working out mm per rev for gearing

[Adam_Lock]

hi
i've seen people using a mm per rev ratio for gear and was wondering how this is done??
and how you us eit properly?

cheers

[burgie]

I asked a very silly question about this once!

look here http://www.rcracechat.com/forum/index.php?board=48;action=display;threadid=4545

once you have finished laughing, then you will find the answer. Most good model shops can sell you a roll-out chart which will give you the correct pinions and spur gears you need to achive the correct gearing for a given tyre diameter .

[DA_cookie_monstA]

(pi*D)/gear ratio=mmpr

This allows you to work out finer gear changes.

[Mark Stiles]

You can get gear charts from the internet. Check out the corally, or Milton keynes website.

[Adam_Lock]

cheers fellas, had a look at the link i'll have a read and see what i come up with.

[Jonny_H]

As to "how to use it properly": when you're using foam tyres, which change size every run, the only way to compare gear ratios is as a mmpr figure. Anything else is meaningless ;)

It also helps if you're comparing, say, a buggy to a touring car or a stadium truck...

[Adam_Lock]

say if i have me wheel size?? how do i work out the mmpr???
and compare the figures to get a gear rations??? ??? ???

i have the gear ratio charts for the car i have but am still comfused!

[marcoski]

There is a thread about this in general discussion by cookie monsta, its explained there.

[Mark Stiles]

If you have a gear chart, then concentrate on the table with ratios for your wheel diameter (eg 53mm). Find what pinion and spur combination you have, and trace the rows/columns down and across, until you arrive at a number. This is your mm per rev.

Running less mm per rev increases acceleration, but reduces top speed. Increasing mm per rev does the opposite. Decreasing mm per rev also increases your battery run time, so if you're dumping reduce your rollout.

To work it out on a calculator;

pi x wheel diam, divide by spur, x by pinion.

You if you have a 75 spur, with an 18T pinion, and 53mm wheel diam, then your rollout is about 40mm/rev.

[MikeS]

If you have a gear chart, then concentrate on the table with ratios for your wheel diameter (eg 53mm). Find what pinion and spur combination you have, and trace the rows/columns down and across, until you arrive at a number. This is your mm per rev.

Running less mm per rev increases acceleration, but reduces top speed. Increasing mm per rev does the opposite. Decreasing mm per rev also increases your battery run time, so if you're dumping reduce your rollout.

To work it out on a calculator;

pi x wheel diam, divide by spur, x by pinion.

You if you have a 75 spur, with an 18T pinion, and 53mm wheel diam, then your rollout is about 40mm/rev.


Where is the internal transmission ratio Mark?
That one just happens to work for a 1/12th scale (or any direct drive car) since it is 1:1, but no other cars are!

[Mark Stiles]

Sorry. He mentioned mm per rev, and I assumed he meant 1/12th scale, as I'm not racing touring cars etc.. (yet!)

[PDW]

It works like this with an internal ratio:

First, work out the distance the tyre goes in one revolution - Pi x D is the equation. 67mm dia tyre (D) x Pi (3.142) = 210.5mm

Now work out the overall gear ratio as follows:

First, motor - spur and pinion: 69 (pinion) / (divided by) 33 (spur) = 2.09.
Second, internal ratio (use this thread www.rcracechat.com/forum/index.php?board=2;action=display;threadid=7083) which for my TC3 is 2.5
Third, multiply the two together = 5.23

Now, divide the overall ratio by the distance the tyre travels in one revolution and you have 40.25mm/rev

What you have done is calculated how many times the motor has to go round to make the wheel turn once (5.23) and divided that into how far the wheel goes when it turns once (210.5mm) So, for every turn on the motor, the car travels 40.25mm. Bingo - QED!!

It is only viable to compare one car with another in the same class, because all sorts of factors come into play - not least weight. So, I run a 10x2 in 4-cell 1/12th on about 42mm/rev, but a 19-turn Spec in a 6-cell TC3 on 40.25mm/rev.

However, you can now compare directly with your mates who run different cars by working out the mm/rev. It gives a guide (ONLY!!) as to what (say) a TC3 is running compared to an X-Ray in TC. HTH :)