Trying to figure it out: How do you determine the gearing?

Why is it that women need "relationships" first before anything else?

You are asking for the whole bible, man! It's quite complex because you have so many factors to consider.

However, I am sure the engineering wizards here could enlighten you to the correct areas.

 

http://www.hewland-engineering.co.uk/

 

I have a book packed away somewhere that was written by a Formula One race driver back in the late 1960's. He wrote about engine torque, and how its applied to the road in a way I had never seen before. I understood the math, i just never visualised it well in motion.

He used the starting example of an engine that produced 100 foot pounds of torque. Using first gear in the gearbox, he used some random ratio, say 3:1. In this case the motor turning at 3000 rpm would input the gearbox, and the output shaft would rotate 1000 rpm. But the torque is multiplied X 3. Now you have 300 Lb.ft.. The propellor shaft goes into the rear axle, and lets say you have 4:1 gears. 1000 rpm in, 250 rpm at the wheels. 300 lb.ft. is multiplied...300 X 4 = 1200 lb.ft.. But torque is split equally in the spider gears, so each wheel is fed 600 lb.ft. of torque.

To keep it simple, he used a 24 inch tall tire. This gives a one foot arm from the center line of the axle to the road surface. Now you can plainly see each tire pushing the car forward with (potentially) 600 pounds of force.

You can change the numbers around to fit your car, but the math is the same regardless. Keep the motor at the upper end of peak torque, or climb slightly above it so that when you upshift you fall back into the peak torque band.

 

i found a link once awhile back that does calculate this for you. it requires a bit of info and math. transmissions are torque multipliers so the force at the wheels is very high, higher than you'd think. i'll see if i can dig it up again, it was very neat as it showed the optimum shift points as well.

 

+1 read the site. This is why proper track car guys change thgeir gearing for various tracks/drivers. Some of us even have well over 15 sets of gears at hand.

 

You are asking a bunch for questions all with different answers. With a common core. That core is touque turns the wheel and the force to the road is wheel torque/tire diameter.

For best acceleration, like a ¼ mile, you want the highest average force. General that will come when you’re geared to be just touching redline in top gear at the ¼ mile distance. If the engine’s hp peak is more than about 1000 rpm blow redline, shifting at redline probably doesn’t work out the best, but simply plugging the torque curve and gear ratios into excel to calculate the force in each gear at every rpm will tell you exactly when to shift. You shift when the next gear gives a higher number than the lower gear and that will always come after the hp peak.

At the track, usually, redline in top gear at the end of the main straight is the fastest. The “right’ gearing is different at every track. On a high speed track where drag plays a big roll or you need to sustain top speed for a while, gearing for the hp peak instead of redline is probably faster….some math is needed to know for sure.

Top speed will come when the engine is at the hp peak…it can not be calculated without drag numbers for the car.

Street gearing is usually chosen for best mileage and smooth operation. Low rpm normally, but it’s all based on measured data. Ferrari is an exception; their cars seem to come geared for best top speed.

 

"anything" must mean nookie...

 

To get it "perfect" you have to decide what you want the car to do. The factory engineers are given certain tragets by marketing, ie 30mpg of 6.2 0-60, wheel/tire size. The engineers then use those targets to set hp, power band and gearing. There simply is no more a "right" gear set-up than there is a right color.

For maximum acceleration, the first thing you need to do is plug the torque curve into excel. Once you have that, you can start comparing the available ratio options looking to put the highest average force to the ground. The only thing to really watch is overloading the tires if you have a lot of hp. That is exactly how the engineers at the factory do it...but they probably have written neat little programs to calc the time to distance with various options. There is no magic, just basic engineering compromise.

Does that help even a little bit?

 

First you must know the parameters you are working in. If, for example, you have 200 BHP available and your drag will not allow speeds in excess of 150 MPH with 200 HP then obviously there is no point in gearing for a top speed of 160 MPH.

I know that my race car will not exceed about 140 MPH with the 200 HP I have so at the longest straight I have ever raced on (Kent) I gear for about 135, and figure to float on the limiter for a few yards.

What I’m trying to say is that you need more information than you are disclosing.

Once you choose a top gear matching available HP with drag, you can choose intermediate gears spacing them so that the drop in RPM between gears keeps the engine in the sweet zone, that area between max torque and max HP.

For track use it is sometimes better to gear for particular turns, it’s all a trade off and you have to pick the most advantageous for a particular track.

Engineers have HP, rolling resistance, drag, and anticipated use values available before picking ratios.

As a general rule the drop between ratios gets less and less as you go up gears so when plotted on a gear chart there is a raising line of shift points. This is due to available torque and rising HP demands at higher speeds.

See the graphs
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The general rule is that the X axis is the independent variable or the input and the y-axis is the dependant variable or the output...it makes no real difference, it’s just convention.

The graphs ferraridriver posted are the type you need to make.

A good example of the compromise is the final drive ratio. On a race car you want the highest mph on the straight, but on a street car the engineer usually tries to get the highest mpg at 65 mph for the window sticker or on a luxury car will go for the lowest engine vibration. This leads to street cars generally having their higher mph in something less than high gear and having sub optimal acceleration because all the gears aren’t actually used….but it gets good mileage.


Another example in 5 speed vs 6 speed. If you have a wide power band, a 5 speed or even 4sp may be faster than a 6, but marketing says the car will have 6 speeds, so it does….and that works out good because 6th is too high and only good for cruising anyway