Gear Ratio's

[Debbie]

Hi All

I understand how to calculate gear ratio's and that a larger Pinion gives higher top speed and a smaller pinion gives better acceleration.

You can get very simular ratios with hugly different Pinion/Spur sets

e.g (internal ratio = 1.83) 64dp

32 pinion 108 spur = ratio of 6.18

35 pinion 118 spur = ratio of 6.17

What I haven't worked out yet is which of these would be better if at all? and when would you use one or the other? Surely .01 ratio is not noticable is it? :confused:

[Andy71]

In my opinion a final ratio is a final ratio no matter what the combination of spur and pinion.

However, different combinations will have a knock on effect on motor positioning and hence weight distribution. I also think that the more teeth that are engaged the more efficiant the drivetrain will be, so you could run them with a little more slop between the gears.

You will notice a difference when the jump is around 0.25.

Just my opinion.

[Robz]

Personally i run larger spur gears and larger pinions, this I think gives better feel when racing 27t stock. It seems to give less motor braking and smoother feel. I know this is probably not ideal as rotating mass is higher but I will continue doing this. On the Pro4 it is a 98t spur which with 2mm chassis sits flat to the deck.

[Remo]

Robz... your ME degree have served you well... :rolleyes:

How manytimes do I have to shout angular momentum.... :D

[Debbie]

Thanks for your reply's guys ! :D

See you all Sunday ! :p

[Remo]

Will be at Barham SLCC meet... since West London is currently blitzed by Robz... :o

[nismo]

i agree ,

has anyone seen that 19x1 arm in his epic can yet :eek:

joking rob

now all i need to do it get robz skills ..... :rolleyes:

[soto]

now all i need to do it get robz skills

fat chance!!

joke :)

[drahcir]

Hi

Haven't raced in about 10 or 11 years, but I think this should still apply...

Smaller gears = lower rotating mass = better accelleration

But then you all seemed to know that already... ;)

In my experience (granted, somewhat dated experience), after racing for a while and getting to know the tracks I raced on, what I realised that I really wanted to be able to do is duplicate past performances. So I did 2 things. Firstly, whenever I did well on a particular day or race, I'd scribble some stuff in my notebook...

a) battery used
b) motor & brushes used
c) tire compound
d) tire diameter
e) pinion gear
f) spur gear

For the purpose of this reply, I'd like to point out that (d) is something I ended up considering fairly often... time to grab your calipers, and consider what we ended up referring to as "roll-out". Firstly, you need to know a basic math formula for calculating circumference of a circle.

"Tire Circumference" = pi * Diameter

Roll-Out is the distance covered per revolution of the motor, so is simply

"Roll-Out" = "Tire Circumference" / Ratio

If your calculator hasn't got "pi" on it, you can substitute "22/7" for a reasonable approximation.

Let's consider some numbers then... let's say we start the race-meeting with a tire diameter of 52mm, and during the heats, we wear off 2mm. We then realise that our first heat was when the car "felt best" or "performed best", so we want to get back to a similar setup. Let's assume, for the sake of argument, that we have 32,33,34 and 35 pinions available, and 108, 112, 115 and 118 spurs, and we have been running with a 33 pinion and a 112 spur (excuse me if my numbers are way off as I haven't touched a car since 1994 or thereabouts)...

We can work out the following:

"Roll-Out" in the first heat is calculated as follows:
: Ratio = 112/33 = 3.394
: Circumference = pi * 52 mm = 163.36 mm
: Roll-Out = 163.36 / 3.394 = 48.13mm

Our tire diameter is now 50mm going into our final (due to the 2mm wear), so let's work out what ratio we would need to keep the desired roll-out... By another little bit of algebra, we fiddle the roll-out calculation and end up with:

"Desired Ratio" = "Tire Circumference" / "Roll-Out"

So all we need to do is figure out the current tire circumference...

Circumference = 50mm * pi = 157.08 mm

Plugging in the numbers into the "Desired Ratio" calculation, namely our desired roll-out and the current tire circumference, we get:

"Desired Ratio" = 157.08 / 48.13 = 3.26

So then it's just a case of working out which spur / pinion combination will give us the closest to that ratio, and it turns out to be a 33 pinion with a 108 spur coming in at 3.27

I know what you're thinking... "WHOA! Complicated!"

Well, it is a little bit more complicated than just dividing your spur by your pinion, but you'll be suprised at how you will be able to more accurately tune your car to maintain a certain level of performance.

I ended up simplifying this a little bit by creating myself a spreadsheet in Excel which gave me a few pages of "magic numbers" for my set of spurs and pinions where all I had to do to calculate the current roll-out was multiply by my current tire diameter. I basically printed myself out a little handbook with some easy tables to look it all up in quickly, and always had a calculator in my toolbox.

Anyway... I guess what I'm saying is that there are always more factors involved than you'd expect... For example, even if you follow my advice above and take roll-out into account when setting your car up, you'd get better performance in the final heat with the smaller tires anyway... the top-speed should be almost exactly the same since the roll-out is the same, but you'd get better acceleration since your tires are 2mm smaller in diameter which reduces your rotating mass.

Argh!!! I was contemplating getting back into R/C again, but now that I've been through and explained all that, maybe I'll stick to trying to learn some more about rocket science instead.

(by the way, I ignored the internal ratio you mentioned... I'm a direct drive man :D )

[Racer1]

Quote:
now all i need to do it get robz skills fat chance!! joke

Well at least he is willing to learn some skills, as opposed to just beating people off the track.

Not a joke. :mad:

[Debbie]

Thanks Richard

That was a well thought out and informative responce :cool: , and after all that you want to go back to Rocket Science ? :eek: ;)

Robz skills, well Rob is certainly the main man in 27t this year so a good example to follow. :)

[Robz]

Thanks for the Kind words guys. It was a really good meeting at the weekend and was really pleased to blitz the track record by so much.
It takes practice and perserverance but everyone can get there!


Rob Miller

-------------------------------------------------
C-SRacing Apex Models TIR Racing

[nismo]

well rob we were hoping you or spencer would take a win on the day. support the club members.

see ya all on sunday