Drive Train: RR vs MR, Which do you prefer?

[TamiyAddict362 60]

I've just bought a MST TCR-M chassis which can be configuated to both RR and MR layouts (and FF!), just because:

its MacPherson suspension system looks sexy (although I wonder how much damping the "spring-only" shocks will have), and;

its brake disc and caliper design is very good looking.

 

For the price similar to a M05 but which doesn't even have bearings included, I think it is unbeatable. At least it does look good, and I've heard some positive comments re. MST this Taiwan brand.

This car will mostly sit on the shelf and may have some light running on a proper track.

 

I'm going to build a porsche on this first m-chassis of mine in 20 years of RC history. (I've only got experience withtouring cars, 4WD and FF, and CC01, so rear wheel drive is pretty new and strange to me.)

I prefer the chassis to match the real-car drive train as much as possible, so on a porsche body it is rear wheel drive, but the motor position can still be open for discussion.

Many have mentioned that RR/MR is difficult to drive on RC cars, say M04 and M06 which makes Tamiya even put RWD shells on the easy-to-drive M05 instead.

Comparing RR with MR, what do you think?

(Since this is a Tamiya forum, maybe the discussion can be steered a bit to compare M06 and M08.) Yes I know M08 is much superior in terms of design complexity/adjustability or even the plastics used. But shall we just focus on the handling or performance difference between moving the motor forth or back around the rear wheels?

Any sharing is much appreciated.

[Superluminal 3750]

Thats quite nice looking. It sort of looks like its a hybrid of a pan car with a touring car with that suspension and direct drive.

Do with the brake callipers stay in place when the wheels are turned?

[Wheel_Nut 322]

My guess is the mid-motor will be more balanced in cornering, but the rear motor *may* have better on a standing start.  I would build the Mid motor version.

IMO the handling problems of the M04 are caused by the soft front suspension.   On my M04 I put silicone tubing on the inside of the front shocks, so I can preload the front springs and largely limit the suspension travel.  Less front grip, but more predictable handling. It tends to lift the inside front wheel on turns.

[Mad Ax 9329]

I love the look of that chassis, I've love a fleet in every configuration just to look at!

As for handling - there are probably some more experience racers who can come along to provide more insight, but my understanding is that RR is an older design for lower-traction surfaces and MR gives better balance in modern tracks.

Having the motor way out back puts most weight down onto the rear wheels for traction, and provides best grip under braking.  The drawbacks are a flighty front-end and a pendulum effect if you lose rear traction - the car will be hard to regain control of if you lose it.  For a long time, virtually all competitive RWD buggies had an RR configuration because it worked, but as tracks got grippier, racers found an MR configuration got better laptimes, so we saw more MR race buggies.  It's very common for RR race buggies to lift the front wheels on modern tracks, I wouldn't be surprised if an RR M-chassis would also wheelie with the right tyres, surface and motor.

If you're on a high traction surface, the MR configuration brings the weight further forward to keep the front end down.  Steering should be a bit more precise and mid-corner balance better.  If you do lose rear grip, it should be easier to regain it.

But if you're not actually going to race it, I'd say all the above points are moot.  Both configurations will challenge your driving skills unless you have good tyres and a good surface.  If you've got access to a proper circuit then that's fantastic, and if you're not racing competitively, you could go with RR to match the 1:1 layout and drive it as Porsche intended  

[TamiyAddict362 60]

2 hours ago, Superluminal said:

Thats quite nice looking. It sort of looks like its a hybrid of a pan car with a touring car with that suspension and direct drive.

Do with the brake callipers stay in place when the wheels are turned?

Yes, I've read the manual and calipers are separate red plastic parts which are screwed to the arms/hubs. They should look so real with wheels rotating. These set-ups must be something good to toy with, esp for shelf queens.

I've only seen similar designs on expensive alloy hop-ups designed for drift cars.

[TamiyAddict362 60]

2 hours ago, Mad Ax said:

I love the look of that chassis, I've love a fleet in every configuration just to look at!

As for handling - there are probably some more experience racers who can come along to provide more insight, but my understanding is that RR is an older design for lower-traction surfaces and MR gives better balance in modern tracks.

Having the motor way out back puts most weight down onto the rear wheels for traction, and provides best grip under braking.  The drawbacks are a flighty front-end and a pendulum effect if you lose rear traction - the car will be hard to regain control of if you lose it.  For a long time, virtually all competitive RWD buggies had an RR configuration because it worked, but as tracks got grippier, racers found an MR configuration got better laptimes, so we saw more MR race buggies.  It's very common for RR race buggies to lift the front wheels on modern tracks, I wouldn't be surprised if an RR M-chassis would also wheelie with the right tyres, surface and motor.

If you're on a high traction surface, the MR configuration brings the weight further forward to keep the front end down.  Steering should be a bit more precise and mid-corner balance better.  If you do lose rear grip, it should be easier to regain it.

But if you're not actually going to race it, I'd say all the above points are moot.  Both configurations will challenge your driving skills unless you have good tyres and a good surface.  If you've got access to a proper circuit then that's fantastic, and if you're not racing competitively, you could go with RR to match the 1:1 layout and drive it as Porsche intended  

Wow thanks for your detailed explanation.

I never race. 99% of my RC time is on the table building and stickering. I just do casual bashing (to a very careful and limited extent) to enjoy the beauty in motion. There is a small circuit not far away from where I'm but I just like to drive when there is not much traffic. The least thing I want to see is traffic accidents with others and/or loss of control and then cracking the bodyshell.

BTW I have ordered this 210 wheelbase Colt (again, a Taiwan brand) Porsche body.

[TamiyAddict362 60]

2 hours ago, Wheel_Nut said:

My guess is the mid-motor will be more balanced in cornering, but the rear motor *may* have better on a standing start.  I would build the Mid motor version.

IMO the handling problems of the M04 are caused by the soft front suspension.   On my M04 I put silicone tubing on the inside of the front shocks, so I can preload the front springs and largely limit the suspension travel.  Less front grip, but more predictable handling. It tends to lift the inside front wheel on turns.

I've never driven one. So do you mean having smaller suspension travel in the front helps handling?

My usual set-up for a 4WD basher is softer shocks at the back with more spring preload. So basically the rear end sits lower a bit and it tends to be more sticky to the ground. So it should be the opposite for a RWD?

I'm not a racer. My aim, again, is just to keep the car on the road and not to damage the bodyshell.

[Wheel_Nut 322]

4WD cars tend to understeer on power, so its common to use use a rear stabiliser bar or firmer rear springs to make the car more responsive to the steering.

Rear wheel drive cars normally don't have that problem, and will be more likely to snap around if you are too hard on the power or brakes.   The first rule is to do everything you can to maximise the grip on the rear which normally means soft springs and dampers.   Its not the opposite of what you described.  

For the M04, I'm using the stock front springs, but pre-loaded so its hard up against silicone tubing.   The front compliance is mostly in the tyres, except on impacts caused by bumps.     If you can get shorter and harder front springs, you would not need to preload in the same way and have potential for good performance.

[TamiyAddict362 60]

1 hour ago, Wheel_Nut said:

4WD cars tend to understeer on power, so its common to use use a rear stabiliser bar or firmer rear springs to make the car more responsive to the steering.

Rear wheel drive cars normally don't have that problem, and will be more likely to snap around if you are too hard on the power or brakes.   The first rule is to do everything you can to maximise the grip on the rear which normally means soft springs and dampers.   Its not the opposite of what you described.  

For the M04, I'm using the stock front springs, but pre-loaded so its hard up against silicone tubing.   The front compliance is mostly in the tyres, except on impacts caused by bumps.     If you can get shorter and harder front springs, you would not need to preload in the same way and have potential for good performance.

Thanks for your detailed explanation. I have not much idea about these physics as I was just bashing around. Learnt a lot. Appreciated.

[SlideWRX 785]

5 hours ago, Mad Ax said:

If you've got access to a proper circuit then that's fantastic, and if you're not racing competitively, you could go with RR to match the 1:1 layout and drive it as Porsche intended  

 

1 hour ago, Wheel_Nut said:

Rear wheel drive cars normally don't have that problem, and will be more likely to snap around if you are too hard on the power or brakes.  

Yep, the Porsche was known for twitchy handling on/off the throttle.  I agree, go for RR and learn to love the spinout.  

[Dakratfink 967]

6 hours ago, Wheel_Nut said:

4WD cars tend to understeer on power, so its common to use use a rear stabiliser bar or firmer rear springs to make the car more responsive to the steering

Way back in the day I drove 4wd buggy (Yokomo yZ10) what you describe above is exactly how my dad set up the car. He also but a 1 way diff in front so I could kick the rear ended out like a 2wd when I slammed on the brakes. 
I kinda learned to drive based on that set up . Brake late and slide the rear in the turn , then hard on the gas to let the 4wd  natural Tendency to under steer power the front out of the corner. 
would setting up the car and learning to drive smoother have made me faster ? Probably. But kicking roosts around ever my turn sure was fun. 

[Matty36 267]

I quite like the chassis config, I nearly bought one earlier this year. I would say that just driving the car in a fun way, you probably wont notice a big difference in how it handles, but get it on a track with other racers, then it will be down to grip versus understeer. My 4wd touring car would tend to oversteer and be a bit drifty, which would be harder to do with a rwd non drift chassis. It might be nice as fwd.

[markbt73 5316]

I was looking at these earlier. In FWD configuration, they remind me of the old Kyosho 1/12 FWD chassis, from the Honda CRX and Peugeot 205 models. Pretty cool stuff. I'll be curious to hear what you think of it!

[Juggular 4964]

I was surprised to learn that M08 with more mid-ship design tends to spin out more easily than M06.  Maybe M-chassis need more weight on the tail if you are going with RWD. 

This MST chassis may have similar issues.  Tamiya did try to get the rotational reaction to the rear wheel.  Who was it, was it @TurnipJF or somebody who talked about it before M08 came out? (If it was Turnip, it's great, if not, I apologize to whomever I should give credit to.) Basically Tamiya made the pinion turn at the same direction as the rear wheel, so the reactionary force from the motor's turning would make the chassis twist down toward the rear wheels. 

Maybe this force wasn't enough because there are 5 gears turning. I would guess the weight of an armature to be about 70g?  That's about 12 quarter coins in US, 12 twenty-cents for EU, or 12 ten-cent coins in Australia, 13 fifty-cent coins in NZ, or 14 one hundred Yen coins.  It's not nothing.  But maybe the rotational-reaction is too temporary (0.2 second?), so it may matter less than a 180g motor pressing down from the tail end.  Hence what I heard: M08 is easier to spin out than M06.  

MST is direct drive.  So I have a feeling that this force Tamiya prominently displays on the box of M08, might be little more pronounced on the MST chassis.  If that were the case (again, I don't know exactly how much impact this would have), FF and RR would be better than MR. Unfortunately, as interesting as MR is, it might turn out to be the slipperiest configuration.  

[1] FF would put more force on the front wheels, and it has the weight on them too. Making FF the most sure-footed.  [2] RR would put more weight on the rear, but the reaction would go against it.  [3] MR would have less weight on the rear. On top of that, it would try to lift the rear wheels. My guess is that it could make MR the most slippery configuration. (Blue arrows are gear rotations. Reactions are in gray arrows --that's the direction chassis would want to go. All 3 would try to lift the rear tires off the ground at the moment of acceleration).  

All that is only about motor location and grip of the driving wheels. Not about cornering behaviors.  

If I were a youtuber who has money coming in through Patreon or something, I'd get it and test it out.  But I'm not a youtuber. And I already have 2 NIB kits waiting to be built.  Anymore, I'll be trading my happiness with my wife's. She isn't the type to hold things in.  So I'll tread lightly for now, and wait to hear what @TamiyAddict362 says about his MR configuration.  

 

[Fabia130vRS 2098]

From my experience on GT (GranTurismo) a playstation game. The MR cars have more superior handling over RR.

 

but honestly… is the lower chassis out of aluminium? I am not really a fan of the design os the chassis, now that I know there are no regular dampers… To me, this chassis looks like a display chassis for rc bodies. I dont see how it justify the  high price.

[Wheel_Nut 322]

3 hours ago, Juggular said:

My guess is that it could make MR the most slippery configuration. (Blue arrows are gear rotations. Reactions are in gray arrows --that's the direction chassis would want to go. All 3 would try to lift the rear tires off the ground at the moment of acceleration). 

 

Thanks for doing analysis and posting your diagram.  However I want to question your conclusion that all 3 "would try to lift the rear tires off the ground at the moment of acceleration".   If you add up the total rotational inertia of the motor, gears, wheel, tyres all combined, I think the net overall reaction may be the opposite.   Because the wheels and tyres have larger diameter and significant combined weight, they will more than cancel out the reaction force due to the motor.    So I'd expect all three are applying increased force on the rear wheels during acceleration.   It will be closer to a neutral situation where opposing forces are partially balanced.

When it comes to cornering stability, I'm more concerned about what happens when you stop accelerating, or when the braking occurs.   That's when all 3 will try to lift the rear wheels off the ground.    In the case of Tamiya M08, they have made the "weight transfer" more exaggerated by having the motor rotate in the same direction of the tyres.   I thought it was a very unusual design choice to be honest.  Now I'm going 'out on a limb' to suggest that's the reason why people find it inherently unstable, or more unstable than the M06.  Do you know any other successful racing buggy using a rear wheel drive chassis that has the motor rotation in the same direction as the tyres?    The ones I can think of are 80s Tamiya buggies, and some drift cars, so that's not a good reputation for corning stability.   I'm happy if you have more evidence or counter examples that can prove or disprove my suggestions.  

[Matty36 267]

In 2wd buggy racing, they use 3 gear or 4 gear transmission. The 3 gear setup has the motor rotation the same as the wheel rotation, to keep the front wheels on the ground under acceleration, the 4 gear setup reverses that so it will lift the front wheels under acceleration. That is mainly on mid motor cars, the rear motor cars have more grip anyway, but mid motor buggys can have too much grip so the motors were getting closer to the centre of the chassis for balance and even out the grip. I have never driven a rwd touring car to compare it to an off road buggy.

[Juggular 4964]

On 8/4/2021 at 12:34 AM, Wheel_Nut said:

However I want to question your conclusion that all 3 "would try to lift the rear tires off the ground at the moment of acceleration".   If you add up the total rotational inertia of the motor, gears, wheel, tyres all combined, I think the net overall reaction may be the opposite.   Because the wheels and tyres have larger diameter and significant combined weight, they will more than cancel out the reaction force due to the motor.    So I'd expect all three are applying increased force on the rear wheels during acceleration.   It will be closer to a neutral situation where opposing forces are partially balanced.

I respect a thinking man!  In the scheme of things, it doesn't matter whether or not we get it right. I could be wrong at any time.  But as long as we keep thinking, the chance of a thinking man being right would increase over time.  

As for the rotation of the motor, I'm simply going with Tamiya's own diagram. Pinion gear rotates counter-clockwise.  The reaction of that is drawn a s a clockwise arrow.  That turns into the big downward arrow.  

It's only a momentary twist. Just like when you press down on the trigger of a hand drill.  Because of the motor, it jerks to one direction. We are so used to it, we hardly notice this action-reaction.  Only the direction matters (even number of odd number gears would decide the direction).  And of course, it gets neutralized in a fraction of a second.  In old trucks and airplanes, you see the chassis torqueing as the engine turns.  Imagine revving that to the max!  (Is that an Antonov AN-2 engine?)  Engine goes one way, the whole chassis goes the other way.  

M08 likes to fishtail even before braking.  So I think it happens on the starting line.  But you gave me another chance to think about this...

