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Old 11-03-2012, 07:36 AM   #136
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I love the suggestion by jmellipse to run four separate motors under electronic control, making possible completely separate power delivery to each tire. Unfortunately the complexities of making adjustments to the control algorithm might make it too complicated for most drivers, and of course the rules would need to be changed to allow more than one motor.

Electronically controlled differentials, particularly for the front, would give a substantial performance increase. Open diff action on corner entry with progressive lock-up when accelerating out of the corner (just like is done on real cars) gives the best of both worlds. While it is possible to get this effect entirely by mechanical means, using cams and clutch plates operated from the input torque, the design would be difficult to adjust and expensive to manufacture. Using a small servo as an actuator to control the diff lock-up makes things much simpler, and gives more options for control algorithms.

Having said all that, the first thing I would do if I made the racing rules would be to ban all electronic controls of differentials, suspension, etc. These cars are already too expensive!
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Old 11-03-2012, 12:23 PM   #137
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Nothing in the rules bans a centre diff, it's just there isn't a performance advantage to using them. You could try a centre diff, they used to be used quite often in 4wd buggies in the 80s until we started to run more power through them.

With the low weight of r/c cars what happens is all the power ends up being sent to the end of the car with lower grip, so when accelerating the power goes to the front wheels which leads to wheelspin, and under braking it brakes the rear wheels, so locking them up.

R/C cars use exactly the same dynamics as full size cars, it's just that the dynamics don't actually scale. Touring cars in full size would be a 4wd rally chassis, with about 1000hp and the weight of a kart.
The use of a front one-way is the best of both worlds. But as we know, you can't use brakes so they are banned. But if there was a 4 wheel brake system, it can work. Problem with that is, the added weight and complexity might not offer much gains over the traditional spool.

Who knows, with the comeback of the gear diffs, maybe someone is developing a simplified version of an LSD.
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Old 11-04-2012, 03:41 AM   #138
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I made one of these for my JRXSR. Just removed the balls and inserted a couple fabricated pieces of lexan to take up the gap. I hated it, but I am also used to running a front diff.
So does Losi for the JRX. I have a few of them around.

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I love the suggestion by jmellipse to run four separate motors under electronic control, making possible completely separate power delivery to each tire. Unfortunately the complexities of making adjustments to the control algorithm might make it too complicated for most drivers, and of course the rules would need to be changed to allow more than one motor.

Electronically controlled differentials, particularly for the front, would give a substantial performance increase. Open diff action on corner entry with progressive lock-up when accelerating out of the corner (just like is done on real cars) gives the best of both worlds. While it is possible to get this effect entirely by mechanical means, using cams and clutch plates operated from the input torque, the design would be difficult to adjust and expensive to manufacture. Using a small servo as an actuator to control the diff lock-up makes things much simpler, and gives more options for control algorithms.

Having said all that, the first thing I would do if I made the racing rules would be to ban all electronic controls of differentials, suspension, etc. These cars are already too expensive!
Actually that was my suggestion, one page earlier.

And electronics make things cheaper, not dearer. Just look at how many different versions of diffs you need to buy to cover every possibility. Then you have to do maintenance on them, then buy parts to refurbish and eventually spend more money to replace. With electronics, buy the unit once, upgrade the software when you need.

With an electronic control at the touch of a jog dial you could select "spool" or "diff" and then dial in the lock just like you adjust exponentials on your radio. Easy.

But that is achievable more readily with one central unit controlling all four motors rather than four ESCs.

An added advantage is that you could distribute where the power goes, like in real cars where you have a setting for take off that sends max power to the rear because the rear can use it. And so on. As I said, with electronics there are endless possibilities.
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Old 11-04-2012, 04:03 AM   #139
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This would be so great! One central unit which controls the rpm of every tire, giving endless options!
On the other hand it would also be so easy to control stock classes! You could allow nearly every motorstrength just by giving max allowed amps and rpms!

Now we need an engineer and/or manufactorer (and ROAR/EFRA) who is/are willing to take the new step!
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Old 11-04-2012, 04:56 AM   #140
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Actually that was my suggestion, one page earlier.

An added advantage is that you could distribute where the power goes, like in real cars where you have a setting for take off that sends max power to the rear because the rear can use it.
My apologies, I got so excited reading through the thread that I missed that post. You did point out that full-size car designers have pursued the idea of individual motors in each wheel. There certainly are many advantages to doing so.

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This would be so great! One central unit which controls the rpm of every tire, giving endless options!

Now we need an engineer and/or manufactorer (and ROAR/EFRA) who is/are willing to take the new step!
Yes, ROAR would need to change their rules. Section 5.2.3 of the 2010 rule book specifically bans "traction control sensing devices". (It also bans active suspensions, which I didn't realize when I made my previous post!) Using one motor per wheel creates the ability to compare the speed of each motor to the others, which is the basis of most traction-control systems. The wording of the rule bans "traction control sensing devices", whether or not they are actually used for that.

I'm sure there are many engineers among us (including myself) who are both capable of designing the hardware and software for such a system, and would love to pursue the four-motor idea, but there might be concern about making the effort when it is not sanctioned by the ruling body.
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Old 11-04-2012, 06:15 AM   #141
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The main problem right now is such small/wheel motors don't exist yet. Once they come on the market, companies like Tamiya won't shy away from releasing cars that use such a system.
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Old 11-04-2012, 06:50 AM   #142
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The main problem right now is such small/wheel motors don't exist yet.
A quick search through the brushless airplane motors reveals several outrunners of suitable size, but all would still require gear reduction. (It would be nice to have them drive the wheel directly to reduce complexity, but this would of course preclude the ability to adjust the ratio to suit the track.) The Great Planes Rimfire 250 and the SuperTigre 370 are some examples. The 250 might be light enough to mount as unsprung weight.

