handling 17"-->15"

[szw]

will I be seeing big handling differences (and positive or negative!) going from

17" rims, 225/45/17

to

15" rims, 205/50/15

(assuming I'm going for a better tire...from continental contisport to either falken azenis or kuhmo ecsta supra 712)

[THREE40SEVEN]

yup, its gonna suck in the corners compared to the 17's. I saw an article where a guy picked up 6 mph in the corners(no idea what course) from going from 16's to 17's. Thats condisderable..

[MrHeavyFoot]

I know the conventional wisdom is bigger=better, but in the autocross world, everything I've read indicates go for the smallest wheel and tires you can.

I don't fully understand it myself. I know its partly to reduce unsprung weight and rotational momentum of the wheels/tires. This is good for acceleration/braking.

I guess going with the smallest tire you can will reduce the amount of sidewall so I guess that also regains some of the benefits of a larger wheel. Of course, you'll also end up with a smaller overall diameter. Something which isn't so bad on the track, but not something you'd want to do for street use.

I dunno, just trying to pass on/interpret what I've read. I'm sure someone will point out the errors.

[speedracer]

Will you have problem with clearance? (Such as the caliper)

Some benefits that I can see:
Quicker accerleration (Less rotational mass)
Slowing down will be easier
Cheaper tires

If you maintain the overall diameter of the tire
eg. xxx/65R/xx
o turn in will be less resposive
o ride quality will be softer
o speedometer relativley unchanged

If Not
eg xxx/50R/xx
o speedometer will be off (that also means mileage reported)
o ride quaility about the same as the 17" or a little harsher
o turn in responsive increased (or the same as the 17")

[Speed_Dreams]

I read in SCC a while back where they tested three sizes of rim 15', 16' and a 17' in the same rim with the same tire and they got the fastest 1/4 mile time and lap time with the 16' rim....hope that helps

[tegdream]

Originally posted by Speed_Dreams
I read in SCC a while back where they tested three sizes of rim 15', 16' and a 17' in the same rim with the same tire and they got the fastest 1/4 mile time and lap time with the 16' rim....hope that helps


that doesnt make a whole lotta sense...but it also depends on the weight of the rim and what size rims came from the factory

[hinggu]

Originally posted by MrHeavyFoot
I know the conventional wisdom is bigger=better, but in the autocross world, everything I've read indicates go for the smallest wheel and tires you can.



My buddy who is in the bcmr2 club in Vancouver autocrosses and he stuck with 15" when he bought rims. He told me that if the rim is too big, it's not very good for autocrossing. I have no idea why though

[midship]

Actually I think you have to run small rims and Lower profile tires to see the benefit.

1 - lowering the center of gravity
2 - better gearing ratio for acceleration
3 - cheaper tires
4 - weight saving

but then check this out

http://www.grassrootsmotorsports.com/plustest.html

for the same diameter of tires, 17" runs faster than 15"

Need some autoX expert in these thing! :P

[kevie88]

It just makes sense really,smaller rims have less inertia, less gyroscopic effect, less weight, less unsprung weight, cheaper tires (letting you go with stickier tires for less money) ect ect ect. Even if the rims weigh exactly the same in a 17" as they do in 16", the 16 will still have different (lower) inertial and gyroscopic forces.

I really feel that anything over 16 will adversly affect all aspects of performance (unless you have absofrikinlutely huge brakes). Big rims sure do look great tho!!

[MrHeavyFoot]

Originally posted by midship
Actually I think you have to run small rims and Lower profile tires to see the benefit.

1 - lowering the center of gravity
2 - better gearing ratio for acceleration
3 - cheaper tires
4 - weight saving

but then check this out

http://www.grassrootsmotorsports.com/plustest.html

for the same diameter of tires, 17" runs faster than 15"

Need some autoX expert in these thing! :P

Good article. I guess it makes sense that the lower sidewalls and wider tires on the 17" provide better handling, where as acceleration and braking would be somewhat compromised due to the weight/rotational mass.

So the best bang should come with smaller wheels and lower than normal profile tires (and wider). I've gone with a set of 15x7 wheels and 225/50/15 tires for autox this year. Stock is 15x6 and 195/65/16 I think. The overall diameter is slightly less than stock (~5%) so hopefully that will help. I could have gone with 225/45/15 but I wasn't completely comfortable going that small (~10% smaller than stock). Maybe next time.

[szw]

Originally posted by midship

http://www.grassrootsmotorsports.com/plustest.html



I dunno, I just don't like that article.

I know its pretty old, and was a 'cheap' test to do.

But it doesn't seem very accurate..a lot of subjective things "feels faster"...(I haven't read it in a while) but I think they also had different rim/tire combinations for each size right?

Does anyone have that Sport Compact Car article where they did the same kinda thing? Except they used a Mitsubishi EVO and a pro driver, who was 'blind' to the set of wheels he was using at each test trial?

I'd like to read it again, but have lost my issue.

There's a while EVO on the cover, I think the rest of the cover is kinda peachy or something.

[XylathaneGTR]

One other benifit of smaller rims:
It takes less power to drive the wheel, and the wheel actually spins faster, since the rim weighs less, Less horse and torque is required to spin the wheel, and the centrifical force is less. Less power required to actually spin the rim, means more power can get to the ground.

I was watching HorsepowerTV on TNN once (boring saturday afternoon :( ) And they had a project Si going, for some reason. And they installed big BlingBling 17's...And when they dyno'd it, the whp output dropped by like, 10 horse...running the exact same setup as before the installation

Sounds like you've made a good chocie w/ your tires Heavyfoot.

[MrHeavyFoot]

Originally posted by XylathaneGTR

Sounds like you've made a good chocie w/ your tires Heavyfoot.

"Times" will tell :)

[Smartiepants]

Well, rally cars run huge rims, for stiffness sakes

[GTS Jeff]

Originally posted by XylathaneGTR
centrifical force no such thing.

[szw]

no such thing.

There is in Jr High!

[midship]

Originally posted by szw



I dunno, I just don't like that article.

I know its pretty old, and was a 'cheap' test to do.

But it doesn't seem very accurate..a lot of subjective things "feels faster"...(I haven't read it in a while) but I think they also had different rim/tire combinations for each size right?



Same tires and rims combination. Only different is the 14", cos the tire was not available in that size. IMO grassrootmotorsport test much more reliable than SCC test. Just my 2 cents.
:)