Motor direction in a mid motor 2WD
 

Ive started to build a mid motor car based on a B4.

Rather than build a 4 gear transmission or use an idler gear ive moved the layshaft onto the other side of the gearbox so the motor rotates in the same direction as stock.

This seems to be the best direction as this will counteract the torque from the wheels helping the keep the front down under acceleration.

However the atomic carbon conversions all have the motor rotate in the same direction as the wheels. With the motor infront of the rear wheels this will help to push the rear down, as well as lifting the front.
This might also help control the car in the air.

What do you guys think?

I think that the direction the motor spins is not nearly as important as most people like to think.

The inertia of the tyres is far more significant than the inertia of the motor.

If this was not true then off road cars with the motor mounted longtitudinally. - (e.g. most shaft drive 4wds, b44 predator etc.) would all be total crap, and tip over to one side every jump.


My zx5 / losi hybrid 2wd should in theory be worse than any 4wd in that respect because only 1 set of driven wheels = less inertia from the tyres.

But I can't see any noticeable effects from torque reaction at all, and it jumps better than any car I've had previously.

The direction the motor rotates is a very small change in terms of car performance and really not worth spending a lot of time worrying about.

I'm afraid I'd have to totally disagree with above, I think the traction and stability that comes with a 4 gear tranny completely outweighs any negative affects that a 4 gear tranny will have.

Having built and run an in-line motored 2wd car and realised how nervous and even unpredictable they can be on low grip tracks I'd rather go with an easy to drive 4 gear any day.

and any car with a 3 gear tranny just isn't as adjustable when it's in the air and won't have the traction you need ... I think that's why the CR2 has an idler gear TBH ...

4 gear all the way :thumbsup:

and if yours is B4 based then just bolt in an X-6 tranny, the cases are only a tenner !!!

Out of interest I've just done a few rough calcs based on the following assumptions (I haven't got a scales to hand so the weights may not be accurate.)

2wd

Tyres weigh 50g each with 45mm effective radius

rotor weighs 100g with a 7mm radius and a gearing of 10:1

acceleration from 0 to a motor speed of 50000rpm in 3 seconds.


Without reproducing the calcs in full, the answers I get are as follows:


The torque due to inertia of the motor is approx 10% of the torque due to the inertia of 2 wheels, which is actually more significant than I was expecting.

For the acceleration above the torque generated by the inertia of the wheels is 35.3 Nmm.



So for a typical wheelbase of 275mm the instantaneous effect of the torque due to the tyres on a 2wd is the same as taking 13g from the front axle and moving it to the back axle of the car.



So take 10% of 13g and that will be the change in load due to the inertia of the motor. i.e. 1.3g



So changing the motor direction by adding a gear to the transmission results in a maximum difference in load of 2.6g for a 1500g car or 0.2% of the cars weight.

Which is a difference that I personally am not good enough to notice.

Thanks for doing those quick calculations :)

I think I will stick with my current design for the moment.

I may be able to modify the gearbox design to include an optional idler gear for testing.

I'm not a tech person but if you hold a motor in your hand and rev it, the torque certainly don't feel like 2.6 grammes :eh?:

Thats because the calculation is for 3 seconds.

If you rev a motor then its more like half a second (six times the force?)

But the proportion of inertia between the wheels and the rotor will be the same at any acceleration (excluding slipper clutch)

double post

Indeed, when I tried to calculate it, I got surprisingly very low numbers as well.


Acceleration in 3 sec:
Wheels:

I = 1/2 * 0.1 kg * (0.045 m)2
= 1.0125 *10exp-4 kgm2

a = 174,44 rad/sec2

T = I * a
= 0.0176 Nm

"Weight" from inertia wheels at 275mm:
0.0176 Nm / 0.275 m = 0.064 N or 6.5 g


Motor:

I = 1/2 * 0.1 kg * (0.007 m)2
= 2.45 *10exp-6 kgm2

a = 1744,43 rad/sec2

T = I * a
= 0.00427 Nm


"Weight" from inertia motor at 275mm:
0.00427 Nm / 0.275 m = 0.0155 N or 1.5g

But an acceleration in 3 seconds is a little bit slow in my opinion. Especially in the air, I think it's at rev in 0.5 sec at the most.

Also there must be an extra force playing on this matter besides inertia: think of a 4WD car that can make a backflip by holding the throttle open. At a certain point the wheels are at speed, but it seems they keep on helping flip the car. This cannot be pure inertia, because inertia only works when speed changes. It must be friction with the air of the wheels. So I think mechanical losses in the motor can have an effect too.

The calculation is based on 3 seconds as Matt says.

It is also based on a lever arm of 275mm (the wheelbase of the car).

Torque is force x distance so by holding the motor can the lever arm becomes the can diameter so you have a force sbout 5 times higher. so 14.3g for acceleration to 50000rpm in 3 seconds


3 seconds is a figure I pulled out of the air based on a car accelerating down a straight, i.e. motor under load.


The torque generated by brushless motor is ball park 200nmm. this equates to a force of 432g in your hand.

This torque corresponds to 0.03 seconds to reach maximum rpm when no external load is applied.

But add the tyres in and that time is instantly increased by a factor of 10 due to the extra inertia. so it takes 0.3 seconds for the car to max out when in the air. - You're still only looking at a 1% of the cars weight at the wheels.



The main point is that how fast the motor spins the wheels is much more important than the direction of rotation of the motor.

My figures are slightly different because I assumed that all the mass of the wheel and tyre is at a radius of 45mm, wheras you have modelled the tyres as a 45mm cylinder of uniform mass, which halves the inertia. - The real answer will be somewhere between the two.

The thing is I really noticed the difference when I went from 3 gear to 4 gear on low grip tracks. On high grip astro the 3 gear was at least as good as the 4 gear.
Maybe because the suspension is really soft in the beginning of it's movement and a car is "floating above ground" when you're driving, that the little weight transfer the motor causes helps the car into squat during accel.
Maybe the fact that the motor is in the "suspended" part of the car makes the little effect it has more important for weight transfer.

I have both 3 and 4 gear tranny for my X6. In most cases i notice no difference. On high grip, 3 gear has a small advantage, as it push the front down, and i can throttle earlier out of corners.

And what effect will this do then?:bored:
http://www.oople.com/forums/showthread.php?p=414441

I think that this flywheel on a 4-gear car will give more behaviour as a 3-gear, if you disregard the "flywheel" effect and only look at the acceleration of inertias.

Just try and hold on to one of your motors as you whack the throttle open.

3-Gear vs. 4-gear makes a world of difference on the track.

Elvo's the man on this, but here's a couple of things you've not taken into account:

Any car with a longitudinal mounted motor will have a torque reaction. Because Off-Road has so little grip compared to track, you don't notice it. However, the longitudinal mounted motor has disappeared from the Track scene precisely because the torque reaction on acceleration makes the car react badly out of corners. Just because you can't see it in your class, doesn't mean it isn't there!!

Your calculation of the torque reaction only works when the wheels are off the ground - hardly the best situation for them to be in! The torque reaction when they are on the ground is far more important, and for that the direction of the motor is critical. In theory, the mid-motored 2WD cars shouldn't work as well as the rear-mounted - but take into account motor direction and...

Vehicle dynamics is a complex subject where any one asset is traded off with another. It is never as simple as one thing having one effect. If it was, all chassis' would be the same and all cars would handle the same. They don't!! HTH :)

I ran my Atomic CR2 exactly like this and I think that it's much smoother under acceleration and better in the air than having the idler gear mod. I ran the car both ways and went back to the way you are running yours :thumbsup: