does diameter size for spur/pinion make a difference?

[Needler56]

i've been testing and tuning with three spur gears: 96, 100 and 104 (64 pitches) using the same pinion gears for each. i know that it all boils down to the ratios, but i've noticed more favorable speeds with the 96 and the 104 spurs. the only thing i can think of is because they're different sizes in diameter. or am i imagining things?

[BTE214]

The reason they are different sizes in diameter is that you need a larger diameter to fit more teeth on it.

[Needler56]

what about a a 78t 48p as opposed to a 96t 64p? the 78 is bigger, but with less teeth, does that make a difference? i think that's what i'm trying to ask.

[evst 360]

it boiles down to gear ratios as you said. If you keep the same gear ratio, regardless of the size on the pinion and spur, the performance will be the same. :cool:

[awnelson]

For the technical answer; gears are made to a specific diameter based on their diametrical pitch. That measurement is a circle measured between the peak and the valley of each tooth. The higher the d/p, ie. 32, 48, 64, the smaller the tooth itself. the teeth on a 96t 64p gear are the same size as those on 92t 64p. These have to remain constant so that the teeth will mesh with the teeth of a 64p pinion. If you want to find out what size is equivalent in a different pitch, you will need to find out what the pitch diameter is for that number of teeth and find a matching one in another gear pitch. You are better off staying with higher pitch in on-road cars because they are quieter but use lower pitch gears for off-road because they are stronger (bigger teeth).

Hopefully this confuses you and you ask more questions. JK because I am sure everyone else fell asleep during that.

[BTE214]

So what you are asking is if you can change pitch? Stay with the same pitch and change the number of teeth.

[awnelson]

Teeth that are engaged with each other reduce the rotation speed by what is called a ratio. If you have a 96t spur and a 32t pinion, it is a 3:1 ratio, meaning that the pinion will rotate 3 times for every one that a spur does. To witness this, take liquid paper and mark a line across the place where the two teeth engage each other. As you rotate the gears by hand, the smaller gear will move approximately 3 times and the lines will match up again. If you have a smaller pinion (less teeth), it will rotate faster than 3 times for each 1 time the spur turns. To accelerate faster, put a pinion with less teeth or a spur with more teeth. This will increase the ratio to more than 3:1. Depending on where your horsepower curve actually is for that specific motor, you may experience better results by changing the spur or the pinion. The pitch diameter is a relationship between the center of the gear and the area of engagement on the teeth. There is only so much room that a motor can move in relation to the input shaft of your transmission. There are other variables that I won't get into (torque arms and such) but your best bet is to understand the ratios better and ask someone who has your same motor and vehicle what they use for gears and go from there. Tire size makes some difference as well as the type of bearings in the driveline but your best bet is to find out what works well for others.