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PostPosted: March 6, 2019, 3:17 am 
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How do you guys decide how much tire is too much tire? Meaning at a point having more tire does not yield any performance benefits.

Some people use 195 all the way through 295 wide... 13" to 18"

What factors did you take in when deciding tires to use on your lotus 7? It seems like the entire car is designed around the tire size and type of driving.

I am considering VICTRA VR-1 245/40ZR15 and Konig Dekagram 15x10. A square setup for an weekend warrior, autocross and occasional track day use.

Is this too much tire? Is a square setup a poor decision? I like the idea of being able to rotate the tires easily that a square setup provides.

Links
Tires
https://shop.maxxis.com/c/automotive_vi ... ted_post=1
Rims
https://www.good-win-racing.com/Mazda-P ... -2421.html


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PostPosted: March 6, 2019, 9:10 am 
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It depends what motor your running and the HP you have. But yes I believe you can have to much tire. It really relates to your skill level, if I drove a car with slicks it would be a waist as I would not go fast enough to use them properly. If you learning to go fast you want a car that is playful moves around a little so you can get use to the feel. If you have a car that is super stuck the limit is more pronounced and can be tougher to control. Some track days don't allow R rated tires in the novice group for that reason. I am Honda K24A2 powered have 220 rwbhp and run 15x7 front and 15x8 rear, loaded with Toyo R888r's and I am not looking for more width, it has never entered my mind.

Graham


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PostPosted: March 6, 2019, 9:55 am 
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For me, it's been through experimentation. I've run as small as a 245/17 all the way up to a 345/18. My current preference is a 255/17 Hoosier A6 in front and a 315/18 Pirelli in the rear. I run about 22 PSI cold in the front and about 18-20 PSI cold in the rear. My car weighs about 2100 pounds with a 40/60 F/R weight distribution and I'm pushing about 500HP.

I went with the smaller front tires because I don't have power steering and fighting larger tires was making me tired. The wider and taller 18 inch tires put a lot more feedback into the steering wheel. These smaller tires are also less expensive.

For the rears, I run the pressure low so I can get some heat into the tires early. For track use, running big wide slicks in the rear is tricky. When cold, they have basically zero grip. Many people want to take it easy for the first couple of laps to get them warm. All that does is make for more laps of a loose car. You have to drive them harder when they are cold to get them to temp. That takes some intestinal fortitude; usually built up from experience.

Based on your username, I would suggest you buy multiple sets of wheels, and start experimenting. Also, assuming budget is a factor, find out what is available on the used market by you. I've never purchased a brand new set of tires from a dealer. I did find some sticker tires from a private party who was moving away from that size. Almost all of my tires come from local race teams and are scrubs. The last batch I bought I paid $10/each, and those are Pirelli slicks I expect to get a lot of life from.

Ken


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PostPosted: March 7, 2019, 11:25 pm 
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My priorities in tires were:
-cheap enough I can afford to race often. It's no fun driving if you are worried about your wallet all the time.
-readily available and good performance options

Which settled me down to 205/50r15 (Spec Miata size so always going to have options). After driving the car, I would probably do better with wider, but 205 is working for now (and is cheap enough I can buy tires regularly).

Too much tire is when you are in a sweeper, are full throttle, and the car is not accelerating. Until then you could probably use more rubber to put faster lap times down. For example, my stock Yaris with 205 NT01s is almost maxed out (2300lbs/115hp). The extra weight of 225 tires would barely improve my corner speeds and greatly reduce my acceleration/braking. Miatas run about the same.

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PostPosted: March 8, 2019, 8:43 am 
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I've got 14" wide slicks on the back of the Slotus. Can't go any wider, or I have to buy a new trailer!
Attachment:
11 03 18 Loaded.jpg
11 03 18 Loaded.jpg [ 751.95 KiB | Viewed 1041 times ]

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PostPosted: March 8, 2019, 9:04 am 
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AT some point, wouldn't you hit a land of diminishing returns as you wouldn't pick up any more contact area? Wider tires in general, to a point, would be grippier 'cause there would be more contact area under various conditions. But at some point I wouldn't think that you'd be picking up a lot more contact patch?

And of course, you would probably have to re-engineer some stuff to accommodate the wider tires, so it wouldn't always be a bolt-on speed adder. Are you making car wider with the wider tires? Cause all racers like that. Do you change the front/rear bias (oversteer / understeer) by doing this? Does your car have considerable downforce? Cause that would make a difference. And so it would go... A good setup with smaller tires might could run circles around a car with bigger tires.

All bets are off when it gets wet tho.

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PostPosted: March 8, 2019, 10:31 am 
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https://www.youtube.com/watch?time_cont ... gz0qF3CuNA

"Do wider tyres give you more grip? The differences between tyre widths tested and explained."

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PostPosted: March 8, 2019, 11:00 am 
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@Trackslut

I'd say your name give away your primary interest, but I'd think you'd be thinking about more factors if you want a practical tire for street use as well. There are things like rolling radius, sidewall construction, hardness of the rubber, tire noise, etc., etc.

