Sidewall deflection and Rim\Tire size questions?

[SSLance]

While taking some video of my front suspension to make sure nothing was hitting, I noticed that my front tire sidewalls are deflecting under only moderate cornering...and this brought up a question of tire size vs rim width and how much deflection is okay before it is a detriment. So I thought I'd post it up here to see what you all thought about the issue.



First off, here is the video in question. You don't have to watch the whole thing, the deflection starts about the one minute mark when I turned onto a side road.

http://youtu.be/NjsVQ6ldiTg

The turns after were back and forth on a two lane road at about 45 mph or so...not what I'd call aggressive at all.

So what do you think? Does that tire need to be on a wider rim and a narrow tire on that rim for ultimate traction?

For reference, that is a 245/40/17 BFG Rival on a 17x8 rim.

I've been waiting for BFG to come out with a 275/40/17 Rival to put on my 9.5" wide rims out back...but I"m halfway thinking that the current 255 just might be wide enough given the deflection this tire has on the 8" rim.

[Rod]

I'm not sure what the question is? But the 245/40/17 on a 17x8 rim is the correct ratio, all tires have deflection, theres no standard amount of flex if that's what your asking, it really depends on the design the manufacture has done and what reinforcements they are using on sidewall construction

[Schwartz Performance]

What's your tire pressure?

[SSLance]

I had started my last autocross with 40 psi in them, and bled a few pounds off after each run before they stabilized at 40 hot...so there was probably about 36-37 psi cold in them at that time.

[SSLance]

I'm not sure what the question is? But the 245/40/17 on a 17x8 rim is the correct ratio, all tires have deflection, theres no standard amount of flex if that's what your asking, it really depends on the design the manufacture has done and what reinforcements they are using on sidewall construction

I hear the veteran autocrossers talk all the time about which tire size works the best on which rim size. They are mainly talking about stock size rims width on small cars with an inch or two variance in widths and 200 TW tires like the Rivals.

I guess with some tire designs, putting a tire on a rim an inch wider or narrower makes a huge difference in traction, ie: a 195 on a 7" is preferred over a 205 on an 8" with everything else being the same. I don't remember exactly the size of tires and rims in question but I've overheard the conversations several times...certain size tires of a certain brand work better on a X" wide rim than a Y" wide rim.

My question is, on the Rivals, who has tried different widths of tire on the same size rim and what works best. Along with that, have others seen the same sidewall deflection on their set up that I'm seeing on mine?

It almost seems to me that if the sidewalls are deflecting that much, it at some point has to be cupping the contact patch up in the center loosing traction there as well. Maybe not, but I don't know.

I know they stick like crazy to me compared to what I have had on there before, just wondering if it can be any better is all.

[Ron Sutton]

Some tire & wheel basics:
Whatever tire width you put on whatever rim width … there is a “happy window” of tire pressure to achieve a full & even tire contact patch on the pavement. Every tire sidewall acts like a spring and the spring rate is affected by tire pressure. More pressure = higher spring rate. All tires do weird things when you get them out of their optimum spring rate. Until just recently I had & utilized an Intercomp tire sidewall spring rater to test sidewall spring rates of different tire & rim combinations to find their “happy window” & to know at what point they got weird.

When you have the tread width wider than the rim width, you have to run lower tire pressure to achieve a full & even tire contact patch on the pavement ... otherwise you’re “crowning” the tire & using less than the full tread width. This lower tire pressure makes the tire sidewall spring rate “softer.” The softer sidewall COMBINED with the fat tire sidewall bulge, leads to the tire “moving around” on the rim substantially. This is ugly when the car is pushed hard, like in track days or AutoX, but provides a softer ride.

When you have the tread width narrower than the rim width, you have to run higher tire pressure to achieve a full & even tire contact patch on the pavement ... otherwise the tire tread goes “concave” … not utilizing the center tread … ending up with less dyanamic tread width. This additional tire pressure makes the tire sidewall spring rate “stiffer.” The stiffer sidewall COMBINED with little to no tire sidewall bulge, makes the tire substantially “more stable” on the rim. This combination performs best … BY FAR … when the car is pushed hard, like in track days or AutoX, but provides a harsh ride.

.

[SSLance]

Thanks Ron, that is interesting...and exactly the information I'm trying to learn about.



Here is the specs on the 245/40/17s I'm running on my 8" front rims

UTQG

Max Load

Max. Inflation Press.

Tread Depth

Tire Weight

Rim Width Range

Meas. Rim Width

Sect. Width

Tread Width

Overall Diam.

Revs Per Mile

200 AA A

1356 lbs.

51 psi

7.2/32"

24 lbs.

8-9.5"

8.5"

9.8"

9.1"

24.7"

841

So I'm running a tread width of 9.1" on an 8" rim.



I have 9.5" rims for the back, using your theory, these 245s would perform better on the track if I moved the 245s to the back and put 215/45/17s on the front rims.


215/45/17 specs

UTQG

Max Load

Max. Inflation Press.

