THE Wheel & Tire Thread

[bnr32jason]

It's about a fine balance.

Let's take two examples:

1) 225 front, 255 rear when you approach the limit of traction, the front gently washes away so you instinctively let off the throttle and the extra grip provided by the bigger rear tires mitigates the "snap" oversteer. Instead the rear transitions smoothly into an easy to control slide.

2) 225 front, 225 rear the front gently washes away so you instinctively lift off the throttle which then throws the car into a "snap" oversteer situation since the fronts are now loaded with extra weight (more grip) and the rears have the weight shifted forward off of them which causes a spin unless you are a VERY experienced track driver and can handle it. Even then it's tough for the experts to catch too.

It's all fairly simple physics and vehicle dynamics. Having the engine sit over the rear axles has many benefits, but it also introduces a few handling quirks such as this. Most people haven't ever driven a MR car so they don't yet understand.

[Xusia]

What about 245 front & rear? That's far more likely (for me) than 225.

[bnr32jason]

You might want to check some other builds before you commit to a 245 front, it might not fit.

But the same thing will still occur, it's about the balance between front and rear grip.

[Xusia]

I get the idea of balance, but the outright amount of grip also applies. A 245 should provide more grip than a 225, raising the point at which the problem could occur. If the front also has a 245, then as long as we aren't exceeding rear grip, the car tracks, right?

[bnr32jason]

What you say has validity, but remember that grip is dynamic term. The transition from understeer or neutral steering to oversteer is still going to be much more violent when you do reach that limit. Or when you get nervous and lift off the throttle suddenly. We all like to say we would never do that and would keep our cool, but in emergency situations fight or flight takes over and we may or may not make the correct decision, even professional drivers are capable of doing it.

[C.Plavan]

Guys, guys, guys.... You all are over-thinking everything. Lets wait to people start really building/ driving the car to actually see what works.

[bnr32jason]

Well some testing has already been done by Road & Track. They confirmed that the development Factory Five did and the setup that was on the 818R for testing works very well. The transition into a "lazy slide" is a very good thing for amateurs like myself who want to visit the track and backroads for some fun. So we know what works already.

[C.Plavan]

Well some testing has already been done by Road & Track. They confirmed that the development Factory Five did and the setup that was on the 818R for testing works very well. The transition into a "lazy slide" is a very good thing for amateurs like myself who want to visit the track and backroads for some fun. So we know what works already.

So you plan on using Hoosier R6's on your car like they did in the article????

[Frank818]

Wow there's a lot of discussions going on here. lolll Maybe in the end it will make it as a sticky loll

Guys, guys, guys.... You all are over-thinking everything. Lets wait to people start really building/ driving the car to actually see what works.

And who crashes theirs lolll

[C.Plavan]

Wow there's a lot of discussions going on here. lolll Maybe in the end it will make it as a sticky loll
And who crashes theirs lolll

That's the truth, but the nut behind the wheel will be the biggest factor.

[Xusia]

We all like to say we would never do that and would keep our cool, but in emergency situations fight or flight takes over and we may or may not make the correct decision, even professional drivers are capable of doing it.

I ride a motorcycle with a scant 3.7 pounds per rear wheel hp (that's with me on it), and you are right. We do indeed like to think we would react correctly, but that's not always what happens. I know this from first hand experience, and can honestly say I'm lucky to be alive (for the record, I am a VERY well trained rider). knowing what can happen, and that despite good training you can react incorrectly, is why I don't drive, ride, ski, etc. near the limit. It's easier to prevent the situation, than to react appropriate to it...

[Wayne Presley]

bnr32jason, I drive lots of mid engine cars and none really "snap oversteer". You will spin any car that is set up mid corner neutral by lifting the throttle and not countersteering and getting back to neutral throttle. The Elise I drive at the track runs 205/225 tires and a 35/65 weight bias. The set up in the car is neutral mid corner and can be rotated by lifting the throttle when you are at the cornering limit. You give it just a little throttle and the tail comes right back in line. The 818R was run with 225/255 tires and a 42/58 weight bias. The set up in it was mechanically grip neutral, slightly aero tight and the car would come right back in line just by getting back to neutral throttle. Same on corner exit if you gave it too much gas, pull back on the gas a little, slight counter steer and the tail would come right back in line.

