Turning Circle question for you engineer types

[donaldp]

Why does the turning circle have to be so huge for these cars? I know that front wheel drive and all wheel drive cars have a disadvantage for a tight turning circle but what geometry causes these rear wheel drive cars to be so awful? Is it necessary, or an oversight by the engineers? Personally, I feel it is a very important attribute to any car.

These are some of my personal cars I've owned.

2010 Audi S5 AWD - much better than my M3
2006 Dodge Charger RWD - Phenomenal
2007 Chevy Corvette RWD - Awful
2008 Lexus IS RWD - Unbelievably small turning circle - best ever
1991 Acura Legend FWD Longitudinal eng/tranny - phenomenal

[onetrader]

I HATE that about my M3. LOVE the car, turns like a pontoon boat.

[ItsaGoose]

It has to do with the width of F80 wheels. The turning circle for F30 is much better

[Yessir]

Actually it has to do with the gearing of the steering rack. Usually a car with quick steering response has a terrible turning radius because of the gearing. The opposite is true when a car has slow steering response;slow response has a good turning radius

[F83]

Originally Posted by ItsaGoose

It has to do with the width of F80 wheels. The turning circle for F30 is much better

I think it's mostly this. The price you pay for zero under-steer PLANTED front tires. They gotta be fat and the wheel well is only so big.

[Spoonman929RR]

I also had a 2010 S5, which felt like a semi when trying to make a u turn.. And I thought my Focus ST before that was bad.. Guess I'll really have to be mindful of it in the F80

[TRZ06]

I can't explain the technical reason, but directly related to how much front-end bite the car has. The better the bite, the less the turning radius.

The Corvette may have awful turning radius , but it has the best/most front-end bite of any car I have ever owned

I'm not sure if this is the right principle, but maybe ankerman angle or effect, or something like that.

It's so huge on the Corvettes, that (at least on the GS and Z06, at full lock the front tires will scrub and skip at low speeds, BUT, during cornering , they turn-in and bite like no other.

Always trade-offs when you are dealing with a high performance variant of any car.

[AlterZgo]

Originally Posted by Yessir

Actually it has to do with the gearing of the steering rack. Usually a car with quick steering response has a terrible turning radius because of the gearing. The opposite is true when a car has slow steering response;slow response has a good turning radius

I'm not so sure about this. I had a Nissan 240SX which had an insanely tiny turning circle - something like 29 feet vs. the close to 40 feet turning circle of the F8x and the 240sx had great steering response and feel.

I think things like our M's 110"+ wheel base, wide 255 or 265 mm width front tires, wider front and rear track, beefier suspension components that push the wheels and tires out further, which also prevents the car from being able to have the front wheels be turned as much, all contributes to the large turning circle.

Conversely, my 2003 Honda Pilot, which drove like a pig with shit steering response, had an excellent turning circle.

[OutlawX3M]

I think it's the turn angle of the front tires and the wheelbase that primarily affects turning radius (curb to curb). The ratio only affects how fast you get from lock 2 lock. Not all vehicles have the same lock 2 lock either, it's clearly dependant on how far the wheel can turn. So tire size and wheel well also play a role.

[CanAutM3]

Originally Posted by Yessir

Actually it has to do with the gearing of the steering rack. Usually a car with quick steering response has a terrible turning radius because of the gearing. The opposite is true when a car has slow steering response;slow response has a good turning radius

I fail to see how the gearing of the rack can have an effect on turning radius; and if it does, it should have the opposite effect of what you describe.

[CanAutM3]

The answer lies in the relatively long wheel base of the F8X combined with the suspension geometry that limits the amount of possible steering lock.

[Yessir]

Originally Posted by AlterZgo

I'm not so sure about this. I had a Nissan 240SX which had an insanely tiny turning circle - something like 29 feet vs. the close to 40 feet turning circle of the F8x and the 240sx had great steering response and feel.

I think things like our M's 110"+ wheel base, wide 255 or 265 mm width front tires, wider front and rear track, beefier suspension components that push the wheels and tires out further, which also prevents the car from being able to have the front wheels be turned as much, all contributes to the large turning circle.

Conversely, my 2003 Honda Pilot, which drove like a pig with shit steering response, had an excellent turning circle.

Originally Posted by CanAutM3

I fail to see how the gearing of the rack can have an effect on turning radius; and if it does, it should have the opposite effect of what you describe.

Alright gentlemen, I am no scientist or engineer but here is where I assume it has to do with the gearing. First the pic will show how the bigger teeth on the rack will enable the quicker steering response on the f80 compared to the f30's smaller teeth. And yes you are correct CanAutM3 about how it would have the opposite effect, but if you look at it from a finite stand point, the range would be limited by the steering wheel lock to lock. If the lock to lock from the steering from the f30 and f80 are equal this is where the limitation of turning radius would stem from. I do see how you can question the gearing but all other things equal, the gearing is the only thing that could effect the turning radius. As far as an engineering standpoint, why would the M engineers limit the turning radius by not adding more teeth to the rack? I can't answer that. Page 63 in the PDF attached below is where they start the chassis and suspension overview.

[Racer20]

Mainly it's related to how far the wheels and tires can turn before the tires hit something. Keep in mind that OEM's have to account for worst case production tolerances as well as all possible bushing and tire deflections, and they have strict minimum clearance requirements between the tire and any other component. "Not hitting anything" isn't good enough, there has to be some minimum amount of clearance.