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 Post subject: Six Wheels?
PostPosted: April 15, 2016, 7:15 pm 
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Joined: November 21, 2008, 9:13 pm
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Location: Grand Junction, CO.
So what about a car with six wheels? I've had this idea rattling around my head for a while now, inspired by the few experiments teams created in F1 in the 70s and 80s. To be clear, I'm referring to the two front, four rear setup, like the March 2-4-0 or Williams FW08D, seen here: http://www.ultimatecarpage.com/gallery/19249/Williams-FW08B-Cosworth.html

This may seem like an outlandish idea, but it has some merit. The earlier Williams FW07E version set a lap record at Paul Ricard circuit in testing, beating out contemporary F1 cars of a standard four wheel design, and that's on a car hastily adapted to running six wheels, not a car designed from the ground up. The FW08D set "stunning" times at Donington, reportedly, before six-wheeled cars were promptly banned. Now the thing is for F1 cars, the main advantage apparently was not in the traction offered by the larger contact patch area and 4wd, but rather aerodynamics. This was in the ground effects era mind you, where most of the downforce was generated under the car. The longer wheelbase allowed a longer, more efficient venturi design, producing more downforce, while the use of four smaller rear tires instead of two large tires lowered the frontal area and reduced drag. These advantages were apparently enough to overcome the weight penalty, and with some development, who knows how well the design could have worked!

Now in a street/track day car, these aero advantages wouldn't be as apparent, but it would be neat to have a car with very low-slung sides on small tires with a meaningfully-large contact patch. And I believe the rear-four-wheel setup would have some key advantages in mechanical handling. Not only would the amount of rubber on the road be greater for more maximum grip, the stability could be increased. Imagine hitting a bump mid-corner, one of the tires could absorb the bump while the other is unfettered, and by the time the trailing tire hits, the leading tire could be settled back down, or closer to it anyways. While the overall unsprung weight would go up, each wheel and tire could be smaller, resulting in less unsprung weight each, making the traction advantage even greater over bumps. This would be further enhanced by the fact the load of the rear end would be spread over four wheels, meaning lower spring rates could be used for the same overall front-to-rear bias, with adjustments for the extra weight of course. Wear on rear parts like wheel bearings, brakes, and the tires themselves may be lessened. The same size tires could be used all around, instead of larger rears as is typical in mid-engine designs.

Braking could also potentially be improved. With more weight on the rear, the rear tires could do more work to slow the car(this is why Porsche 911s are historically so good under braking), and with two extra contact patches the stability at the rear should go up as well. As for understeer, the extra weight due to the third axle and mid engine should make the rear weight bias pretty high naturally, making the actual grip-to-weight ratio similar for each tire and leading to a neutral handling balance.

On the negative side, obviously this is a very unusual arrangement and a builder would pretty much be on their own for setup. Who knows what sort of alignment settings, brake bias, etc., would actually work best? The cost of the project would clearly go up compared to a traditional setup, due to purchasing six wheels and tires, six sets of A arms, uprights, coilovers, brakes, etc., not to mention potential trial and error waste. Durability may be an issue if the layout introduces any vibration issues in the link between transaxle and rear end, which could be exaggerated by the super short driveshaft. Overall weight is going to go up, potentially by several hundred pounds, and rotational inertia will go up too, dulling acceleration. The overall length would go up by a couple feet of course, meaning it won't be as compact and nimble. This wouldn't exactly be a KISS build, and I'm sure numerous issues would pop up during the build and while running the car. Additionally, handling might actually become too stable and it could be more difficult to steer with the throttle.

In the end though, I believe this could be a very effective setup, especially for a track car, and would be a fun challenge to take on. It would certainly be a conversation starter!

So what about a donor? That's actually pretty simple. Nearly any front-engined AWD car would work, especially transverse setups. I initially thought to use a Subaru drivetrain, but the length from the front(now mid) driveshafts to the output shaft would result in a prohibitively large gap between rear axles. Pretty much every longitudinal setup would suffer the same issues. Anything like an Evo, DSM, etc. would work well though. Personally I'd like to go with a Golf R32 VR6 drivetrain. Not for any practical reasons mind, I just love the sound they make and think the VR6 is a neat design. Might as well go with a very out-of-the-box engine too, right? Hooking output shaft to rear end could be as simple as one U joint or short shaft with two, with rigid mounts all around to prevent much movement between the parts.

To recap, pros:

-Increased lateral grip.
-Increased stability, especially over bumps.
-Great "launch" off corners similar to AWD, while maintaining steering purity.
-Decreased wear on certain parts.
-Potentially lower aero drag.
-Improved braking stability and performance.
-Readily-available donors at various cost points.
-Same size front and rear tires.
-It's freaking cool.

