Up to 100% of Torque to the wheels? BS

"HammerheadFistpunch" (hammerheadfistpunch) 06/12/2015 at 15:35 • Filed to: Tech, Rants

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A huge Pet Peeve of mine is when marketers/salesman/ads use the term “transfers up to 100% of the torque (power, etc) to a single wheel!” when referring to brakes based traction control systems.

I realize its just me being pedantic, but no...you are NOT transferring 100% of torque to a single single wheel or axle unless you lock the diff. I know it has the “effect” of a locking differential, but it doesn’t actually work with respect to torque in the same way for one simple reason: Open differentials !!!error: Indecipherable SUB-paragraph formatting!!! , !!!error: Indecipherable SUB-paragraph formatting!!! , split torque 50:50. (minus a small amount of internal friction)

Any differential can only apply as much torque as the to the ground as the traction will accept. In the case of the open diff if you have one wheel that can accept 500 ft-lbs of force and one wheel in the air receiving no torque then no torque is applied to the ground and the car stays put. An open diff will only be able to use (at max) the lowest torque value that doesn’t spin the wheels. If you spin a wheel at 20 ft-lbs on ice for example, you can only apply 20+20 (for each side) and so you don’t move.

What helped me get this concept was the idea that torque is an applied force and so if there is no application there is no torque. i.e. it doesn’t matter that your engine figures say it produces 500 ft-lbs of torque at whatever rpm, if it can’t apply it to the ground via the wheels its not producing torque (which is why engine dynos have to be under load). So if you are thinking to yourself (like I was) “well where does all my engine torque go?” Well it never gets applied. It doesn’t matter how strong you are, you can’t torque a spinning bolt, for example.

If you brake that spinning wheel you are applying artificial applied torque up to the limit of the 50/50 split and the traction limit on the grounded wheel. i.e. if the good wheel can accept 500 ft-lbs before slipping and you can apply 500 ft-lbs of clamping force to the braking wheel for a total input of 1000 ft-lbs (half in the brakes, half on the ground) and after that the wheel looses traction. In this case it gets you moving like a locked diff but still only sends 50% of the available torque to any given wheel. If you had 2000 ft-lbs of engine force to apply you could still only apply 500 ft-lbs to the good wheel before it slipped and then you are back to being only apply to 1000 total ft-lbs (500 until the wheel slips and 500 to the other side). There is never a scenario with an open diff that allows you to send 100% of supplied torque to one wheel.

Man, I didn’t mean for this to be an engineering rant, more a rant on the phrase “up to 100% of torque”...but here we are.

Now, a locked differential CAN vector torque up to 100% and there are LSD’s that lock up but if its an open diff don’t let a marketer or salesman feed you this “up to 100% of torque” BS. Its not their fault they don’t get physics but you...you don’t fall for it.

HammerheadFistpunch > HammerheadFistpunch 06/12/2015 at 15:46

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I will concede (begrudgingly) that in a technical way, that you could argue that “power” is an acceptable term since the wheel has to be moving for work to be done (and thus power to take place - power = rate of work) and so all the work is being done by 100% of the wheel with motion (not being braked) but at the very least its a misleading term.

KusabiSensei - Captain of the Toronto Maple Leafs > HammerheadFistpunch 06/12/2015 at 15:48

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You expect marketing to know the first thing about how a car works?

HammerheadFistpunch > KusabiSensei - Captain of the Toronto Maple Leafs 06/12/2015 at 15:50

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As a marketer I agree.

crowmolly > HammerheadFistpunch 06/12/2015 at 15:51

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You expect the truth from the Mr. Mojo Risin?

Joking aside, it sure seems misleading. If you are clamping a brake down to prevent wheelspin then you are doing so to fight torque.

HammerheadFistpunch > crowmolly 06/12/2015 at 15:52

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That makes the assumption that wheelspin is a result of excess torque, which isn’t the case. The closest analogy I can make would be that a spinning wheel is like a lightbulb running on 10000 Volts but only 100 milliamps, its still 100 watts of light but getting more light would be easier cranking down the volts and bringing up the amps. Braking the spinning wheel, in this analogy, raises the amps.

FastIndy > HammerheadFistpunch 06/12/2015 at 15:55

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Came here to post that. Thunder stolen. Leaving disappointed.

=(

HammerheadFistpunch > FastIndy 06/12/2015 at 15:57

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bwahaha!

duurtlang > HammerheadFistpunch 06/12/2015 at 15:58

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Unrelated, but I have to spam you with this picture I took today of my new purchase. A convertible from the 1980s that weighs less than a tonne.

