Learning how to build Lotus Seven replicas...together!
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PostPosted: March 11, 2019, 12:20 am 
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Joined: January 31, 2012, 12:49 pm
Posts: 1711
Location: Louisville KY
horizenjob wrote:
...but the zero pressure should have given me an infinite contact patch.

Well maybe, but the tire diagonal from that was enjoying significant lift off of the pavement!

Geek49203 aka
Tim Wohlford
Louisville, KY
Hayes front, S10 +2 rear, Lalo body.
Girlfriend thinks I'm nuts for building this....

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PostPosted: March 11, 2019, 5:43 am 

Joined: December 18, 2010, 3:29 pm
Posts: 122
horizenjob wrote:
If the car slows down when you're turning, the tires might be too big.

it's a fact that your contact patch is very simply vehicle mass divided by tire pressure

I don't think that is even close to being true. There's all sorts of stuff going on here. My first event at Pocono I felt threatened by another driver so I put him in the infield coming off the big oval turn ( my bad ). He took one of my tire valves with him though. I could tell something was wrong, but the zero pressure should have given me an infinite contact patch. My memory on this is a bit hazy now, but I do remember trying to give the car a bit of a weave at 135 on the way to infield turn in. Something was up I could tell, I was tip-toeing. :rofl:

Isn't the point of using wider tires to be able to run lower pressure? With lower pressure you can use softer rubber. You can use thinner rubber ( and belts etc. ) to give a much thinner carcass.

DId you run the same tire pressures on your 8" and 6" tires?

I don't really mean to be terse here, Alex. That's a great post. Honestly the tire sizes are spec on my FF, so I've never invested any thought into this.

Terse doesn't bother me, this is fascinating stuff if you love vehicle dynamics. I agree it's a tough concept to swallow but I literally just had this conversation over lunch with a group of tire engineers from a major tire brand because I still thought carcass structure had something significant to do with it. Of course, totally flat tires are a whole different story, there are limits, for sure. Given some basic restraints that we all operate under like pressure somewhere between 10 and 100psi and sidewall of about a 20 series or higher they were adamant that your contact patch acts just like a thin balloon in terms of size, it's just mass/pressure.

Shape is a different story, you can do all kinds of interesting things with tire construction there, cones, footballs, etc, but overall area is pretty clearly just pressure related.

In the FSAE experiment we really just wanted to prove this concept so I know we kept pressure constant, 15 or 16 psi, IIRC, but that was over 20 years ago so I don't remember the exact pressure. But you are totally correct, bigger tires can allow somewhat lower pressure but not close to in an even ratio with the width increase, most of the grip comes from changing the shape of the patch. I didn't mean to imply there weren't other factors, the shape change is just the dominant one and I didn't want to write a book!


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