The big disadvantage is that these motors are all sensorless, which is fine for planes, but not ideal for cars. Using sensored motors with output signals compatible with car motors would make prototyping easier.
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Old 11-04-2012, 04:50 PM   #143
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The problem of using one motor per wheel is that it will add a great deal of unsprung weight to the car. By removing the single motor weight out of the chassis and replacing it with the weight of four small motors at the wheels, it will have a detrimental effect on the handling.
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Old 11-04-2012, 05:53 PM   #144
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The problem of using one motor per wheel is that it will add a great deal of unsprung weight to the car. By removing the single motor weight out of the chassis and replacing it with the weight of four small motors at the wheels, it will have a detrimental effect on the handling.
Not necessarily. The motors don't have to be in the wheel, it could still be mounted inboard. Even if mounted outboard, you eliminate driveshafts, possibly some other bits and pieces to help compensate. You would still see some increase but if done right, wouldn't be bad.
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Old 11-04-2012, 06:53 PM   #145
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It's just a matter of design and development. If a suitable market is identified and volume savings are adequate, it will be done. One possibility is that part of the motor be actually in the wheel (say the motor coils or the magnet). Just needs to get small enough that it can be managed by clever software that will offset the unsprung weight problem. Current motors even if small enough have not been designed for this, but they can be redesigned, I am sure. This can take care of the gearing need as well.
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Old 11-04-2012, 07:19 PM   #146
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Here are some ballpark numbers: The unsprung weight on each front corner of my XRay TC is about 65 grams. That includes a Jaco slick, upright, hub, drive shaft, and half of the upper and lower arms. It doesn't include the shock shaft and piston, or any portion of the spring or sway bar. The Rimfire 250 weighs 20 grams. That's a substantial increase, but not necessarily prohibitive. Any necessary gear reduction would add more.

While i'd like to eliminate the complexity and weight of a drive shaft, it still might make sense at this point to mount the motors inboard. It would probably be easier, at least for prototyping, to modify an existing chassis with the motors inboard rather than outboard. But I think the final goal should be outboard, using purpose-built motors.
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Old 11-04-2012, 08:39 PM   #147
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Hudy's latest column has some good info in it about what went into the design of the T4, and why they made the decisions they did.

Of particular interest to me was the part about the car stopping in the corners. That's what I felt every second I drove the T3'12, and Martin explained it perfectly. What's odd is many other people told me they never felt anything of the sort. So maybe it's an issue that's exposed by certain driving styles? I dunno. That car always felt so weird mid-corner. Perhaps that's fixed on the T4 now.
Yeah interesting column, it's really neat to see all the prototypes, but I do not understand the reasoning for the steering stop. I understand that it's best to limit the steering, but I don't agree that racers are forced to limit it through design. It's as if they are saying we are too dumb, thus we will limit you so you can't screw it up. Really? How does limiting steering through the radio setting not prevent the desired steering? If anyone can care to elaborate I would be interested. As I've mentioned, I would rather have access to the screw from the top, eliminate washers, and get better tie rod angle. I hope they will release new post so we can have the option to run it either way. That would be nice.
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Old 11-04-2012, 09:21 PM   #148
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Really love the idea of four individual motors. Hell, with software, you can even do away with expensive steering servos as you can vary the speed of the turning wheels. Sounds good and all, but how much are the electronics going to cost after a company prototypes everything??? You would need sensored motors and a esc that can read four different sensors and make adjustments on the fly. All that can be done....but you guys are forgetting something with putting motors in wheels......WE SHIT SHIT. How fragile would a setup like this be??? How fragile would it be if it's ran on asphalt??? How about the off road guys with the debris and the impacts they suffer?? To me if a system such as described can be made, it would have to be made of such high grade materials that it would put it out of reach of 99.9 percent of the guys on rc-tech. If it's not made of unobtanium, then you would have to change a motor every time you tap a wall. Forget tweaking your chassis, you're changing electronics.


To me, the biggest leap in on road, hell even off road that can be made is shocks. Heating element in the cap that can vary the thickness of the fluid and can be adjusted individually.
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Old 11-07-2012, 04:05 AM   #149
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Really love the idea of four individual motors. Hell, with software, you can even do away with expensive steering servos as you can vary the speed of the turning wheels. Sounds good and all, but how much are the electronics going to cost after a company prototypes everything??? You would need sensored motors and a esc that can read four different sensors and make adjustments on the fly. All that can be done....but you guys are forgetting something with putting motors in wheels......WE SHIT SHIT. How fragile would a setup like this be??? How fragile would it be if it's ran on asphalt??? How about the off road guys with the debris and the impacts they suffer?? To me if a system such as described can be made, it would have to be made of such high grade materials that it would put it out of reach of 99.9 percent of the guys on rc-tech. If it's not made of unobtanium, then you would have to change a motor every time you tap a wall. Forget tweaking your chassis, you're changing electronics.


To me, the biggest leap in on road, hell even off road that can be made is shocks. Heating element in the cap that can vary the thickness of the fluid and can be adjusted individually.
To have turning (i.e. steering) wheels you need to have a steering servo.

The setup needn't be fragile. Real cars already have this setup and they're working fine. All you need to do is encapsulate a magnet segment in the plastic of the rim and that's your rotor.

Not that cheap right now, but it shows it can be done. Like I said. Create/find the market and it will happen.
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Old 11-07-2012, 06:25 AM   #150
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