HammerheadFistpunch > duurtlang 06/12/2015 at 16:00

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I expect the next picture of it to be pulling a 3 tonne trailer.

Textured Soy Protein > HammerheadFistpunch 06/12/2015 at 16:14

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Differentials and torque splits and such can get complicated quickly.

I think the easiest way to describe how an open diff/traction control combo works in off-road situations is:

“Keeps going as long as you have 1 wheel on the ground, in theory.”

Daily Drives a Dragon - One Last Lap > HammerheadFistpunch 06/12/2015 at 16:17

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Slightly off-topic but I like the way these look.

duurtlang > HammerheadFistpunch 06/12/2015 at 16:19

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I’d expect something like this:

HammerheadFistpunch > Daily Drives a Dragon - One Last Lap 06/12/2015 at 16:19

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Me too. If it wasn’t for their questionable durability/reliability concerns it would be a very great little light duty off roader to consider. I’ve been pretty impressed with how they off road so far.

HammerheadFistpunch > crowmolly 06/12/2015 at 16:22

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I think i just re-read your comment and it makes more sense to me now. What you are saying is that on the face of it, when someone tells you that 100% of the torque is going to a single wheel that it totally flies in the face of the need to brake a wheel. Im on the trolley now.

Daily Drives a Dragon - One Last Lap > HammerheadFistpunch 06/12/2015 at 16:23

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I’ve seen a few around and I like them a lot more than the Compass.

HammerheadFistpunch > Daily Drives a Dragon - One Last Lap 06/12/2015 at 16:25

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stepping barefoot on LEGOS is better than a compass. What suprised me about them (the renegade) IRL was how big they looked on the outside. Cars are getting so much bigger on the outside without getting bigger on the inside.

Daily Drives a Dragon - One Last Lap > HammerheadFistpunch 06/12/2015 at 16:27

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Am I seriously the only person who doesn’t think stepping on Legos hurts?

HammerheadFistpunch > Daily Drives a Dragon - One Last Lap 06/12/2015 at 16:30

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There are probably as many of you as there are that loved the look of the compass enough to buy one.

Daily Drives a Dragon - One Last Lap > HammerheadFistpunch 06/12/2015 at 16:33

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True.

Textured Soy Protein > HammerheadFistpunch 06/12/2015 at 16:43

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The Compass is one of those cars that when I see one on the road I wonder what was wrong with that person that led them to say,

“Why yes, good sir, I will take the Jeep Compass. Here are several thousand of my hard-earned dollars.”

See also:

HammerheadFistpunch > Textured Soy Protein 06/12/2015 at 16:46

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I understand 2nd hand and used rental buys (cause every car has its merits, even if that merit is cheapness to buy) but the ones that went it to a JEEP dealership and said “This is exactly what I’ve looking for, a FWD Jeep that looks like crap inside and out and is universally panned by anyone who’s driven it.” and then follow through AFTER a test drive... I got nothing.

and 100 more > HammerheadFistpunch 06/12/2015 at 16:55

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Right, because, discounting friction loss within the drivetrain and assuming that the 4/AWD transfer case (read: PTU in this instance) is locked; if you have 100% of torque coming out of the transmission, a maximum of 25% of that can go to any wheel, and is then either used to propel the vehicle or not, depending on traction conditions at each wheel. But all said, each wheel ONLY ever receives 25% of the torque output.

What they’re REALLY saying is that you can still move the vehicle even with traction at only one wheel. But they way they spin it sounds more impressive, because of all the numbers they get to throw around.

Am I understanding you correctly?

thereisnospork > HammerheadFistpunch 06/12/2015 at 17:02

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On that point, taking a hypothetical car producing 100hp and 100ft/lbs of torque in steady state operation with one wheel locked up and the other strapped to a dyno. Shouldn’t that dyno read 100hp (ignoring drivetrain losses) as the wheel is the only moving part doing work other than the differential spinning around itself? Where my head is doing a loop though is for the dyno to read 100hp, just as it would under normal operation with two wheels, it must be ‘feeling’ a total of 100ft/lbs of force on the drum(s) even though that is now only being delivered through one wheel.

HammerheadFistpunch > and 100 more 06/12/2015 at 17:07

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Partly right, in a vehicle that can lock both axles and the center (my land cruiser for example) I can technically have (aside from friction losses) 100% of my torque being applied to one wheel. Lets say 200 ft-lbs @1000 rpm (its more than than actually but i like simple math) x 4 (simple first gear) x 2 (low range multiplier) x 4 (axle gear ratio) means that at 1000 rpm in low range I have 6400 ft-lbs to apply. With all my axles locked if I had 3 wheels in the air and the 4th on a surface that could accept all that torque (and not spin the tire) than all of it could then be applied to that wheel. With open axle diffs but locked center (assuming same numbers) and traction control I would only be able to apply 3200 ft-lbs to that wheel (100% torque to the axle with grip because its locked and 50/50 side).

