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Advantages of mid-motor vs. rear-motor in a RWD buggy?

Advantages of mid-motor vs. rear-motor in a RWD buggy?

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Old 01-12-2016, 09:56 PM
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Default Advantages of mid-motor vs. rear-motor in a RWD buggy?

I have an Arrma Raider buggy that I use when I want to go fast and corner hard on pavement, but I've started entertaining the idea of getting another RWD buggy for off-road use. (why RWD? I dunno, for the challenge I guess. Also for powerslides.) I was looking at the Durango DEX210v3, and I noticed it has a reversible transmission, so it can be run in rear-motor or mid-motor configuration. I'm quite familiar with the behavior of rear-motor vehicles, good acceleration and good braking, but a tendency to wheelie, but I've only driven one mid-motor RWD vehicle and its performance was pretty crappy. It accelerated okay, but when braking the gravitational-force vector of the transmission tilted away from the rear wheels' contact patch instead of toward it, so it just tended to skid and spin around a lot.

So...in what circumstances would I want to consider running a buggy in a mid-motor configuration, especially if it's going to be run off-road a lot?
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Old 01-13-2016, 07:31 AM
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are you looking to race with this buggy or just cruise around 'off road'?
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Old 01-13-2016, 10:00 PM
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Originally Posted by fyrstormer View Post
the gravitational-force vector of the transmission tilted away from the rear wheels' contact patch instead of toward it, so it just tended to skid and spin around a lot.
Is this a thing!?

More to do with setup IMO. Sounds like excessive droop on the rear, which would have allowed the rear of the car transfer too much weight to the front and you lose traction that way. Same can happen with RM cars

RM vs MM for me is more behaviour in rotation. In a straight line, RM and MM cars just need to have shocks setup properly
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Old 01-14-2016, 12:54 AM
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Originally Posted by WagwanBumba View Post
Is this a thing!?
Yes, it's a thing called physics. Imagine little invisible arrows pointing straight down from every atom of the vehicle; they represent the force of gravity on the vehicle. When it accelerates, they all tilt backwards, when it brakes, they all tilt forwards, and when it turns, they all tilt to the outside of the turn. More mass in a given location on the vehicle equals more little arrows pointing down and tilting every-which-way as the vehicle moves, and where those arrows intersect the ground determines whether the vehicle will hold its line, oversteer, understeer, wheelie, or traction-roll.

Originally Posted by WagwanBumba View Post
More to do with setup IMO. Sounds like excessive droop on the rear, which would have allowed the rear of the car transfer too much weight to the front and you lose traction that way. Same can happen with RM cars

RM vs MM for me is more behaviour in rotation. In a straight line, RM and MM cars just need to have shocks setup properly
Weight-transfer is the common expression for the physics I described before. We're talking about the same thing.

Yes, that one mid-motor RWD vehicle I drove was poorly tuned, and I eventually sold it because I couldn't get it stable. So, how does rotation differ between mid-motor vs. rear-motor?
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Old 01-14-2016, 12:56 AM
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Originally Posted by motoman811 View Post
are you looking to race with this buggy or just cruise around 'off road'?
Just off-road bashing, but I'd still like to know in which circumstances mid-motor is better vs. rear-motor.
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Old 01-14-2016, 01:16 AM
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A 4WD will drift circles around a RWD if that's what you are looking for.
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Old 01-14-2016, 02:29 AM
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Mid vs rear will shift the weight balance of the car. Mid is generally better suited for medium high grip. Rear for low grip conditions, there's more weight on the back and less on the front.
Setups can only do so much. Setup is all about weight transfer control but there's a limit to it. You just can't compensate a 200gr+ weight transfer on the back with it.
Tracks with extremely low grip are getting rarer nowadays so most people go with either mid-rear or mid center motor position.
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Old 01-14-2016, 04:29 AM
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I'm guessing you tried a 4 gear transmission mid motor car? The motor reactions with a 4 gear add to the weight transfer. Suggest trying a 3 gear mid, they have less weight transfer.

The main idea of mid motor is centralizing the mass, reducing the moment of inertia. Less effort to turn the car and less to straighten it back up, responds quicker.

Last edited by Dave H; 01-21-2016 at 12:04 PM. Reason: Polar doesn't apply, wrong moment, my bad
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Old 01-14-2016, 06:25 AM
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If you look into the b5m thread there is a ton of discussion on this exact subject.

The 4 gear is a better choice for low grip because of exactly what your describing. IT transfers weight to the rear.

The 3 gear is better for high grip , it transfers weight forward.

The team Durango website also has some great animation showing this in action in the 2wd buggy description.

http://www.team-durango.com/blog/201...onfigurations/
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Old 01-14-2016, 10:26 AM
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Originally Posted by fyrstormer View Post
Yes, it's a thing called physics. Imagine little invisible arrows pointing straight down from every atom of the vehicle; they represent the force of gravity on the vehicle. When it accelerates, they all tilt backwards, when it brakes, they all tilt forwards, and when it turns, they all tilt to the outside of the turn. More mass in a given location on the vehicle equals more little arrows pointing down and tilting every-which-way as the vehicle moves, and where those arrows intersect the ground determines whether the vehicle will hold its line, oversteer, understeer, wheelie, or traction-roll.

