

OK, out with the calculator, or pencil and back of an envelope.

Now, knowing your overall gear ratio - say 5:1 and working with say 1000 engine rpms in top gear
(1:1 in the gearbox, conveniently)
the back wheel will turn 1000 divided by 5 in one minute = 200 rpms.
If your back wheel is say 80 inches around, and doing 200 rpms, it does x feet min, which is y mph.

I figured out my dommie does 1264 ft min per 1000 rpms = 14.4 mph
which neatly means its doing 100.8 mph* @ 7000 rpms. (14.4 x 7).
And, the top gear in an iron dommie is 4.78 : 1 ( think ?)
I rather suspect I've used a fairly small sized rear tyre too in these calcs, which may make them a bit low-ish.

I think, the page of calculations with this little chart is missing, so I'm reconstructing this from memory.
I don't believe I've ever seen this in a formula anywhere, although it would be simple enough to do.


*Except an iron dommie won't ever do 7000 rpms in top, so 85 to maybe 90 on a good day @ ~6000 rpms.
And Smiths speedos aren't generally that accurate, unless they have been especially calibrated.

Its worth commenting also that different brands of tyres are different sizes (diams) although not by much,
and tyre pressures and tyre wear alter the numbers, although not by much,
and even rider weight does alter things a little.
So the numbers are always going to be +/- a little bit, maybe a few to 5 percent even.
And another possibly 5% at least for the Smiths, so thats maybe +/- 10% accurate, mostly ??

I've been looking to add a tacho to a dommie, for many a year, and it may just happen 'sometime soon'.
Just out of curiousity, to confirm these numbers ??
https://s17.postimg.org/lfs2zulq7/M7tim ... odrive.jpg

Hopethishelps.

Rohan
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Joined: Thu Aug 26, 2010 5:30 pm


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