What they are trying to say is in essence what you say in the second paragraph, although it may not technically be true. with my land cruiser I have huge torque and big torque multiplication on my side, but if you scale down available engine torque and then start dividing it up you could see how you could reach a point where you have traction to support all the available torque, but it might not be sufficient to move you forward. You can see it starting to be an issue here on Patriot FDII

HammerheadFistpunch > thereisnospork 06/12/2015 at 17:09

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hp = (torque x rpm)/5252

half the torque, half the hp.

and 100 more > HammerheadFistpunch 06/12/2015 at 17:46

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Gotcha.

(DISCLAIMER: I own a Patriot, and frequent a Patriot-oriented forum)

People are often impressed by how capable the FDII Patriots are (because, frankly, nobody expects much from them), but Brake Lock Differentials and traction control are simply NOT up to par with a well-sorted mechanical, locking 4WD system, as many owners have realized. You can program to traction computer to do some pretty incredible stuff, but you’ll never match the capability of a true 4x4. In my opinion, this is due mainly to the fact that a brake lock differential (eLSD, as some call it) subtracts available power by its very nature, as its mode of operation.

One major issue some of my more adventurous counterparts have discovered is that the Patriot really, really dislikes climbing sandy hills. The intermittent lack of traction at each wheel that can occur causes the AWD system to brake at each corner, which essentially robs the wheels of ALL power, and the car literally revs without moving. The Patriot/Compass exhibits this, and its descendents - the new Cherokee and the Renegtade - do as well.

Topped off by a distinct lack of torque multiplication in the final drive (14:1 in FDI models, and 19:1 in FDII), the Patriot/Compass is great as a trail runner and a beast in the snow, but decidedly NOT a 4x4. The final drive ratios in the upper-level models of the Cherokee and Renegade are better by a fair margin, but still, an eLSD will never outperform a true 4x4 system.

We do have a member who bought an extra Patriot/Compass rear diff from a junkyard in hopes that he could get measurements to a company willing to make a true locker, but nobody he spoke to seems interested in developing for this platform.

thereisnospork > HammerheadFistpunch 06/12/2015 at 20:27

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Certainly, but where is the other 50hp going, where is the energy sink? The motor is producing 100hp as per the stated conditions, and we all appear to have agreed it isn’t going to the locked wheel. So where is the other 50 that the dyno doesn’t see? Nominally I would suggest the differential is boiling its fluid into vapor, but the gears in the differential should only turn at a fixed ratio(?) to the driveshaft. An overall speed which hypothetically might be very slow.

Edit: does the loose wheel perhaps turn twice as fast relative to the engine - half the torque but double the rpm. It seems reasonable that might happen but I don’t know enough to say for sure.

HammerheadFistpunch > thereisnospork 06/12/2015 at 22:03

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If I read you right I think you are feeling the same misconceptions that I had a tough time getting my head around at first and that is that engine power isn’t a “thing”. Its a tricky concept to get around at first but once you do it makes total sense. My original thinking was that my engine chart tells me that at 1000 rpm i make 240 ft-lbs so naturally I will always be producing 240 ft-lbs at 1000 rpm no matter what my wheels are doing; spinning, gripping, alternating,etc. The engine produces this force at that speed. That’s the wrong way to look at it.

Its all about work; Which is force (and distance) applied. i.e. even though my specs say I should be producing 240 ft-lbs at 1000 rpm, if I put my truck in neutral than no matter what I’m only producing enough torque to equal the friction of spinning my gears in oil. Its like the spinning bolt analogy, you can be the strongest person with the longest lever but if the bolt doesn’t resist your efforts you have no torque and without torque you don’t have work, without work you don’t have power. Likewise if you apply a high force but the bolt doesn’t move you have torque but no distance, thus no work and no power. Make sense so far? Torque and HP aren’t things, they are just tools to measure work done.

So when we talk about a 100 hp engine, its not really anything except a measure of the work being performed at a rate. i.e. a 100 hp engine is only that when its doing 100 hp worth of work. So if an engine is hooked up to a dyno and the dyno reads 100 hp than 100 ft-lbs are being applied at a rate of 5252 revolutions per minute. if there is no load, there is no torque and it could happily spin around at 5252 rpm all day long making only the hp required to overcome friction losses.