[Frank818]

I ride a motorcycle with a scant 3.7 pounds per rear wheel hp (that's with me on it)

I couldn't resist that one, but Xusia, 3.6 pounds of that 3.7 is you. lolllllll
Sorry dude I had to say it.

[Frank818]

The set up in it was mechanically grip neutral, slightly aero tight and the car would come right back in line just by getting back to neutral throttle. Same on corner exit if you gave it too much gas, pull back on the gas a little, slight counter steer and the tail would come right back in line.

That is AWESOMELY fun to drive!!

[wleehendrick]

bnr32jason, I drive lots of mid engine cars and none really "snap oversteer". You will spin any car that is set up mid corner neutral by lifting the throttle and not countersteering and getting back to neutral throttle. The Elise I drive at the track runs 205/225 tires and a 35/65 weight bias. The set up in the car is neutral mid corner and can be rotated by lifting the throttle when you are at the cornering limit. You give it just a little throttle and the tail comes right back in line. The 818R was run with 225/255 tires and a 42/58 weight bias. The set up in it was mechanically grip neutral, slightly aero tight and the car would come right back in line just by getting back to neutral throttle. Same on corner exit if you gave it too much gas, pull back on the gas a little, slight counter steer and the tail would come right back in line.

Thanks for chiming in Wayne... you're one of the few who can really contribute anything beyond pure speculation at this point.

By 'slightly aero tight', I assume you mean that the rear generates more downforce than the front, so that at speed the aero tend to shift the balance towards understeer?

One takeaway from your comment is that since the 818 is set-up to be neutral with a staggered set-up, this implies that with the same size tires all around, the handling bias will shift towards oversteer. Is the same applicable to the S? was is set up neutral with staggered tires? Or was a bit more understeer dialed in for 'safety'?

My only concern is that there are some rear/mid-engined vehicle with a known reputation for swapping ends (early 911s, 2nd gen MR2...). It sounds like the 818's suspension geometry and tuning is sorted well enough that this is not an issue, and the only oversteer characteristics are those which are unavoidable due to the laws of physics, as you describe!

[Xusia]

I couldn't resist that one, but Xusia, 3.6 pounds of that 3.7 is you. lolllllll
Sorry dude I had to say it.

LOL - I'm a lanky dude. 6'1" and 165 pounds.

[bnr32jason]

bnr32jason, I drive lots of mid engine cars and none really "snap oversteer". You will spin any car that is set up mid corner neutral by lifting the throttle and not countersteering and getting back to neutral throttle. The Elise I drive at the track runs 205/225 tires and a 35/65 weight bias. The set up in the car is neutral mid corner and can be rotated by lifting the throttle when you are at the cornering limit. You give it just a little throttle and the tail comes right back in line. The 818R was run with 225/255 tires and a 42/58 weight bias. The set up in it was mechanically grip neutral, slightly aero tight and the car would come right back in line just by getting back to neutral throttle. Same on corner exit if you gave it too much gas, pull back on the gas a little, slight counter steer and the tail would come right back in line.

Thanks for the input, from what you have written it has completely verified my suspicions that running the same size tires front and rear will cause problems due to the way the car is setup from Factory Five.

My experience on the track is mostly from when I was ages 7-19, starting in karts, then moving to Formula Ford, and ending in Formula Continental. I do have some experience tracking a Ferrari 308 and a mid 90's MR2. Both of these would easily go into a "snap oversteer" situation if you weren't careful, so that's what I'm basing my judgements on. The 308 was a car that was too expensive to mess with suspension tuning, so the cheapest way to change the handling characteristics was changing wheel and tire sizes. We went from the stock setup 205 front and rear to a 225 rear and the car became so much easier to drive it was ridiculous. The MR2 I had a little more freedom with and with an adjustable sway bar setup along with spring changes and tire changes I was able to get the car to handle very neutrally.

I haven't been fortunate enough to drive a modern mid-engine car like the Elise or a modern Ferrari sports car, but my experience with the old stuff is that mid-engine + same tire size F/R = disaster in amateur hands.