Cons:
-Increased build cost and time.
-Complexity/serviceability.
-Durability(?).
-Setup headaches.
-Weight, both total vehicle and rotational.
-Vehicle length.
-Too stable/lack of "playful" balance.
-Builder would be really on their own for design and troubleshooting.
-Having to constantly explain when people ask; "Why?"

So, what say you lot? Any merit to this design for a home-built track car?


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 Post subject: Re: Six Wheels?
PostPosted: April 15, 2016, 11:42 pm 
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BIG con for the F1 guys was tire diameter. The smaller tires couldn't handle the rpms and would disappear in an instant.

Solution, don't use itty bitty tires up front.

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 Post subject: Re: Six Wheels?
PostPosted: April 16, 2016, 12:58 am 
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Joined: April 23, 2006, 8:26 pm
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And had tiny brakes.

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 Post subject: Re: Six Wheels?
PostPosted: April 16, 2016, 1:12 pm 
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KB58 wrote:
And had tiny brakes.
Yeah, tiny, but they had lots of them.

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 Post subject: Re: Six Wheels?
PostPosted: April 17, 2016, 8:31 am 
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Joined: February 28, 2009, 11:09 pm
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carguy123 wrote:
BIG con for the F1 guys was tire diameter. The smaller tires couldn't handle the rpms and would disappear in an instant.

Solution, don't use itty bitty tires up front.

Sorry, but I'm not tracking with that statement. I don't see how rpm's are a problem. As I see it, the problem is tread speed and centrifugal force, both of which are a function of vehicle speed.

What am I missing?

Bill


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 Post subject: Re: Six Wheels?
PostPosted: April 17, 2016, 12:58 pm 
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Joined: February 28, 2009, 11:09 pm
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BBlue wrote:
carguy123 wrote:
BIG con for the F1 guys was tire diameter. The smaller tires couldn't handle the rpms and would disappear in an instant.

Solution, don't use itty bitty tires up front.

Sorry, but I'm not tracking with that statement. I don't see how rpm's are a problem. As I see it, the problem is tread speed and centrifugal force, both of which are a function of vehicle speed.

What am I missing?

Bill

Constant acceleration greater with smaller arc?


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 Post subject: Re: Six Wheels?
PostPosted: April 17, 2016, 2:26 pm 
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Smaller radius less centrifugal force....

less unsprung weight less spinning mass

stiffer tire "spring rate" due to less internal volume.

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 Post subject: Re: Six Wheels?
PostPosted: April 17, 2016, 2:46 pm 
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But...... Given a target road speed, while radius of a smaller tire reduces, the rotational speed increases by the same rate. But the centrifugal force is related to the square of that increased speed. Therefore CF of a smaller tire is larger than a larger tire at the same road speed.

At least that is what I think.

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 Post subject: Re: Six Wheels?
PostPosted: April 17, 2016, 4:52 pm 
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The itty bitty tires are spinning at much higher rpms than a traditional sized tire. Notice that small diameter trailer tires are rated for much lower speeds than larger ones. That's what happened to the tires on the Tyrell. The small tires simply couldn't handle racing speeds and disintegrated regularly.

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 Post subject: Re: Six Wheels?
PostPosted: April 18, 2016, 2:29 am 
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All an interesting discussion, but not really a relevant issue to the 2F4R design. The Tyrell used four tiny tires up front, but the designs I listed used normal sized fronts and rear tires of the same size.


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 Post subject: Re: Six Wheels?
PostPosted: April 18, 2016, 8:38 am 
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I guess the big question is what are you going to do with it. If it is to be a track car I would find out where you can race it. I know trikes are basically useless as a track car as insurance is usually based on 4 wheel cars. Not sure if 6 wheel car would be covered, you would have to check with each club you expect to race with. The other problem is you would ave no one to race against. If you you were fast in any class you would be protest (and rightly so) for unfair advantage.

If however you just want the car for fun and you can find a club that would allow you to do HPDE (High performance Driving Experience) or similar then I say go for it. That would be an amazing feat of engineering.

If you are just looking to be different building you own car is enough. While everyone on here makes there own cars, in the real world there are very few. I my travels I have only met 2-3 people who race a car they built from scratch. Maybe a dozen more who have built a kit. At EVERY event I have a constant stream or people coming to check out my car. Some look, some talk, take photos, some tell you what you should have done (usually the guys who could not identify an oil filter). 6 wheels would certainly draw a crowd.

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 Post subject: Re: Six Wheels?
PostPosted: April 18, 2016, 9:28 am 
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Here you go http://toledo.craigslist.org/cto/5528014339.html

Seriously, four rear tires is nuts. If traction is lacking, buy racing slicks and be done with it.

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