3 or 4-gear?

[Richard Lowe]

Originally Posted by Paul_Sinclair

No, the position of the motor relative to the rear axle doesn't matter when considering the weight transfer effects of the motor's rotational inertia.

I think I'm going to side with Elvo and Roger (!) in that the position of the motor in the car does change the magnitude of the weight transfer.

Bear with me here as I might not use exactly the correct terms as I don't study/work in engineering...

The force the motor exerts on the chassis on acceleration/deceleration is a twisting force, trying to alter the pitch of the chassis, which pivots at the rear tires. If you think about it the closer the motor is the the diff the more pronounced the effect will be. Imagine if you had a car with the motor a foot further back than normal, but it magically had normal weight distribution and pitch inertia, is it going to want to wheelie more or less than normal?


Back to the 3/4 gear question... I'm not a big fan of 4 gear cars, the drawbacks outweigh the benefits IMO. The good bit is you get a lot more forward drive, but the bad bit is... you get more drive Every 4 gear I've driven has had almost no on-power steering, and had quite bad lift-off oversteer as the weight is thrown forward and backward again. I'd rather have the consistency of a 3 gear car and change the overall balance as needed

[Paul_Sinclair]

There are two "force-relationship-things" going on here, and I don't think I was clear enough in my posts above distinguishing between them. First, there's the relation between the motor's torque and the reaction torque in the chassis. Then, there's how that reaction torque is realized as weight transfer between the chassis and the ground. The first of these has nothing to do with the location of the motor relative to the chassis (as long as they're rigidly connected), and I can prove it to you. The second is affected by weight distribution, suspension set-up, and everything else, and its this second part that most drivers are more familiar with, I think.

The first of these relationships, between the motor and the chassis, is independent of the motor's location in the car as I laid out in my first post. More simply: on any acceleration, the motor exerts a torque, or twisting force on the car. The motor and the car are rigidly connected; since the motor can't twist in the car the car has to exert an equal and opposite torque on the motor. Since these are equal and opposite, the length between them cancels out of the equation - the length term doesn't matter. The torque interactions between the car and the chassis - including whatever effect changing the motor's direction has - are constant no matter where the motor is located.

The second interaction, or torque-relationship, or whatever you want to call it, occurs between the chassis and the ground. If there is no ground, the chassis' torque is realized as rotation - the "gyro effect". On the ground though, we have a similar situation to the above, this time the car exerts torque on the ground and the ground "pushes" back. The difference here is that the two are not rigidly connected - there's suspension, tires, etc. between them. Because they can move relative to each other, moments of inertia (essentially the weight distribution, including where the motor is), spring rates, suspension settings, and all that other stuff become important.

So... the location of the motor does not affect the magnitude of the torque between the motor and the chassis. It does, however, affect how this torque is realized through the suspension and in the way the car drives.

Whew. That's been about an hour or so, but I'm confident about it - I hope it makes sense. Time for bed.

Oh, and Roger, thanks for the kind comments! Very much appreciated, glad you liked it.

[Big E]

Originally Posted by Richard Lowe

Back to the 3/4 gear question... I'm not a big fan of 4 gear cars, the drawbacks outweigh the benefits IMO. The good bit is you get a lot more forward drive, but the bad bit is... you get more drive Every 4 gear I've driven has had almost no on-power steering, and had quite bad lift-off oversteer as the weight is thrown forward and backward again. I'd rather have the consistency of a 3 gear car and change the overall balance as needed

Not on mine it doesn't!!! It's so safe it's unreal. You need to try a good one Rich and then you will really know, I know you have tried a few but it is all down to the set up and what we came up with this winter is very good.

E.

[Northy]

On my 'home brew' mid motor B4 thingy I tried 3 gear (without having to reverse motor direction) and 4 gear. Of the two I prefered the 3 gear for the reasons Rich has stated, and we have chatted about it in the past.

The last time I drove an X-6 it was a 3 gear, now I have returned I am driving a 4 gear. I have to say it feels very much as I remember, I'm not sure why the X-6 is less affected by the change (well I have a few ideas), but if I'm honest I don't care. The car is good and I'm just going to get on with driving it now........

........ or trying to fit a 5 link rear end!

G

[Richard Lowe]

Sorry Paul, I didn't realise you wer talking about two different things there, I was thinking of them both as one if that makes sense

The X6 is about the only mid-motor 2wd I havn't had a chance to try properly, anyone with a spare car they don't need for a bit?

[Chris Doughty]

I have only skim read through the recent posts,

but I would have thought that the motor kick effect would be more pronounced if the motor was mounted at (or as close to) the cars CoG and it it was a way forward or backwards from that point

[Lee]

Agreed Chris, i was on the understanding that if the motor is centrally mounted then it will make the chassis pitch more when on or off power. if it is 80% to the rear it will transfer more weight to the rear end on power but less to the front off power, possibly 30% less?

Maybe my way of thinking is wrong??

[Chris Doughty]

my theory is that its easier to apply the torque force at the 'pivot' (which the CoG will be most like the pivot when in the air) than trying to twist something that is off centre to the pivot to turn the pivot

note - not 'lever' something, 'twist' the handle the apply the force

hope that makes sense -

[Jonny_H]

Erm... basic free-body-diagram theory here. A moment is a moment is a moment (or torque, or couple, as you prefer). It has exactly the same effect no matter where it is applied - the reaction moment arm, for this discussion, is the distance between the front tyres (where the reaction force is reduced) and the rear tyres (where it is increased by an equal amount) - i.e., the wheelbase - which doesn't change.

The only influence that the position of the motor has is on weight distribution and moments of inertia.

If you could build a car with two motor mounts, one in front and one behind, and you could fit a motor in both positions but only fit a pinion to one of them, then it wouldn't matter which one was spinning.