Having a tire that has grip to 1.3G's is nice, but if you use that capability on the street, you're probably going to get arrested. :wink:

Cheers,

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PostPosted: March 8, 2019, 11:06 am 
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more questions...

1. How much HP do you have? I mean, if you're running a Citroen 2CV w/ 30 hp, then I'm thinking that putting on F1 style Bridgestones won't exactly help. I would think that your speed bottleneck is "elsewhere".

2. Size... or compound? I'd think that at some point, when width is at a diminishing return, that compound would be more important.

And as the video points out... the balance of the thing.

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PostPosted: March 8, 2019, 11:43 am 
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The wider the tire, the more sensitive it is to camber change, including uneven or worn road surfaces.

Wider tires also make it harder to get good turning geometry as far as scrub, wheel offset, steering lock, and kingpin inclination. So you either make serious steering compromises, or join the "it doesn't make any difference anyway" camp.

Wider tires are also heavier, which means more unsprung weight and more mass to spin up and down every time the car brakes and accelerates.

Wider tires are also more sensitive to pressure, which has to be balanced across four axes - staight line, cornering, center wear, and edge wear. What's best for wear is usually not what's best for lap times. (nor is tread temperature, at least below the point where the tire "goes off.")

If you're racing, you just keep throwing money at the question until lap times tell you which configuration of available tires is best.

I used to be a "steamroller" fan, but I've since moved to the "narrowest tire that will do the job" camp, as I value light, precise steering without kickback and predictable handling on bad surfaces more than (theoretical) absolute grip. At my (admittedly not Nuvolari) skill level, my lap times stayed the same, but the cars were much more pleasant to drive.


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PostPosted: March 8, 2019, 12:26 pm 
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Next we will cover how the largest exhaust pipes aren't always the best....

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PostPosted: March 9, 2019, 9:55 pm 
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There's a mix of art and science here that is pretty interesting. On the science side keep in mind that you get zero extra rubber on the road with a wider tire. As much as your brain wants to ignore the math it's a fact that your contact patch is very simply vehicle mass divided by tire pressure, that's it. That's IT. What you are changing is the shape of the contact patch. A wider tire puts a short, wide rectangle of rubber on the road vs a narrow tire with a long, narrow rectangle of rubber. Trust me, in the vehicle dynamics world we've done the math and made these measurements 100 different ways, you don't put any more rubber down until you lower the air pressure (which you really don't do substantially until you're way oversize on your tire and there's a lot of other problems when you get to that point).

SO, given the amount of rubber is the same the extra grip comes from the shape of the patch. If you're building a deck you don't lay the joists on their side and the same is true of a tire contact patch. For a lot of interesting reasons a short, wide contact patch improves your cornering capability. A great, readable book on this subject is "the racing and high performance tire" by Paul Haney. He's not an engineer but a motorsports journalist with a lot of engineer friends so it's quite a good read. The most explainable aspect is that when providing any amount of side force a tire is operating at some slip angle, its never rolling straight along when you're cornering above about 0.3G. With a long, narrow contact patch a large part of the contact patch is already sliding geometrically at lower cornering forces (think about the double rear axles of an 18 wheeler and how they slide around intersections with their combined 4 foot long contact patch) A slipping piece of rubber is not as grippy as a static piece so you loose a lot just due to most of your contact patch sliding before the actual car is sliding (which happens when the last little bit of the contact patch finally lets go). On the short, wide patch of a wider tire much less of it is slipping as the tire operates with any slip angle so it can all grab statically right up until the bitter end when the whole tire lets go. Again, great drawings, etc in Pauls book.

The art comes in balancing the extra grip you get with a wider tire with the extra feel you get with a nice, long contact patch that you can feel transitioning to slip much easier (since parts of the contact patch start sliding much sooner in the cornering process). There's also a point where you can't put heat in the wider tire at all since it does't slide enough. There's a lot more interesting reasons why you reach a point of diminishing returns with added width that are probably too much to type here.

On our FSAE car we found identical lap times for our top drivers with 6" wide vs 8" wide tires but the car drove completely differently with each setup. Novice drivers were much faster on the narrower tire. Like FSAE cars, our cars are exceedingly light relative to the typical design considerations for pass car tires so you have other tire design aspects working against you when you go to really wide pass car tires designed for 3-4k lb cars.

Another consideration is wheel width. I don't advocate for stretched tires like you see on stancenation but you generally want to go with the widest rim on the tirerack rim width column for your specific tire. (they usually have about 1.5" variation, it's just data from the tire manufacturer that they are kind enough to pass along in the tire specs page) At GM we tuned the balance of the Camaro SS 1LE by using the same 305 tire front and rear but a half inch wider wheel at the rear. That gave the rear a bit more grip and much better precision. We did the opposite on the ZR1, we used the same tire as the Z06 but widened the front wheel to handle the extra weight of the bigger engine. You certainly don't want to do what we all probably did in high school and cram the widest tire you can on whatever wheels you have, it will drive sloppy and lack grip relative to the tire width. Either figure out how to go wider on the rim or check the ego and drop tire width.

