RDX_NRG

Hey guys,

If you would give me your opinions and thoughts on this..

I am debating between two tire sizing options:

1) 245/45/20 with an offset of 20

or

2) 255/45/20 with an offset of 40

The stock OEM tires and rims provide the RDX with a Static Stability Factor (SSF) of 1.26. The formula for SSF is SSF = T / (2*Hcog) where T is your track width and Hcog is the height of the center of gravity.

From my calculations, it looks like the first will produce a SSF of 1.28 while the second option will produce an SSF of 1.25

However, since the second option are wider, i am assuming this should give more grip than the first. I guess my question is: does anyone know what the relation is between SSF and lateral grip? Is there any at all or does SSF only have a relation with roll-over safety? or which tire option 1 or 2 do you think would give more lateral grip (now that SSF is taken into consideration)?

Also, the first option would stick out 30mm (each side) over the stock while the second option only sticks out 15mm (each side) over stock. I plan to put Eibach springs on my vehicle and i'm concerned that the first option will rub against the fenders? what are your thoughts on this?

Thanks for your thoughts guys!

BigHatch

the wheel width and offset will determine what sticks out more than the tire itself.

255/40/20

45 will be too tall..issues using your spare.

rohit.

I'd run a 255/40 like BigHatch said and go with the +40 offset wheel; +20 is a bit low for the RDX. How wide is the wheel?

RDX_NRG

The wheels i would use in both cases are 8.5" wide.

The reason why i'm debating between the two sizes in the original post is because 255 / 40 tires are much too expensive and there is lack of selection from discounttiredirect.

so my choices are:

Option 1) sticks out too much due to +20 offset

Option 2) is a little too tall @ 3.2% taller than stock

which is the lesser of two evils? lol

I plan on getting a set of Eibach's too.

BigHatch

well my 9 inch +40's still sit in just a hair. Some might use a 10-15 mm wheel spacer to push em out just a bit more...

1"= EXACTLY 2.54 cm.

An 8.5" rim would remove 1/2 total or 1/4" from each side = 6.35mm from each side. the +20 would push em back out 20mm, so the ~6+ mm, plus the balance, or 13.65mm.

InfamousJamous' 20x10 +40's sit about as perfect as I've seen. That 12.7mm farther out than mine...You're probably gonna be ok with the 0.95mm difference. I cant speak for IJ, but I don't recall him ever mentioning any rub. I certainly don't have any. We both have eibach's.

I used to have 255/40's, but yes, selection was very limited. I'm very happy with my 275/35's now. ~$1200 for the tires this time around at America's tire, but great ratings and much quieter than my last set (Toyo Proxes STII). They are a hair shorter than the 255/40 - 27.7 vs 28.03

That might be the magic formula. I would have liked my wheel design in the +20.

RDX_NRG

I've always thought it's the tire width that will determine how much stick out you have. Is this a wrong assumption? is it the width of the rim that determines how much sticks out?

According to this thread on acurazine, it shows that the amount of stick out is due to the tire width.
https://acurazine.com/forums/1g-rdx-tires-wheels-suspension-362/wheel-size-chart-783651/

ie: since you have a 275 wide tire, that should be the measure of how much it sticks out because i think your tires bulge over your 9" wide rims.

BigHatch

The tire width will only determine anything in relation to the rim itself.

The width of the rim will obviously be the main thing to consider when thinking of how much space is under the wheel well side to side.

The offset of the wheel is the second component of the what will fit question. The section of any wheel that bolts on to the hub is not "one" standard. (+20, +40, etc.. ).

Think of it this way. The wheel has two parts.. the hoop and the face. Where do the two connect? All the way at the outside of the hoop? At the inside? It's actually varying places near the middle. +40 means 40 mm towards the outside (less lip on the hoop), while +20 would be closer to the center. As the face is stationary against the hub, you could imagine the hoop "sliding" further in or out depending on where (in relation to the center of the hoop) it attaches to the face.

RDX_NRG

Thanks for all that info BigHatch. After a bit more thinking and research and you're quite right. Rim width determines the protrusion/intrusion more so than tire width.

Now i have a different dilema.. WEIGHT of wheels. The stock 18" rims and tires weigh around 55 lbs. I've read that in this forum somewhere. So now i have two choices..

245/45/20 with TSW Interlagos (20 x 8.5 with 20 offset) - total weight of combo is 51.25 lbs. This combo is 3.75 lbs lighter per corner than the stock.

or

275/40/20 with TSW Interlagos (20 x 10 with 40 offset) - total weight of combo is 55.7 lbs. This combo is 0.7 lbs heavier per corner than the stock.

Both of these options will lead to the tires being flush with the fenders, so aesthetically, i think they are almost equal.

Now, in your opinions, would the lighter weight be more beneficial in terms of handling/ride comfort (even maybe more grip than the 275 as lighter wheels stay in contact with uneven road surfaces better than heavier wheels) or would the wider wheels be more beneficial for handling due to the 30mm width difference (as there is almost a negligible weight gain from the stock of 0.7 lbs)? Both sizes are going to be with the same tires, Hankook EVO V12.

BigHatch

Nice looking wheel.

The lighter weight will prob do more for you performance wise. I'm sure you'll have plenty of grip for everyday driving.

I'm assuming you've calculated the overall diameter on that 245/45. You might like the look of the deeper bend in the lug area too.

Triple check everything... You don't want to be wrong on this big of a buy. I can only speak for my 9" +40's. This was with 255/40, and already lowered:

rdxjockey

hey Hatch, that pic is with a 255 or 275? Im confused by what you have posted in your specs 275/35/20. Thanks!

BigHatch

pic was with my old tires with 255. I currently have 275's. I'll try and get a new pic.

BigHatch

Here's a couple with my current 275's. More pics of em in my albums, including one looking up from within my rim on the LF.

Here's LF:



and LR:

RDX_NRG

thanks for the pictures bighatch.. really helpful.

i'm still torn between rims that are either 20" x 8.5" wide with 20 offset or or 20" x 10" wide with 40 offset.

i'm concerned that the the 8.5 with 20 offset would hit the flanges in the rear wells while doing a hard corner due to body lean.

RDX_NRG

I'm also concerned that the 20 offset would hit the brakes/callipers.. any thoughts on this?

BigHatch

I think there's actually more room to spare in the back. I've never hit anything at all under cornering.

I don't imagine you'd hit the caliper. The hoop is what would "move" farther out, On that wheel design it wouldn't change the shape of the spokes. The angle maybe, but all the pics I saw only showed a lil deeper bend in the lug area with the outer end of the spokes remaining connected at the face, and pretty much the same angle of the spoke. I may be mistaken, so yes you should check the actual +20's. I dont recall seeing any with a lip. The center part at the hub may just be thicker. Check all possibilities before settling.

BigHatch

You may have plenty of vertical room too.

This was with the 0.13 inch taller 255/40 too.

andygirl

^ love. I have yet to see any rdx in all of florida modified properly.

andygirl

I have stock 19's with yoko avid envigor. I want to go wider, but I am not as savvy in all this tire speak. What is the widest i can get on there and still have it look proper?

Mr Marco

OP, it's an RDX, it's way too top heavy to take the "static stability factor" into consideration. Just go with the advice from Hatch and you'll be fine.