[Evan78]

To anyone that says that will not approach the limits of the car while driving on the street: skill, driving responsibly, looking ahead, and experience will help keep you out of trouble, but remember that you are operating in an uncontrolled environment. Are you comfortable with the setup of your car in the event that you need to jump on the brakes while coming around a corner or some other emergency avoidance maneuver?

[Xusia]

Hi Evan,

You say "To anyone" but I've been pretty vocal about that, so that comment seems directed at least partially at me. What you say is true, but what piques my curiosity is why you felt the need to say it in the first place. Have I somehow come across as immature, naive, or overconfident? I'd really like to know if I've somehow given off an incorrect impression.

[bnr32jason]

I don't think it's about coming off as immature, naive, or overconfident.

For me it's just hard to believe that someone would spend ~$20k building a Factory Five 818 but then say they aren't going to drive it like it's been designed to be driven. I'm not saying to push it to 95% of the limit on the street, or even on the track either. Something just isn't adding up, you've got a picture in your avatar of you on a ZX-6/10 pushing it decently hard through a corner on a public street, but you say you won't do the same thing in a car. It's just not adding up.

Think about it in motorcycle terms, in your avatar if you had come up to a disabled car stopped in the middle of the road mid-corner your first instinct is to let off the throttle which pushes you wide. Or you just go in a little too hot, an experienced rider stays on the throttle and leans in more, but our first reaction is to let off the throttle as well, which again pushes us wide. In 818 terms, especially one with equal size tires front and rear, pushing it the same amount through the corner as you are your avatar (around 65-70%) you end up spinning if you abruptly let off the throttle or even brake in an emergency situation because now the front tires have tons more grip than the rear which have been unloaded. So even though you weren't pushing it hard, an emergency situation still proves to be challenging.

But you said you were going to try to run wider tires on the rear if you could anyways, which is going to be the safest setup for handling these emergency situations.

[Xusia]

Ahh. I see the disconnect. You are associating my statements about limits and getting a preconceived notion of how I will drive. You don't really know me, so I think your impression is likely inaccurate. Not pushing the limits doesn't mean a Sunday stroll! LOL. It just means keeping a margin of safety (i.e. for the uncontrolled environment and such), which is below the car's limits and within my own personal limits (This is how I ride motorcycles too). I do plan to drive it aggressively on back roads and such, just with an appropriate margin of safety.

As far as my avatar goes, I had to laugh a bit when I read your comments. Bragging isn't my style, so I don't ramble on about how good I am or what my experience is, etc. As a result, you seem to have formulated an incorrect assumption as well as underestimated my riding skill (not surprising since we only know each other through this NON-motorcycle forum!): I'm not even close to pushing it in that pic (that was only at about 65 and I take that corner at 85 when I decide to ride closer to the limit). Seriously, I could taken a hand off the handlebar, turned my body completely around, waved to the guy behind me, and taken a sip of tea (if that were possible with a helmet on). I'm not even leaned over far enough to get my knee down. The only reason I'm even leaning off the bike is because it's fun. The photographer was a friend who wanted some action shots in his portfolio, so I thought I'd have a bit of fun while we were out and was just screwing around as I took various runs through the corner. That one just turned out to be a really good pic, so I use it as my avatar.

Not really related, but I thought I'd clarify something you stated incorrectly. I don't mean or want to offend you; I only want to ensure accurate information is presented in case there are other motorcycle riders reading this: Letting off the throttle doesn't push you wide. The front fork compression caused by using the front brake changes the bike's geometry and results in the bike wanting to "stand up" (the more forceful the braking, the more pronounced the effect). If you LET it stand up, THAT can cause you to go wide. If you maintain lean, however, you actually drop into the corner more, tightening your line. This can be dangerous, however, because braking while actively maintaining (or increasing) lean consumes additional front traction. And you don't want to exceed your finite amount of traction! That puts you on the ground and ends your fun.

Letting off the throttle causes engine braking which primarily affects the rear tire and shock. The front forks and overall geometry are not appreciably affected. So as long as the engine braking isn't enough to compromise rear traction, letting off the throttle causes you to drop into the corner more, tightening your line with minimal impact on traction.

