ijnek

the nex time i c u,i'll give u a couple of punches

STLNLST

iTrader: (32)

In theory the larger spur gear would have more punch.

Mike D

Is it a theory or a rumor that a larger spur gear gives more punch? I say rumor...

schumacher

its not a rumor its fact:

if you increase the size of your spur gear you are increasing the overall gear ratio, so the torque at the wheels increases.

Since Force = mass x acceleration

the Force at the wheels increases and this produces a faster rate of acceleration.

the other thing to note is if you can obtain the same gear ratio with a smaller spur gear, then this will be punchier because you have less moment of inertia and thus less energy wasted in getting the spur gear rotating in the first place. This is small but nontheless should not be neglected.

sanj

schumacher

i should just add that the above post is based on the pinion staying the same size

sanj

Mike D

schumacher - The rumor is that a larger spur gear with the same final ratio will give more punch. (i.e. 30/120 vs 24/98) I think there is no difference but some will disagree. You also need to take into account the efficiency of gear meshing as the diameter ratio between pinion and spur changes. It is more complicated then simple moment of inertia.

schumacher

you are quite right about the gear meshing but the losses are neglibile compared to the losses through increased inertia.

If anyone is even suggesting that a bigger spur gear with same overall ratio gives more punch I feel they need to get a book on transmission ASAP

sanj

johnbull

Hi guys. Greetings from Malta.

I always try using the smallest spur and pinion - of a given ratio - that will fit in the car. In other words if I am testing and find 100 spur by 50 pinion (2 to 1) gives the correct performance, then I will fit 80 and 40 or smaller, if they will fit in the car.

The reason as I see it is, as Schumacher says, that smaller gears are lighter gears, have less centrifugal force and mass, and require less power to get them spinning.

The theory with heavy flywheels works in drag racing where once you get your engine spinning you want to keep it spinning. That's why we run light flywheels on circuit racing cars and heavy flywheels on dragsters.

But all this has very little to do with drive frequency!

But it's interesting anyhow, so keep it rolling.

Regards

Joe from sunny Malta.

imataquito

ijnek ...... did i say sumting wrong?



hm ..... about the bigger gear smaller gear issue ....
heres imho theory

like schumacher said force = mass x acceleration

so if its spur and pinion is bigger it will ahve more rotating mass making it WANT to go foward or continue moving foward (momentum?)

think of this a heavier flywheel will not slow you down on slopes while a lighter one will.....
and vice versa ... so the gear we have on our car can be said as flywheel (maybe)
this is because the heavier flywheel will have more momentum to keep the torque up ..... therefore the engine is less stressed but acceleration is compromised


think of a toy car which uses inertia or momentum to keep it moving ... it has a unbalanced disk which works as a motor to keep launching the car foward .....
or think of this

take a lorry's wheel and rotate it with hands on a wheel balancer and compare it with a sedans wheel

the lorry's wheel will be harder to rotate but will rotate LONGER !!- heavier flywheel / bigger spur and pinion

and vice versa for sedan wheels

hehehehe

Cole Trickle

Due to design, many cars will have the motor sit lower with a large spur/pinion, compared to smaller pinon/spur with same ratio.

A low CG are nice !

schumacher

well i like to think of it as the stone string analogy.

imagine a rock tied to a piece of string and you swing it round yr head, if you make the string shorter its easier to swing. this ist he same as using a smaller spur gear.

cole, the weight of the motor is neglible compared to the gyroscopic force it generates on the wheels, you are quite correct in that it is advantageous in some cars but you have to weigh up which is more important for you.

sanj

Cole Trickle

The motor itself generates gyroscopic forces itself - the armature are pretty heavy compared to a spur gear.....

John Stranahan

iTrader: (27)

In a Losi XXXS I was able to duplicate gear ratios exactly using either an 86 tooth spur or a 90 tooth spur. I could not tell the difference in the car. I use different size spurs to get the exact final drive ratio that I wan't. A difference of .05 is noticeable.

The effects of rotational inertia of the arm are magnified by the gear ratio, The motor inertia has 7 times greater effect than the inertia of the wheels and axles. 4 times higher than the inertia of the spur. Using too low a gear ratio (smaller pinion) will reduce acceleration of the car because accelerating the motors rotational inertia will become the liminting factor.

imataquito

so..... can anyone come up with the conclution ?

schumacher

and your point is what?

you don't have any control over the mass or speed of the armature (unless you wind it yourself) you do have control over the spur/pinion combination.

if you have a very close match in spurs say 90 and 86, the difference in punch will be neglible, if you go down lets say 10 teeth in 48pitch, the difference will be noticable and you'd probably notice it more ducring braking.

sanj