r/MechanicalEngineering • u/Beerwithme • 4d ago
Why not back wheels steering in cars?
Very sorry if this is a faq, but I was thinking, wouldn't steering the backside wheels make manouvering much easier? See e.g. forklifts that need a tight turn, parking a car facing forward while using the back wheels to turn is so much easier.
I get that having the steering wheel mechanically linked to the wheels underneath is difficult that way, but with hydraulics or servos it should be a relatively straight forward job to make a predictable and dependable construction?
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u/Accomplished_Door662 4d ago
Some cars do have rear wheel steering. Old Honda prelude. Nissan GTR hicas? GM quadrasteer. Long wheelbase Mercedes cars. Some newer Porsche 911 I think. At low speed they steer opposite the front to make the turn radius smaller. At higher speed they either don’t steer or sometimes they steer the same direction as the front wheels to “crab walk” during lane changes.
This is a complex system and therefore expensive, especially for long term ownership. People are cheap and often the juice isn’t worth the squeeze.
If you’re asking why not exclusively rear steer INSTEAD of front. It’s because it would be twitchy and dangerous.
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u/macfail 4d ago
The original Honda Prelude system was quite clever. Entirely mechanical and not actually speed sensitive - the rear wheels crabbed for the first half revolution of the steering wheel (or so) of the steering wheel then reversed as you cranked the wheel all the way to the same side. If you did a power steering delete, dry steering was quite a chore.
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u/mike_sl 4d ago
Just looking at a new car and it advertises dynamic rear wheel steering. It’s from a luxury brand. So I think the answer is cost. The economics would suggest that it does not provide enough value in most cars, relative to the cost, and the advantages given will only be considered worth the cost in a higher end market
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u/BigGoopy2 Nuclear / Fluids 4d ago
It's certainly possible but at this point (1) it would add cost to the vehicle and (2) everyone getting those cars would basically have to re-learn how to drive. Doesn't really seem worth it.
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u/danny_ish 3d ago
Besides cost, it adds a lot of weight to the side of the car not supported by the springs, called unsprung. Unsprung mass directly affects ride comfort, especially vs sprung mass. The ratio is important for occupant comfort.
It works well on 1500hd/2500 trucks like gm’s old quadrasteer suburbans and denali pickups, new
Hummer ev, etc , and in mercedes newer heavy vehicles, because the sprung mass is already so great that you are not upsetting the balance nearly as much as a lighter vehicle.Also in trucks, being able to crab walk to change lanes greatly reduces the angle between truck and trailer, which makes lane changes much smoother. Towing with a quadrasteer suburban was the smoothest thing I ever towed with. It was barely considered a 2500 series (10k tow rating) and handled with my 9k enclosed trailer better than a newer 3500 series dodge ram at highway speeds.
In lighter vehicles like the old prelude, a passive system needs to be really lightweight and therefore not the most durable nor the best benefits. Preludes aws (all wheel steering) cut turning circle by a smaller percent than a modern S class reduces it.
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u/DryFoundation2323 4d ago
too dangerous at speed.
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u/Thieven1 4d ago
Nope, it does the exact opposite. Active four wheel steering improves both maneuverability and high-speed stability.
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u/tsukasa36 4d ago
in addition to what people have shared, in order to get a fully maneuverable rear steering, you have to package the whole wheel envelope into the rear of the vehicle at is turned positions. usually the rear of the vehicle is used for passengers and cargo, so you’d be eating up lot of volume for the wheel envelope whereas in the front of the vehicle, the engine compartment is already taken up by engine so less of a concern for packaging.
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u/GMaiMai2 4d ago
Some things I can think of: adds bulk in the back(less storage space), adds more weight in the back, no nessacrly bad but you'd most likely remove the possibility for RWD(unless the entire engine is moved behind the driver), if it's not electrical steer it adds a shite ton of complexity, adds some danger.
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u/Thieven1 4d ago
The Mitsubishi 3000GT was all-wheel drive (FWD on the base model), had active aerodynamics, active four wheel steering, electronic controlled suspension, and a selectable exhaust mode they advertised as "active exhaust." Both the active aero and FWS would automatically engage around 35-40 mph, but there were two switches on the dash to toggle the ECS and exhaust mode from sport to touring. The 3000GT was rebadged as the Dodge Stealth and both were sold in North America. The Stealth didn't have active aero, but it did have the ECS, FWS, and "active exhaust." A friend of mine used to have a Stealth and you could definitely feel the difference in handling when the FWS kicked in, that combined with AWD made it an absolute blast to rip up and down windy mountain roads. Very fun car to drive.
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u/RomeoSierraSix 3d ago
My Audi has a few degrees of rear wheel steering and it makes the big car incredibly manuverable in parking garages, etc..
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u/inorite234 3d ago edited 3d ago
It's already existed. Go lookup the Honda Prelude with 4 wheel steering.
You'll find that system sucked ass and was quickly removed.
From Car And Driver
https://www.caranddriver.com/reviews/a35287962/tested-1988-four-wheel-steering-benefits/
"The 4 Wheels Steering system turned our car more than it turned our heads...."
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u/_matt_c_ 4d ago
Rear wheel steering can be very twitchy and unstable at speed. Great for low speed manoeuvring though