Lastly, leaning more AND using the throttle is a bad idea in an emergency avoidance maneuver because it has a much greater likelihood of overcoming available traction than other options. Leaning/steering and accelerating both consume traction, so if you do more of them, you are in effect consuming more of your precious, finite traction (both front and rear). If you encounter an emergency situation and you need to tighten your line, the best response is to let off the throttle (but avoid excessive engine braking) and push on the inside bar until the you have achieved the necessary tighter line. This consumes the least amount of additional traction.

The following was taken from Sport Rider Magazine (link):

Your bike's trajectory or curved path through a corner is primarily determined by two factors: your speed and your lean angle. Increasing lean angle quite obviously tightens your cornering line while increasing your speed has the opposite effect. In other words, once you've set your lean angle you can tighten your line by simply reducing your speed or widen your cornering arc by increasing it; without ever changing your lean angle. Keeping your engine rpm between 60 and 80 percent of redline also enhances its throttle response, though the higher rpm also demands greater smoothness.

While I'm not as experienced with high performance driving as I am with riding, I am a sharp guy and quick study. And based on what Wayne said, the car is well mannered when being driven at less than the limit. So, I think I got this!

[blueoval_bowtie_guy]

When I build mine, I'm going to use the same size tires FactoryFive used. This is what the suspension was 'dialed in' with.

[bnr32jason]

While I'm not as experienced with high performance driving as I am with riding, I am a sharp guy and quick study. And based on what Wayne said, the car is well mannered when being driven at less than the limit. So, I think I got this!

I understand, and I didn't mean for this to become a war of words. It's just a simple fact that the car will handle differently and be more prone to oversteer if you decide to run the same size tires front and rear. Wayne's comments about the 818 throughout this forum apply only to the setups he has driven and track tested. If you take an 818 and start messing with the setup, everything goes out the window. It's simple enough to turn an 818 into a handling mess if things aren't approached the right way. Running equal size tires front and rear is step one towards making it a mess.

[DodgyTim]

The analogy with the sportbike and a 818 is a good one, they are both going to be too fast to have genuine fun with on public roads. Too many cops and distracted drivers for me. For $120 I can go to the track and pretend to be Valentino or Sebastan for the day!

[longislandwrx]

http://www.1010tires.com/Tools/Wheel-Offset-Calculator

Is a great tool for testing fitment.

[Frank818]

When I build mine, I'm going to use the same size tires FactoryFive used. This is what the suspension was 'dialed in' with.

You mean from the Open House?

[wleehendrick]

And you don't want to exceed your finite amount of traction! That puts you on the ground and ends your fun.

Been there, done that, front wheel washed out... four broken vertabrae.

I have no memory of several minutes before/after my accident, but looking at the scene afterwards, it was clear that I exceeded the traction limit of the front tire and I low-sided. I slid for a while, which would have been fine since I was in full gear, but then there's the guardrail. I always try to leave a significant safety margin on the street, but having no memory, I can't say what was responsible... did I come in too hot? possible. There was gravel in the corner that I may not have seen. I just don't know.

That experience is why I want my 818 to be as well balanced with as predictable handling as possible.

[Xusia]

...I always try to leave a significant safety margin on the street...
That experience is why I want my 818 to be as well balanced with as predictable handling as possible.

I hear ya and agree. I'm sorry to hear that happened to you. I had a similar experience in the past that has also shaped my thinking and behaviors. I wasn't injured, but the distinct potential of death was undeniable (had things gone another way, which they easily could have).

I think it's been forgotten, but my intent IS to run as wide a tire as possible on the rear, AND a narrower tire on the front. I understand that's the desired set up and what is best for handling. I would only run wider fronts IF I HAVE TO. Hopefully, I won't have to.

[wleehendrick]

I hear ya and agree. I'm sorry to hear that happened to you. I had a similar experience in the past that has also shaped my thinking and behaviors. I wasn't injured, but the distinct potential of death was undeniable (had things gone another way, which they easily could have).

Thanks... I'm fine now, but it does change your perspective.

my intent IS to run as wide a tire as possible on the rear, AND a narrower tire on the front.

That's my thought as well... just don't take it to an extreme, or you end up with this:

deltawing-takes-off-inline-turns-photo-460519-s-original.jpg

http://www.caranddriver.com/features...ce-car-feature

[Frank818]

Ok, so if we try to make this simple, can someone plz complete the following line?