I wondering if this force makes the tail jump?  From 0.1 to 0.2 second, the tail gets pushed down.  From 0.3 to 0.4 second, it rebounds slightly and make the tail spin out. At 0.5 second on, it settles down, maybe?  

M06 has 4 gears.  Which means the tail would go up from 0.1 to 0.2 second. But from 0.3 second on, it would just settle down.  It would look like a slight delay in the acceleration, but less fishtailing.  

I wish I had a high speed camera to see how it happens. 

 

[Wheel_Nut 322]

When it comes to control and traction of the M08, you may have a point about the rotational intertia caused by wheel spin.    This is a dynamic system, so it seems a bit too simplistic to expect that "motor reaction force = powerful drive under acceleration".  Motor acceleration and the vehicle acceleration will only be matched while there is perfect grip from the tyres.   When you have directional stability issue there is also the gyroscopic reaction force, and lateral "weight transfer".    If the car spins to one side, the gyroscopic reaction will apply additional load to the outside loaded tyre, which is a destabilising effect.   As the car will break traction at the rear, its will over-rotate and the gyroscopic reaction force becomes even greater.   That seems less of a problem when using front wheel drive because it causes understeer, which will slow the yaw rate.   I thought it was logical to have the motor rotate in the opposite direction of the tyres so these forces can be cancelled as much as possible.

[Re-Bugged 10159]

I’ll be interested to hear how you go with this @TamiyAddict362, as I was only looking at one of these chassis’s the other day as I saw that there was an option to extend to 245mm W/base with another chassis plate for a Street Scorcher project I had in mind.

My first M chassis was a bog stock M05, turned out to be a little underwhelming. I looked and compared all the M chassis’s after that as the M07 & 8 was around by then until my head was exploding with info. Then I came across this race between an M05/7 & 8, 

turns out in the right hands and well  set up car they are all virtually the same.

So I bought an M08 as I like RWD and I also wanted to see the difference between a basic & higher spec chassis. I was not at all disappointed in the M08. With that I hopped up the M05 and is now turning out the same lap count in the Postal Racing and a nicer experience. Super Grips on the back of the M08 did make a difference and although they are as quick as each other now if I had to choose 1 it would be the M08, it just feels a bit more fun to me.

[SupraChrgd82 1425]

I have nothing useful to contribute regarding "MR vs RR".  Having recently been exposed to the M07, M08, and the direct drive Legends, I much prefer FF to MR.  Perhaps it is the dusty asphalt on which my friends and I drive, RWD cars are too loose in the rear.

However, a McPherson strut RC is a fascinating prospect.  I hope to see lots of photos of the build and driving action. 

[Nicadraus 6201]

FF - would be easier to drive. The chassis will follow where the front wheels take it but requires a proper driving style when handling a FWD RC to reduce understeer.

MR - Think of it like a mid-engined super/sports car. Well balanced with RWD action.

RR - Expect a lot of fish tailing and oversteer, although this will be very good with drifting.

Nonetheless, all three configurations will have their own fun factors.

[Tizer 606]

Back in the days when I was racing my M04 I created a bracket that let me flip the gearbox around to make it RR. The reverse rotation of the motor didn't help the performance of the motor itself giving me less top speed, but the handling was much better. I felt I could tune it like I would with a 2WD (RR) buggy.

As for the motor rotation and the effect it has on traction.. This is a very common thing to play around with in RWD RC drift cars, and there you do really notice the effect! My current competition drift chassis, an Overdose GALM, has their counter-drive kit installed, which basicly just adds an extra gear allowing you to mount the motor from the other side and get this effect of the motor rotation pressing down on the rear wheels when accelerating. It is my favorite of their drive line options. They even make a floating motor mount system with rubber O-rings dampening the movement of the motor mount. Interesting stuff!

Either way I've driven and played around with the normal MST TCR and it's actually really good! I drove it in FF form with a Civic shell on it.

[alvinlwh 5895]

Hi @TamiyAddict362 how did your build go? I am also building one at the moment as a FF but intend to build it into a 2 motor 4WD in the future, either with a second kit or parts.