So, in the end, that's how I'm choosing tires for the stalker. I can fit a 9" wheel on all 4 corners. A typical 40 series 245 tire has a wheel width range from 7.5" to 9" so 245 is the answer. Again, most people would say you can cram up to a 275 on a 9" wheel but that would drive like garbage and likely not go any faster.

Hope that's interesting, there's a lot more to talk about but get Paul's book, he does a much better job.

Alex


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PostPosted: March 9, 2019, 10:14 pm 
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BTW, SAE wants $225 for the book. Paul sells it on his website for $55 here http://insideracingtechnology.com/bookorder.htm

There's also a free (well, you have to give an e-mail address) webinar that I have't listened to but is said to cover a lot of the book content here http://info.motorsportreg.com/race-tires-webinar-offer

Now that I've mentioned the book twice I should say that I have no affiliation with him whatsoever, never met the guy, just thought it was a good book pertinent to this discussion.

Alex


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PostPosted: March 10, 2019, 11:16 am 
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ajmacdon wrote:
There's a mix of art and science here that is pretty interesting. On the science side keep in mind that you get zero extra rubber on the road with a wider tire. As much as your brain wants to ignore the math it's a fact that your contact patch is very simply vehicle mass divided by tire pressure, that's it. That's IT. What you are changing is the shape of the contact patch. A wider tire puts a short, wide rectangle of rubber on the road vs a narrow tire with a long, narrow rectangle of rubber. Trust me, in the vehicle dynamics world we've done the math and made these measurements 100 different ways, you don't put any more rubber down until you lower the air pressure (which you really don't do substantially until you're way oversize on your tire and there's a lot of other problems when you get to that point).

SO, given the amount of rubber is the same the extra grip comes from the shape of the patch. If you're building a deck you don't lay the joists on their side and the same is true of a tire contact patch. For a lot of interesting reasons a short, wide contact patch improves your cornering capability. A great, readable book on this subject is "the racing and high performance tire" by Paul Haney. He's not an engineer but a motorsports journalist with a lot of engineer friends so it's quite a good read. The most explainable aspect is that when providing any amount of side force a tire is operating at some slip angle, its never rolling straight along when you're cornering above about 0.3G. With a long, narrow contact patch a large part of the contact patch is already sliding geometrically at lower cornering forces (think about the double rear axles of an 18 wheeler and how they slide around intersections with their combined 4 foot long contact patch) A slipping piece of rubber is not as grippy as a static piece so you loose a lot just due to most of your contact patch sliding before the actual car is sliding (which happens when the last little bit of the contact patch finally lets go). On the short, wide patch of a wider tire much less of it is slipping as the tire operates with any slip angle so it can all grab statically right up until the bitter end when the whole tire lets go. Again, great drawings, etc in Pauls book.

The art comes in balancing the extra grip you get with a wider tire with the extra feel you get with a nice, long contact patch that you can feel transitioning to slip much easier (since parts of the contact patch start sliding much sooner in the cornering process). There's also a point where you can't put heat in the wider tire at all since it does't slide enough. There's a lot more interesting reasons why you reach a point of diminishing returns with added width that are probably too much to type here.

On our FSAE car we found identical lap times for our top drivers with 6" wide vs 8" wide tires but the car drove completely differently with each setup. Novice drivers were much faster on the narrower tire. Like FSAE cars, our cars are exceedingly light relative to the typical design considerations for pass car tires so you have other tire design aspects working against you when you go to really wide pass car tires designed for 3-4k lb cars.

Another consideration is wheel width. I don't advocate for stretched tires like you see on stancenation but you generally want to go with the widest rim on the tirerack rim width column for your specific tire. (they usually have about 1.5" variation, it's just data from the tire manufacturer that they are kind enough to pass along in the tire specs page) At GM we tuned the balance of the Camaro SS 1LE by using the same 305 tire front and rear but a half inch wider wheel at the rear. That gave the rear a bit more grip and much better precision. We did the opposite on the ZR1, we used the same tire as the Z06 but widened the front wheel to handle the extra weight of the bigger engine. You certainly don't want to do what we all probably did in high school and cram the widest tire you can on whatever wheels you have, it will drive sloppy and lack grip relative to the tire width. Either figure out how to go wider on the rim or check the ego and drop tire width.

So, in the end, that's how I'm choosing tires for the stalker. I can fit a 9" wheel on all 4 corners. A typical 40 series 245 tire has a wheel width range from 7.5" to 9" so 245 is the answer. Again, most people would say you can cram up to a 275 on a 9" wheel but that would drive like garbage and likely not go any faster.

Hope that's interesting, there's a lot more to talk about but get Paul's book, he does a much better job.

Alex


Great post, with experience based comments. Thanks.

The most interesting part to me is the comment about different drivers liking different tire sizes, even though the lap times were the same. This type of experience is why I suggested the OP try different combinations. Getting something you are comfortable with, is the most important part for me. If I like the way the car responds, it increases my confidence and allows me to drive harder, for longer periods of time with less mental fatigue.

Ken


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PostPosted: March 11, 2019, 12:10 am 
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If the car slows down when you're turning, the tires might be too big.

Quote:
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.

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