mhax

I have a set of R888 tires installed on 15" RPF-1 wheels with a 41mm offset. I installed the rear wheels and checked the fitment. The tire is quite close to the trailing arm. Actually, there is about 1/4" of space between the trailing arm and the tire (at the closest point).

Now, I know that the tires move a little when cornering. Would the 1/4" space be enough or will the tire touch the trailing arm during heavy cornering?

Tight-R

Only one way to find out. Not sure how anyone could predict how far your tire will move under cornering. If they hit, get some 5mm spacers.

Tight-R

And I wasn't trying to come off as being arrogant, just FYI!

TunerN00b

This is the truth.

The amount of pressure ran and the traction developed on the surface you run on will be different than someone else on the same tires, which will cause a different amount of tire deflection.

OP you're just going to have to try it and see.

mhax

But, wouldn't it be dangerous for the tire to try it on the track? Also, I'm not sure if I could hear it on the track (engine + other track sounds). Would it results in vibration?

I know I was able to hear a tire hitting the inner fender, but I have the feeling that the trailing arm, which is a solid piece of metal is not that forgiving for a tire rotating at full speed.

mhax

You are right! I know the best way would be to try them on the track. Adding a 5mm spacers would result in a space of around 1 cm between the trailing arm and the tire.

Xian

With 1/4" you should be fine... I'd wager that the tire will contact the RTA some and "polish" it under heavy cornering load. This is common nearly any time you shove a big wheel/tire combo in the back and something that I've seen on several EF's as well as a couple DC's that I've owned.

PS
I've tried the R888's in the past and wasn't a fan. YMMV and all that but I didn't think they were that much better than the current cop of street tires (Rival/RS3/Z2). If you're bound and determined to run a second tier R-comp, maybe check out the NT-01 next time.

mhax

I had a nice deal for 2 sets of used R888, so I grabed them. Could I just grind some part of the RTA?

I tried to find some formula for calculating the lateral deformation, something similar to this :

,where the value "e" is what I'm looking for.

B serious

iTrader: (1)

Stop doing equations. There are quite a few variables to account for.

1/4" should be fine. Check the RTA after a few laps. Your tire isn't going to explode upon touching the RTA. It will take hundreds or thousands of repeated contacts to wear thru.

mhax

Hehe, I kind of like equations and physic. I found an equation for determining the tire stiffness:
K [lb/in] = Max load (lbs) / ( (Overall diameter / 2) - Static Loaded Radius)

I found these specifications for the R888:

225/50R15
Max load = 1356 lbs
Overall Diameter = 23.7 in
Static Loaded Radius = 10.9 in
So K = 1427.368 lb/in (this assumes max pressure)

225/45R15
Max load = 1201 lbs
Overall Diameter = 22.8 in
Static Loaded Radius = 10.5 in
So K = 1334.444 lb/in (this assumes max pressure)

After that, I supposed that the lateral load (force) would have values between 300 to 900 lbs during cornering.

So at 900 lbs, the maximum deflection would be 0.63 in for the 225/50 and 0.67 in for the 225/45. So considering that my tires are at pressure lower than this, I wouldn't be surprise to see a deflection of 1 inch.

But it is just an approximation, I'm no tire engineer and I'm not 100% sure my calculations are exact (maybe someone can help)? For example, I would have expected a lower deflection for the 225/45 compared to the 225/50.

B serious

iTrader: (1)

Deflection of 1" at the contact patch. Not at the RTA...which is about 60° away from the contact patch. You'd have to find out how much deflection there is there.

Bushings/wheels also deflect. Are you calculating cornering load at the contact patch? Or at the LCA? at the wheel? Somewhere else on the chassis?

Find that...then try to equate TIRE deflection at the RTA.

Or just go drive it.

Xian

**** me. Seriously? LOL.

And that ^^^^^. Eleventy Seven Brazillion guys have run similar wheel/tire combo's before... you'll be FINE. Now put down the calculator and go drive it...

mhax

Indeed, I would approximate a 1" deflection at the contact patch. The resulting deflection at the RTA would be much lower. I don't know how to evaluate that yet. Also, I think my equations assume an evenly distributed weight on each wheel, which is not the case on the DC2. So it means that the deflection will be more important on the front wheels.

mhax

I know you are probably right. Don't worry, I'll track the car as is. I'm just curious about everything that is car physic related, that's all. The tire physic and design is quite complex and I've learn quite alot just by trying to find out how to calculate de deflection.

Xian

Yep, there's (obviously) a ton that goes into tire design... and your deflection will be dependent upon the total weight being transferred laterally across the rear plus the coefficient of friction of the tire (variable depending on temp and age) and then depenedent upon the surface and external conditions. Oh, almost forgot tire size relative to wheel width! Anyway, trying to calc something like that in a vacuum just isn't worth the skull sweat that goes into it.

You're right that the front will deflect more... this is at parking lot speeds/grip levels:



And a better shot of rear deflection:

B serious

iTrader: (1)

Look at it like this:

You're never going to find the deflection in every scenario because there are literally infinite scenarios.

Weight distribution in that car is 60/40 STATIC. That's not dynamic. There could be more or less than 40% on the rear axle during cornering. That could change every millisecond.

G force spikes are also impossible to predict. A pebble on the track surface affecting your instantaneous G force is an unpredictable factor, for example.

Production design principles say that you should design for the worst predictible situatuon and then multiply a requirement by 3 (factor of safety). Good luck with that on a track car.

Simply put, heres what you're trying to derive:

Pure lateral deflection OF THE TIRE AT THE RTA. Based on all forces present.

X amount of force is required to flex the tire X amount.

But....if you apply that much force at the tire, what else moves around. X deflection of the tire means X deflection of the bushing. Another component is deflection of the wheel vs. The tire, twist of the RTA vs. The tire, etc.

The tire is not the only thing absorbing the load.

To get any of those things, you will need insane amounts of actual data. Even then, you won't be totally accurate.

You can throw in the earth's rotation if it makes you happy.

Only way to know for sure is to use it. Nothing wrong with flexing brains. But don't count too heavily on any of those calculations.

mhax

I'm mostly trying to get an approximation, I'm clearly not trying to calculate the exact value. Actually, I'm would like to find the maximum deflection possible in the worst case (which would be a good information to know and still an approximation).

361 accord

color your trailing arm with crayon / grease pencil and go rip on the car. see if your markings got rubbed off

the tire deflects mostly at the bottom anyways, not all the way up the tire

mhax

That's a good idea!

Runnerdown

1/4 is plenty. I'm closer than that on mine. 15x7.5 40 offset 225/45/15 r888. Plus paint mark etc are useless. If it rubs the trailing arm the wheel will be very obviously scratched and damaged.

mhax

I used this to compare both setup:http://www.rimsntires.com/specspro.jsp

Your setup - 15x7.5 40 offset 225/45/15 r888
My setup - 15x7.0 41 offset 225/45/15 r888

You have a backspace value of 149 mm compared to 143 mm on my setup. 6 mm almost correspond to 1/4 inch (6 mm = 0.24 inch). So I believe you when you say that you are closer to the RTA with your setup.

Runnerdown

I was almost sure that they would rub with some hub deflection but they never did. I am swapping them into my newer gsr I bought and will probably clearance them just to be 100% sure they never can. As it is right now I may be able to slip a credit card into the gap but not much more.