Turning radius impact with 18x9 wheels and steering rack spacers

[Brian76]

I have purchased FR500 wheels for my build. The FR500 front wheel has a +30mm offset compared to Halibrand +24mm offset. I decided to delay purchasing tires until I can confirm clearances. I created cardboard cross sections of 245-40-R18, 245-35-R18 and 255-35-R18 tires and placed them on the wheel. It appears that none of the tires will allow full steering rack travel without touching the F panel. The car is on jack stands so its not a completely accurate test.

I shimmed behind the wheel 6mm to simulate Halibrand wheels. I noticed a small improvement in rack travel (steering angle) before contacting the F panel, but not significant.

The 245-35-18's will allow the most steering rack travel before contacting the F panel. I know I can add steering rack spacers to the rack to prevent rubbing, but I'm curious if the reduction in turning radius will impact everyday driving? I can't tell by just looking at it.

Do these cars still turn on a dime even with 1/4" spacers on each side of the rack or do I try to eek out every degree of steering angle I can get?

[Big Blocker]

Hopefully you'll never have the need to crank your wheels to full lock to maneuver into a parking spot . . . these cars have a pretty good turning radius and you shouldn't really need to worry about another 2 to 3 degrees of angle for normal street driving.

Not knowing your lock-to-lock turns, or if you have PS or not, it doesn't take much to switch lanes on a highway, make a right or left turn thru an intersection, pull into a driveway . . .

when all else fails, turn the wheel in the direction you want to go and punch it . . . just kidding.

Just my 2¢

Doc

[Brian76]

when all else fails, turn the wheel in the direction you want to go and punch it . . . just kidding.

Just my 2¢

Doc

LOL, I like that option!

I do have the FFR power steering. I took some measurements with a protractor. Results are attached. Comparing full locked position to a 245/35/R18 tire I'm only sacrificing 5 degrees of turn radius. Full lock to a 245/40/R18 I give up 12 degrees. My method is not scientific but it helps me understand how tire size and offsets impact turn radius. Unfortunately I don't have real world driving experience in a cobra to know if 12 degrees makes any difference when making tight turns. Based on your experience it seems to be a non-issue.

116628

[CraigS]

I wouldn't worry about it at all. The only time it will be noticed is in a parking lot or driveway. Looking here at the dimensions for your sizes (scroll down a little and clicks 'specs'. (I picked these because we have been using them on my wife's Mustang-other brands may have slightly different specs)
https://www.tirerack.com/tires/tires.jsp?tireMake=Continental&tireModel=ExtremeContact+DWS+06&partnum=435YR8DWS06XL&vehicleSearch=false&fromCompare1=yes
it is obvious you don't want the 245-40 due to it's diameter and also it gives up some tread width compared to the 245-35.
The popular 255-35-18 gives you .6 more tread width and only .2 in larger OD. Later you can install spacers if you like. BTW what tires are you looking at? The range gets pretty narrow when you look for a 315-30-18 for the rear.

[Jeff Kleiner]

I wouldn't worry about it at all. The only time it will be noticed is in a parking lot or driveway.

Exactly. Virtually every car I get in here whether on 15s, 17s or 18s will kiss the splash panel at full lock but you'll never reach that point unless you're trying to do some really tight parallel parking.

Jeff

[Brian76]

I wouldn't worry about it at all. The only time it will be noticed is in a parking lot or driveway. Looking here at the dimensions for your sizes (scroll down a little and clicks 'specs'. (I picked these because we have been using them on my wife's Mustang-other brands may have slightly different specs)
https://www.tirerack.com/tires/tires.jsp?tireMake=Continental&tireModel=ExtremeContact+DWS+06&partnum=435YR8DWS06XL&vehicleSearch=false&fromCompare1=yes
it is obvious you don't want the 245-40 due to it's diameter and also it gives up some tread width compared to the 245-35.
The popular 255-35-18 gives you .6 more tread width and only .2 in larger OD. Later you can install spacers if you like. BTW what tires are you looking at? The range gets pretty narrow when you look for a 315-30-18 for the rear.

Thanks guys. I've attached the tires and sizes I'm looking at. Sorry...I'm an excel nerd. I'm mostly looking for 245 or 255 up front and 295 in the back. The FR500's in the back have a +12mm additional offset to the halibrands AND is only 10" wide so I'm a little nervous going with 315. I'll have to add the Continentals to my list!

116634

[CraigS]

I always get lost w/ offset specs. Do you have back space specs? I am thinking the 1/2 in less width may make up for the offset. IMHO, 315r35x17s have been the most common rear size by far for at least 10 yrs. That 315 size may not be perfect on 10x18 in wheels but it is close enough.

[Brian76]

I always get lost w/ offset specs. Do you have back space specs? I am thinking the 1/2 in less width may make up for the offset. IMHO, 315r35x17s have been the most common rear size by far for at least 10 yrs. That 315 size may not be perfect on 10x18 in wheels but it is close enough.

The FR500 18x10 backspace is 6.3". I think 18x11 Halibrand is 6.46".

[CraigS]

The FR500 18x10 backspace is 6.3". I think 18x11 Halibrand is 6.46".

So that means that the 315 tire will fit fine inside the fender. I'd run them in a heartbeat.