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Discussion Starter · #1 ·
A question which is often asked of me is what gear ratio is best for a track.

I usually ask Roly and pass on his answer to make me look clever, but is there an easy way of calculating it at all?

I would just experiment until I found the ratio which provided the lowest lap time, but surely there is an easier way....

Over a set distance in a straight line, in theory, you should reach your top speed as you touch the finish line. That way you are not ever not accelerating, but you also don't have more acceleration to come.

This method won't work on a circuit as there are many straights of different lengths and various radius corners to contend with.

So how do you choose the best one?

Now I think about it, I don't think that I have ever seen a slot car going at full speed, as I think they are always still accelerating at the end of the longest straight when you have to start braking. This would mean that they are overgeared because you are missing out on acceleration even on the longest straight.

Another point to consider is traction. It is not infinite and gearing for too much acceleration can often lead to wheelspin and therefore slower acceleration.

Homework: Gear Ratios, Discuss

McLaren
 

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There is a mathematical way.

First find these values:

Vmax - top speed (in a straight line)
VR1 - top speed to take a radius 1 curve
VR2 - ditto for radius 2, also VR3 and VR4
BD - braking distance from top speed to stop
BT - braking time as above
AD - distance taken to go from 0 to Vmax
AT - time taken to go from 0 to Vmax

get figures for each of these values for a range of gearings, motors and tyre compounds for each of your cars

Then for each lane of the track work out the distance of each straight, and what radius they become, how long they stay on that radius, and what that becomes so you have a distance map of the whole lane showing straight, R1, R2 etc.

Tell me when you have got all that and then we can get on to the tricky bit.
 

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Al Schwartz
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QUOTE Tell me when you have got all that and then we can get on to the tricky bit.

The method that you are proposing was reviewed in Acta Mathematica Monegasque by Prof. C.R.A. Quenshaftt and show to have a fundamental error. When Vmin, the minumum velocity term, goe to 0 , as in the case of a car at rest, a "divide by zero" term is introduced in the expression for the second derivative. This is a forbidden mathematical operation as it "blows up" the desired ratio to infinity.

EM
 

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mac pinches
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Astro, i lost you just after "there is a" but i notice there is no mention of driving style or reaction time variables, or are these "pure" lap times we are after. i know its not the most mathematicaly correct approach but as the man said its "different strokes for different folks" mac p
 

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Graham Windle
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just gear it so it peaks about a foot before the end of the main straight trial and error ,if its then too punchy on the twisty bits or spinning the wheels drop a tooth at a time off the spur until its drivable . moral of the story is the available power should go onto the track and not be lost in wheelspin
 

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Trial and error is the best way I find !

Just get a full set of crowns , fit a 9 tooth pinon and just play around. They
are so simple to change (assuming you are running slot.it running gear).

I start by doing the classic 3-1 ratio and pump in some good laps , I then
go for one with higher acceleration and less top end and do the same.

Then go for one with slower accleration and higher top end and once
again drive some laps.

Take a look at your times and then decide.

However DO NOT base it on the fastest lap
- but rather the time over 10-15 laps.

Some ratios do make for a quicker lap but worse overall performance due to them
being more difficult to drive.
i.e.
With the higher accleration you can find your times can be affected by overcooking
out of the bends.

With higher top end you can find you are trying to carry to much speed through the
bends to take advantage of that new top end.

I have at times altered cars in a way that gives me a WORSE fastest lap - but over
15 laps I am a hell of alot quicker cause I can drive it close to 100% the whole
way.
 

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Mclarens post made me start thinking, and by the time I got as far as listing the relevant variables, my head was hurting too much to continue!

The next bit in one method involves making a graph, as well as some simplifications.

The model is an approxiamation, and so I have assumed a constant (or slower) speed in the curves, acceleration and deceleration occuring at the start and end of straights (special exception with compound curves)

Make a graph of a lap (a middle lap, so no start or finishing, though you can do start and finish laps too if you want)

vertiacal axis is speed, horizontal axis is time. Area under the plotted line is distance.

First work out each curve - VR1, with the area under = the distance of that bend

finally make the straights, using the gradients of the acceleration and deceleration to make slopes up to Vmax form VR, and then back down to the next bend velocity. The area under the line is the distance, the length is the time.

Simple?
 

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I just do what Grah1 does. Wing it until it screams then tame it. Then any other cars to be used on that track start off with the same gearing and adjusted from there. Then when you race on another track you start from scratch. Every track takes different gearing.
 

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I've read through it all but can't remember if anyone has said that a lower gear will give better engine braking if you don't have brakes
 

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Graham Windle
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a 4-1 ratio will brake better than a 2-1 etc just like in the real world but something else to take into acount is the weight of your car compare carrera braking to proslot on the same ratio the carrera takes much longer to stop
 

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Some good points above. I`m with Mac on this one as it depends on your personal driving style. I think it would be good to see a complete table of gear ratios available and a brief description of related performance. Graham , any chance mate?
 

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Gear Ratios
..........................................................Hub
..................24 Teeth................26 Teeth......27 Teeth........28 Teeth.......30 Teeth
Pinion
8....................3-1......................3.25-1............3.375.........3.5-1...........3.75-1
9....................2.66-1.................2.88-1.............3...............3.11-1..........3.33-1
10.................2.4-1.....................2.6-1...............2.7...........2.8-1...........3.0-1
11....................2.18-1..............2.36-1..............2.45............2.54-1.........2.72-1

Low Ratio : High Top Speed , Slower Acceleration , Less Brakes
High Ratio : Lower Top Speed , Higher Accleration , Increased Brakes.

To take your first stab at what ratio you need , look at the track.

Circuits that are twisty or only have short straights you aim for higher ratio
Circuits that have some good long straights it might be worth trading off some
acceleration for more top end.
Circuits that are in the middle - go for a middle ratio.

3-1 is the standard balance between top speed , acceleration & brakes.
However your car or circuit may mean you need to go either way.
I have never gone above a 3.5 or below a 2.6 to be honest - but I am sure there
are circuits and cars out there that would demand it.
 

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Hi

What I do is NOT the conventional approach. Simply this, I am a poor driver. Thus, I rarely change gears from track to track.

I do better if the car drives the same way on every track, changing the gears gives me something NEW to learn about the car. While I am learning the track, the drivers and the changing conditions. So, I don't. I want the gear ratio to produce an easy to drive car in any corner. If I gear for one type of corner, say commercial track long corners, I will make mistakes. Rather, at any time, I want the car to be controllable from start to top and have good brakes.

Thus, for me, gear ratios for a car are FIXED once. With a given car, a short twisty club track and a big commercial sweeper...doesnt matter. I want a driveable car in the corner.

That means driving the car...nothing more.

Fate
 

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Discussion Starter · #19 ·
Cheers for the info guys...

I agree with the trial and error thing, it's got to be easiest so long as you can guess the starting ratio pretty accuratly.

@ Fate - But how do you arrive at your first gearing?

@ fouldsc - I can understand what you mean about putting in one quick lap, but not being able to do it consistantly, but it's at that point that I would start to improve my driving again. Learn to drive it at that speed all the time. Good idea, no?

McLaren
 

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Hi

First ratio is fairly arbitrary. If I havent done the motor before, usually start at 3:1.

But at this point, I usually know before hand from experience. Trying to remember when the last time was that I saw a new motor.

And given my prejudices, I often build the motor I want.

Fate
 
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