Weight-transfer is the common expression for the physics I described before. We're talking about the same thing.

Yes, that one mid-motor RWD vehicle I drove was poorly tuned, and I eventually sold it because I couldn't get it stable. So, how does rotation differ between mid-motor vs. rear-motor?
No, that's the resulting force vector of friction (aka grip) and acceleration. The acceleration vector is what changes direction depending on the direction of the force, gravity is always applied towards the center of the Earth, except in Australia...

A rwd buggy to drift and bash? Then it doesn't matter mid or rear motor but if you are looking into the Dex you will have enough possibilities to try. My experience with the TLR 22 is good overall, liked the traction offered in the rear motor configuration and the mid motor was not far but required one or two changes from the setup hey at least it drifted more than the rear motor configuration.
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Old 01-15-2016, 12:38 AM
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Originally Posted by UK.hardcore View Post
A 4WD will drift circles around a RWD if that's what you are looking for.
Not looking for a drift-specific vehicle, just an occasional powerslide.
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Old 01-15-2016, 12:43 AM
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Originally Posted by Dave H View Post
I'm guessing you tried a 4 gear transmission mid motor car? The motor reactions with a 4 gear add to the weight transfer. Suggest trying a 3 gear mid, they have less weight transfer.
Actually the MR RWD vehicle I used to have had a 3-gear transmission, it was just a RR transmission flipped around and installed backwards. What you're saying makes sense, though; the motor builds up a hell of a lot of rotational momentum as it accelerates and decelerates, and that leverages against the chassis which as you say affects weight transfer.

It was just a crappy truck, that's all.

Originally Posted by Dave H View Post
The main idea of mid motor is centralizing the mass, reducing the polar moment of inertia. Less effort to turn the car and less to straighten it back up, responds quicker.
That also makes sense. I guess the tradeoff is sharper handling in exchange for reduced traction, but if low-grip tracks are rare nowadays then the reduced traction doesn't matter much.
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Old 01-15-2016, 12:47 AM
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Originally Posted by 30Tooth View Post
No, that's the resulting force vector of friction (aka grip) and acceleration. The acceleration vector is what changes direction depending on the direction of the force, gravity is always applied towards the center of the Earth, except in Australia...
You are correct. In my explanation I was combining the force of gravity and the force of acceleration into a single resultant vector because it's easier for me to comprehend it that way. It would've made more sense if you could've seen the imaginary visual-aid inside my head.

Originally Posted by Dave H View Post
A rwd buggy to drift and bash? Then it doesn't matter mid or rear motor but if you are looking into the Dex you will have enough possibilities to try. My experience with the TLR 22 is good overall, liked the traction offered in the rear motor configuration and the mid motor was not far but required one or two changes from the setup hey at least it drifted more than the rear motor configuration.
Well, it might not "matter" in the sense that I'm not going to lose a race because of it, but it will still affect the vehicle's handling, so I'm still curious what the effects would be. It makes sense intuitively that mid-motor would drift better, but to be honest I'm not sure I can explain why.
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Old 01-15-2016, 06:43 AM
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Originally Posted by fyrstormer View Post
You are correct. In my explanation I was combining the force of gravity and the force of acceleration into a single resultant vector because it's easier for me to comprehend it that way. It would've made more sense if you could've seen the imaginary visual-aid inside my head.

Well, it might not "matter" in the sense that I'm not going to lose a race because of it, but it will still affect the vehicle's handling, so I'm still curious what the effects would be. It makes sense intuitively that mid-motor would drift better, but to be honest I'm not sure I can explain why.
I sensed what you were thinking, just wanted to make sure you understand that gravity is "always" constant.

I have a 4 gear MM buggy that has more traction than most RM buggy I've drove. It's comparable to my ex TLR 22 RM because weight bias between the two are similar if not equal. The defining factor is polar moment of inertia, this is the biggest difference between RM and MM and easy to see for yourself if you acquire a convertible buggy.
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Old 01-15-2016, 07:54 AM
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Originally Posted by fyrstormer View Post
It makes sense intuitively that mid-motor would drift better, but to be honest I'm not sure I can explain why.
Consider the motor's weight and position. When rear of the rear diff, it's a large pendulum and in a better position to overwhelm the rear tire's side-grip (this is also why adding weight to the front doesn't increase front grip, it decreases it). With the motor ahead of the rear diff, all of the car's mass is now between the front and rear wheels and even if you were to achieve the same front/rear bias, that weight would still be concentrated more in the middle, decreasing the car's polar moment (I may be misusing that term..).

It does change direction more easily, but it also slides more predictably and less violently, partly due to weight location, partly due to a front and rear suspension that are closer to having balance. Tougher balance to achieve, but theoretically better when done, yeah?
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