Back to the differential if we set up your system and have an engine that’s capable of producing 100 ft-lbs at 5252 rpms (so 100 hp) verified and hook it to an open diff, with one end being a dead end and the other attached to a dyno then 50% of that 100 ft-lbs will be used to resit the fixed side, but no work will be done. There is still 50 ft-lbs being exerted but no HP because no work. On the other side of the diff, there is 50 ft-lbs being applied to the dyno: (50 x 5252)/5252 = 50 hp. half the torque, half the work, half the power. The torque applied is the same, but the work accomplished is halved. Its like mechanical leverage right? add a pully to a rope system and you double the work but half the rate at which you do the work - power is work x rate so Work (2) x rate (1/2) is still the same Power as Work (1) x Rate (1). What we are doing in the engine example is keeping the rate the same, but halving the work by splitting the torque by 2.

Once I got my head around the idea that HP and Torque were things or tangible, but rather measuring components for work it got a lot easier for me to figure out differentials.

BaconSandwich is tasty. > HammerheadFistpunch 06/12/2015 at 23:40

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Electric motors at each wheel FTW.

thereisnospork > HammerheadFistpunch 06/13/2015 at 00:40

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I believe you are misunderstanding my scenario; your analogy of a car ‘freewheeling’ at 5252 rpm or in neutral is irrelevant: the vehicle is at WOT and the dynometer is putting sufficient resistance so as to match the engine’s output through the one wheel so as to hold that rpm constant as a condition of the scenario. Note that rather then controlling the engine rpm via throttle (imagine a spherical brick pushing it to the floor), we are doing it via the ‘braking’ effect of the dyno so we can stay at any point on the power curve we want as measured at the flywheel (5252 is just convenient).

Now by definition, I believe we agree our motor at a steady 5252 rpm is pushing/being pushed by 100ft/lbs of torque at the flywheel - which by definition means we are putting out 100hp at the flywheel (100*5252)/5252=100. However for there to be only 50hp at the wheel is impossible, without some other part(s) of the car doing 50 hp worth of work. Work Energy/Sec, and if you are producing 100hp of energy/sec at the flywheel, that 100hp of energy/sec is coming out somewhere. Lacking an energy sink to the contrary the ever pesky law of energy conservation is dictating that all 100hp are coming out the unbraked wheel regardless of what torque that wheel is pushing with. If it has half the torque, then it is spinning twice as fast.

Sorry if this is excessively descriptive but please note that power must be conserved in a system over a given time interval being power is simply energy/sec. If a motor is producing 75,000 Joules/second (as we have so fixed it from the combustion of gasoline) then 75,000 Joules need to be turned into mechanical motion or heat every second. If nothing is getting hotter, and the only thing moving is the wheel - how can it not be dumping 75,000J/sec (100hp) into the dyno? And how can a wheel output 100hp w/o 100ft.lbs of torque other than by spinning at a relative rate of 2x (assm 50ft.lbs)?

HammerheadFistpunch > thereisnospork 06/13/2015 at 12:11

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see what you’ve done here is just substituted ft-lbs for Joules. You are mixing up the concepts of work and conservation of energy. 100hp is ONLY 100 hp when 100 hp of work is being done. That means that there isn’t an engine that always puts out 100 hp regardless of condition, but its the condition that allows work to take place. You mistake is thinking that 100 hp engine is always a 100 hp engine, its not the case, its the amount of work that defines the engines power output not the power output of the engine that defines it. once a work is being done (turning the dyno drum) THEN then law of conservation of energy comes into play.

thereisnospork > HammerheadFistpunch 06/13/2015 at 14:42

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Joules are conserved, by ‘law’. Our 100hp engine IS 100hp because we’ve said it is so. Our dyno is providing such resistance X so as to allow the engine to turn at 5252 rpm at wide open throttle - which if we put a load sensor on the flywheel would read 100ft/lbs of torque and we know this because our dyno is modulating maintain 100ft/lbs at the flywheel. The amount of work/sec an engine does is independent of what is past the flywheel (at constant throttle X RPM) and it doesn’t magically half just because we’ve connected the driveshaft to a half-locked open diff.

Think of it this way: we put a brick on the accelerator, and the dyno adds or decreases resistance on the free wheel to mediate rpm, like a PID controller. If rpm exceeds 5252 the resistance increases and vice versa.

Same as if it were an electric motor turning a generator through a 1/2 locked open diff - power in must = power out (less friction ofc.) only power out is coming through 1 wheel.