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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / December 2004Questions About Acura RL AWD Claim
I was watching TV last night and after one of the short breaks to show programming I became interested in an Acura commercial for the RL sedan. In the commercial the claim was made that the AWD system "accelerates the outside rear wheel in a turn to dramatically improve handling." How does the system "accelerate" one rear wheel? The following is from http://www.acura.com/models/model_specs_index.asp?module=rl "Helps steer the vehicle and enhances its maneuverability. It helps compensate for understeer (push) or oversteer (the rear end feels loose), when cornering. The RL feels agile and nimble and tracks smoothly through turns without unnecessary steering correction. It sends up to 70% of the engine's torque to the front or rear wheels, but it can also send up to 100% of that same rear-wheel torque to either the left or right outside-rear wheel when cornering. It will even accelerate the outside rear wheel to enhance handling while cornering." If you click on the right buttons ("Click to Experience It Now," and then the "Control" tab) they have a little movie illustrating the system in action. The cut away view of the car indicates some sort of complicated rear end. An article at http://www.theautochannel.com/F/news/2004/11/10/277071.html indicates there are magnetic clutches in the rear end, but I still don't see how these can accelerate one of the rear wheels. I can see how they might selectively apply torque to only one wheel, but not "accelerate" that wheel beyond the other wheels. Maybe it is just the way the terminology is used that is confusing me. Is there also a conventional differential in the rear axle, or just magnetic clutches? Other articles: http://www.auto-report.net/index.html?sh-awd.html Regards, Ed White Perhaps there is a computer sensing the turn, and shifting power to the outside rear wheel, like a Mercedes? > Perhaps there is a computer sensing the turn, and shifting power to the > outside rear wheel, like a Mercedes? "Shifting Power" seems meaningless to me. It is also not the same as "accelerating" which implies a change in velocity. Ed >>Perhaps there is a computer sensing the turn, and shifting power to the >>outside rear wheel, like a Mercedes? [quoted text clipped - 3 lines] > > Ed I suspect the problem is terminology. The Acura info was probably written by a marketing type..not an engineer. By applying all the torque to only one of the rear wheels it would tend to help steer the car to the opposite direction. Stan K. > >>Perhaps there is a computer sensing the turn, and shifting power to the > >>outside rear wheel, like a Mercedes? [quoted text clipped - 8 lines] > to help steer the car to the opposite direction. > Stan K. Alright, so they have electrically operated clutches in the rear axle, and they only engage the "outside" wheel in a turn. Is there a traditional differential as well? If so, does another clutch also "ground" or somehow modulate the disengaged axle - which seems like it would casue a whole other set of problems. I'd really like to see a detailed drawing with explanations of the rear axle mechanism. It would also be interesting to see a comaprison test of an RL with the system and one with a traditional rear end. I have to believe it is like quadra-steer, cool sounding but almost worthless in the real world for most people. On the other hand - I like cool things. Everytime I see the RL ad, I have a strange compulsion to go buy one. This despite the fact that I'll bet it doesn't have enough head room for me. I guess I should just go test drive one. Do you think the salesman would let me take it down a twisty road at high speed? Ed >> Perhaps there is a computer sensing the turn, and shifting power to the >> outside rear wheel, like a Mercedes? [quoted text clipped - 3 lines] > > Ed The outside rear wheel on every car spins slightly faster in corners. So does the outside front wheel. Both front and rear outside wheels travel a longer distance compared to the inside wheels when turning. > The outside rear wheel on every car spins slightly faster in corners. So > does the outside front wheel. Both front and rear outside wheels travel a > longer distance compared to the inside wheels when turning. I understand this. But Acura claims that they are "accelerating" the outside rear wheel. A traditional dear differential splits torque evenly beteen the two rear wheels, while allowing them to rotate at different speeds. The illustrations in the Acura RL ads show a very complicated rear end. I want to understand what they are trying to tell me. The rear end has electrically operated clutches and some of the articles talk about a planetary gearset. I want to know what they are doing back there. Are they merely matching the speed of the outside rear wheel to match up with the increased arc lenght (comapred to the inside wheels), or do they really apply additional torque to the outside rear wheel in an attempt to drive the wheel differenty to improve handling. This is what the ads imply. Ed By modulating the brakes on the rest of the wheels. Just like the traction control on the Vette ? Left turn and it slow the left side, that makes the differentials speed up the opposing side.  SignatureSteve Williams > I was watching TV last night and after one of the short > breaks to show programming I became interested in an Acura [quoted text clipped - 38 lines] > > Ed White >By modulating the brakes on the rest of the wheels. Just like the >traction control on the Vette ? Left turn and it slow the left side, >that makes the differentials speed up the opposing side. It's called vehicle stability assist, and is common on hondas and acuras. it uses athe abs to apply the crakes on the other wheels allowing the others to spin faster. Chip > >By modulating the brakes on the rest of the wheels. Just like the > >traction control on the Vette ? Left turn and it slow the left side, [quoted text clipped - 3 lines] > acuras. it uses athe abs to apply the crakes on the other wheels > allowing the others to spin faster. Go look at the references. This system does not work that way - or at least the ads and articles imply that it is something totally different. Clutches and planetary gear sets in the rear end have been added. Ed Then they use the clutches and planetaries to do the same thing, only they added MORE parts and will charge you more to repair it when it breaks. Ads are written by people who probably have even less understanding of the way the system works than folks here do. And after looking at the picture of that rear axle I really hope it never has a failure. What a mess. Looks like they are using the planetaries to alter effective gear ratios to each side of the rear end on the fly. By locking and unlocking the sun gears they can alter the ratios at will. Also looks like they have placed a dual set of planetaries on the input to act as the center differential. The side planetaries are how the accelerate one wheel independent of the other. by unlocking/locking the shells they change the relative gearing across the axles, one side will speed up while the other stays at constant speed. I would also think that there claim of "worlds first direct electromagnetic clutch" is false. That is the same type of clutch used on auto AC systems as well as garden tractors. The magnetic field pulls the plates together and locks the clutch. SignatureSteve > > >By modulating the brakes on the rest of the wheels. Just like the > > >traction control on the Vette ? Left turn and it slow the left side, [quoted text clipped - 10 lines] > > Ed > Looks like they are using the planetaries to alter effective gear > ratios to each side of the rear end on the fly. By locking and unlocking [quoted text clipped - 7 lines] > -- > Steve Right now I cannot remember where I read it, but you have stated things essentially correctly. Indeed, the trick is in using the planetary gear sets and clutches to actually apply more torque and speed to the outside rear wheel, based on the sensor inputs regarding the g-forces acting on the car. Actually sounds like a very slick system. While it does seem a bit complex, I really wouldn't expect significant reliability problems. Planetary gear sets usually last forever as long as properly lubricated etc. Really seems like this system is head and shoulders above the "apply the brakes to the inside wheel systems" Frank |
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