Tread Depth

Tire Weight

Rim Width Range

Meas. Rim Width

Sect. Width

Tread Width

Overall Diam.

Revs Per Mile

200 AA A

1356 lbs.

50 psi

7.2/32"

21 lbs.

7-8"

7"

8.4"

8"

24.7"

844

Am I understanding this correctly? It goes against the normal grain of thinking to believe a smaller width tire will perform better, but in this case, I think it might be justified.

[SSLance]

I was taught early on in my autocrossing days to put shoe polish or chalk on the sidewalls of a tire and make a run to see how far the tire is rolling over and adjust air pressure accordingly. Here is the result of that from two events ago, Started the pressures out at 40 PSI and bled a pound or two out the first two runs and it leveled off at 40 PSI hot.



The polish wore off right at the edge of the tread blocks and I called it good at the time.

This was before I saw the video of how far the tire was deflecting under "normal" driving circumstance at near the same pressures.

[SSLance]

Okay, just a few more pics, then I'll sit back and listen.

These were taken at the same event where I ran the shoe polish above

[Paul Huryk]

There is always a tradeoff of sidewall deflection vs ride vs wheel width.

We always suggest to my customers that the sweet spot is to have a wheel that is about 1" more narrow than the section width of the tire. So if you have a 255mm tire (10.0" wide), you would want a 9" wide wheel to maximize it. BMW for example uses an 8" wheel to mount 225 width tires and a 9" wheel for 255 ones. That being said, the optimal wheel width for a 245 is 8.5". We nudge customers to run a 255 to 275 width tire on a 9.5" wheel.

I had a racer who bought a set of 9.5" wheels from me to replace the 8" ones he was running. He used the same 245-45-17 tires (were 95% new) and noticed a huge increase in sidewall stability and also turn in - do keep in mind that the old wheels weighed 10lbs more each than the replacements. He was super happy and dominated the field by seconds a lap until the trans gave out (during a 7 hour chump race).

Here is a pic:

[Ron Sutton]

Thanks Ron, that is interesting...and exactly the information I'm trying to learn about.



Here is the specs on the 245/40/17s I'm running on my 8" front rims

UTQG

Max Load

Max. Inflation Press.

Tread Depth

Tire Weight

Rim Width Range

Meas. Rim Width

Sect. Width

Tread Width

Overall Diam.

Revs Per Mile

200 AA A

1356 lbs.

51 psi

7.2/32"

24 lbs.

8-9.5"

8.5"

9.8"

9.1"

24.7"

841

So I'm running a tread width of 9.1" on an 8" rim.



I have 9.5" rims for the back, using your theory, these 245s would perform better on the track if I moved the 245s to the back and put 215/45/17s on the front rims.


215/45/17 specs

UTQG

Max Load

Max. Inflation Press.

Tread Depth

Tire Weight

Rim Width Range

Meas. Rim Width

Sect. Width

Tread Width

Overall Diam.

Revs Per Mile

200 AA A

1356 lbs.

50 psi

7.2/32"

21 lbs.

7-8"

7"

8.4"

8"

24.7"

844

Am I understanding this correctly? It goes against the normal grain of thinking to believe a smaller width tire will perform better, but in this case, I think it might be justified.

The challenge with this direction of thinking ... is that it assumes controlling sidewall deflection is a higher priority than the size of the contact patch. I don't find that to be true. Achieving the largest contiguous contact patch is the highest priority. That is why we did all that work on your front suspension geometry ... to get more of your tire's contact patch on the ground when the car is "dynamic" in the corner.

I would not go down in size to 215's. The 1-3/16" less contact patch would be a greater loss than you would gain from decreasing the sidewall deflection. Your 245/40/17s would perform better on 9" rims ... for sure. In fact 9.5" to 10" would be optimum. You would have optimum contact patch, less tread "squirm" and much less sidewall deflection.

For the rear, if you put 245's or 255's on the 9" rims, they would grip well. Maybe too well. They would grip better than your 245's do on the 8" front rims.

The next thing I am going to say will sound weird and go against conventional hot rod wisdom. The car would AutoX best with the same size tires front & rear (either 245's or 255's) ... with the 9" rims up front & the 8" rims in the rear.



The reasons are:

• The front of the car already has a higher weight bias than the rear ... and then when you brake & corner hard ... the weight transfer loads the front tires like crazy. This causes the monster sidewall deflection you're seeing ... and would benefit most from a stiffer sidewall set-up due to a wider rim than tire width combo.
• The slip angle would increase in the rear of the car ... with a moderate to high degree of sidewall deflection ... increasing the rear steer the slip angle already provides ... helping the car to turn better.

But the hot rodder in most of us can't bring ourselves to put a wider wheel on the front.

Are these your nice street rims?
What widths are the rims your slicks are on?

[Ron Sutton]

I was taught early on in my autocrossing days to put shoe polish or chalk on the sidewalls of a tire and make a run to see how far the tire is rolling over and adjust air pressure accordingly. Here is the result of that from two events ago, Started the pressures out at 40 PSI and bled a pound or two out the first two runs and it leveled off at 40 PSI hot.