The bigger the width difference (real surface contact) between front and rear tires, the ________________________________.

[wleehendrick]

less oversteer.

(although it's a bit more complicated than that)

[bnr32jason]

It's tough to simplify it THAT much, but to entertain your question:

"...the more the handling characteristics will shift towards neutral and eventually understeer"

But even that statement isn't wholly correct because there are so many different factors in play.

Track tested 818 with 225 from and 255 rear seems to be quite neutral. Make that a 225 front and 285 rear and then characteristics will shift more towards understeer.

[Evan78]

Hi Evan,

You say "To anyone" but I've been pretty vocal about that, so that comment seems directed at least partially at me. What you say is true, but what piques my curiosity is why you felt the need to say it in the first place. Have I somehow come across as immature, naive, or overconfident? I'd really like to know if I've somehow given off an incorrect impression.

No, you don't come across as any of those things. I just felt it would be beneficial if you had not considered all scenarios that you may want to. I agree with you that there's no issue with equal size tires if you never approach the limit of any of the tires and I don't have any trouble believing you when you say that you will not be driving "ten/tenths" at any time. If everything goes to plan, you'll be fine, but I feel that the plan neglects the unforeseen or you are simply okay with that compromise. We all make compromises, but I figured it wouldn't hurt to bring up this one in case you had not already considered it.

When BNR32Jason mentioned that traction control will not do anything to help with off-throttle oversteer, your reply was "A narrower tire up front isn't going to solve that problem, and a wider tire up front isn't going to cause it." I disagree with your statement. Even if you don't have enough traction to stop without running into something, would you rather go in whatever direction you're steering or would you rather have the back end come around and potentially take you off the road/mountain, onto the sidewalk where pedestrians are, into opposing traffic, etc?

You've recently clarified or reminded us that you intend to run wider tires in the front than the back, if feasible. Even with the same size wheels, you should be able to get at least 20mm difference in section width between them, perhaps more depending on how tall your sidewall is.

[bromikl]

Thanks for the input, from what you have written it has completely verified my suspicions that running the same size tires front and rear will cause problems due to the way the car is setup from Factory Five.
...We went from the stock setup 205 front and rear to a 225 rear and the car became so much easier to drive it was ridiculous...
I haven't been fortunate enough to drive a modern mid-engine car like the Elise or a modern Ferrari sports car, but my experience with the old stuff is that mid-engine + same tire size F/R = disaster in amateur hands.

Given this, I'm surprised FFR hasn't recommended running larger tires in the rear. Maybe they will.

The 818 will drive with the stock setup. But to drive aggressively (and stay on the road), we will need RaceLogic and wider rear tires. Anything else?

[Wayne Presley]

Given this, I'm surprised FFR hasn't recommended running larger tires in the rear. Maybe they will.

The 818 will drive with the stock setup. But to drive aggressively (and stay on the road), we will need RaceLogic and wider rear tires. Anything else?

FFR already uses a 30mm split between front and rear tires... 215/245 S or 225/255 R

If you use the recommended tire sizes and alignment specs, you will have a well balanced and fun driving 818. I've driven the car a lot and it is gentle when the tail comes out. The references to other cars that snap oversteer are just that, references to other cars and not the 818.

Do you need the traction control? No, would it be nice to have? Yes.

[Frank818]

FFR already uses a 30mm split between front and rear tires... 215/245 S or 225/255 R

If you use the recommended tire sizes and alignment specs, you will have a well balanced and fun driving 818. I've driven the car a lot and it is gentle when the tail comes out. The references to other cars that snap oversteer are just that, references to other cars and not the 818.

Do you need the traction control? No, would it be nice to have? Yes.

30mm split, ok great.

So a 40mm split would "theoretically" make it a little harder to oversteer?

[bnr32jason]

Yes, a 225/265 or 215/255 setup would be more towards neutral/understeer.

[Frank818]

Yes, a 225/265 or 215/255 setup would be more towards neutral/understeer.

Would you really feel the difference on public roads under spirited driving? I am not talking about race driving on public roads, just spirited driving.

[Wayne Presley]

Would you really feel the difference on public roads under spirited driving? I am not talking about race driving on public roads, just spirited driving.

Nope, not with the 818...

[Frank818]

Tnx Wayne.