The polish wore off right at the edge of the tread blocks and I called it good at the time.

This was before I saw the video of how far the tire was deflecting under "normal" driving circumstance at near the same pressures.

I do this same thing ... a different way. I take tire crayon and color up a rectangle patch on the edges of the tires. it shows the same thing. Where the color getting worn off ... the tire is using that part of the tire.

I use this as one of many guides for dynamic camber. But it's doesn't tell the whole story on sidewall deflection. Your camera does that. And the sidewalls on your front tires are deflecting a LOT.

.

[Ron Sutton]

I had a racer who bought a set of 9.5" wheels from me to replace the 8" ones he was running. He used the same 245-45-17 tires (were 95% new) and noticed a huge increase in sidewall stability and also turn in - do keep in mind that the old wheels weighed 10lbs more each than the replacements. He was super happy and dominated the field by seconds a lap until the trans gave out (during a 7 hour chump race).

This is exactly what we see in racing too. The tire has more grip and is responds quicker on initial turn in.

[SSLance]

The next thing I am going to say will sound weird and go against conventional hot rod wisdom. The car would AutoX best with the same size tires front & rear (either 245's or 255's) ... with the 9" rims up front & the 8" rims in the rear.



The reasons are:

• The front of the car already has a higher weight bias than the rear ... and then when you brake & corner hard ... the weight transfer loads the front tires like crazy. This causes the monster sidewall deflection you're seeing ... and would benefit most from a stiffer sidewall set-up due to a wider rim than tire width combo.
• The slip angle would increase in the rear of the car ... with a moderate to high degree of sidewall deflection ... increasing the rear steer the slip angle already provides ... helping the car to turn better.

But the hot rodder in most of us can't bring ourselves to put a wider wheel on the front.

Are these your nice street rims?
What widths are the rims your slicks are on?

Not the first time this has been suggested to me, in fact...I've already test fit that scenario and it works...




The ZQ8 wheels my slicks are on are 16x8s with 4.25" back space all four. The A6s I run are 275/40/16s.

[SSLance]

Are these your nice street rims?

The nice street rims are 17x8 fronts and 17x9.5 rears

They were special ordered to get the backspace just right to fit that large of a tire in the stock rear wheel house. Any other rims of a wider width would also have to be special ordered to fit the car. Read... $$$

I don't think the ZQ8 wheels are an option for Rivals either as there just aren't any rivals in 16" sizes, 225/50/16 with a tread width of 8.5" is it.

[Ron Sutton]

The nice street rims are 17x8 fronts and 17x9.5 rears

They were special ordered to get the backspace just right to fit that large of a tire in the stock rear wheel house. Any other rims of a wider width would also have to be special ordered to fit the car. Read... $$$

I don't think the ZQ8 wheels are an option for Rivals either as there just aren't any rivals in 16" sizes, 225/50/16 with a tread width of 8.5" is it.

Gotcha.

Just for everyone's clarity .... I'm not making a wheel or tire size suggestion. I'm just sharing how this stuff works.

But ... if ... if ... if you go with 245's for the rear ... it would interesting to simply swap front & rear wheels at the track some day.

[SLO_Z28]

What you are seeing is normal and is related to the construction of the tire. BF Goodrich wont tell you anything, because the entire industry is like that, but it has to do with carcass construction and gaining available grip over a larger slip angle. A comparable tire would be the Direzza Z2 and while it offers a higher peak torque it falls off much faster at higher slip angles, because of the construction, and wont deflect like the BFG. For a setup like yours the BFG is a clear winner this category.

As far as tire fitment on wheel width, there is a big difference between manufacturers on this one. Some manufacturers run wide on any given size, some run smaller. I cant speak for BFG, but a 235 on a 8 is about the norm. A 275 on a 9.5 should also work the best.

[CA B4C]

This was taken at optima in 2011

This was taken at RTTC in 2012

From the photos you posted, I'd say you are not to far off ... I'll let the car speak for itself

[Craig510]

These are my Star Spec Z1's in action 255/40-17 on 17x9's. I had some 225/50-16's on 16x7's but they were not sticky and rolled over like pigs.




image by Craig 510, on Flickr


image by Craig 510, on Flickr

[SSLance]

Gotcha.

Just for everyone's clarity .... I'm not making a wheel or tire size suggestion. I'm just sharing how this stuff works.

But ... if ... if ... if you go with 245's for the rear ... it would interesting to simply swap front & rear wheels at the track some day.

What I'm leaning toward after much discussion, is sticking to my original plan of waiting until BFG comes out with a 275/40/17 Rival and putting those on my 9.5" rear tires and running them.

What I will do though is, next spring when our club runs a test N tune day at the track, Run several laps with the tires in the current staggered position, then swap them to put the 275s on the front and the 245s on the back and compare times and feel.

Or possible settle for the 255s for the 9.5" rims, we'll see.