rim weights versus tyre (tire) weights
07-29-2010, 10:57 PM
#16
Rennlist Member
ohnKoaWood - I'd be interested in the actual weights of your replicas. Factory Boxster 17's are light rims. The factory hollow 18's are pretty much the same weight which was an accomplishment itself. If your replicas are actually lighter than factory boxster 17's... that's a seriously good find.
07-29-2010, 11:35 PM
#17
Burning Brakes
This might be a little off topic but maybe not. I am wondering how big of a difference the ride would be going from 16" rims to 17" or even 18" rims. Maybe some one who has gone from the stock rims to the 911 turbo 18" with 8" fronts and 10" rears could chime in on the ride and performance changes.
07-29-2010, 11:55 PM
#18
Burning Brakes
When my car was both street and track, I would switch between 16's and 18's. 16's for the street, 18's for the track. I also switch between 16's and 18's during race weekends.
For test and tune I typically run 16's, and for qualifying/race 18's. Lap times are very similar between 16 and 18.
On the street the 18's are much less forgiving with less rubber to absorb potholes etc. From my experience, 18's are much more fragile than 16's, easy to knock out of true or crack. I have experienced at least 3 cracked 18's, all OEM wheels, conversely I have never seen a cracked 16. Again From my experience you feel a lot more with the 18's, which is why with a track set up, the 16's made street driving more tolerable for me.
For test and tune I typically run 16's, and for qualifying/race 18's. Lap times are very similar between 16 and 18.
On the street the 18's are much less forgiving with less rubber to absorb potholes etc. From my experience, 18's are much more fragile than 16's, easy to knock out of true or crack. I have experienced at least 3 cracked 18's, all OEM wheels, conversely I have never seen a cracked 16. Again From my experience you feel a lot more with the 18's, which is why with a track set up, the 16's made street driving more tolerable for me.
07-30-2010, 12:07 AM
#19
Bannana Shine
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When my car was both street and track, I would switch between 16's and 18's. 16's for the street, 18's for the track. I also switch between 16's and 18's during race weekends.
For test and tune I typically run 16's, and for qualifying/race 18's. Lap times are very similar between 16 and 18.
On the street the 18's are much less forgiving with less rubber to absorb potholes etc. From my experience, 18's are much more fragile than 16's, easy to knock out of true or crack. I have experienced at least 3 cracked 18's, all OEM wheels, conversely I have never seen a cracked 16. Again From my experience you feel a lot more with the 18's, which is why with a track set up, the 16's made street driving more tolerable for me.
For test and tune I typically run 16's, and for qualifying/race 18's. Lap times are very similar between 16 and 18.
On the street the 18's are much less forgiving with less rubber to absorb potholes etc. From my experience, 18's are much more fragile than 16's, easy to knock out of true or crack. I have experienced at least 3 cracked 18's, all OEM wheels, conversely I have never seen a cracked 16. Again From my experience you feel a lot more with the 18's, which is why with a track set up, the 16's made street driving more tolerable for me.
07-30-2010, 12:41 AM
#20
Rennlist Member
Just weighed a 9x16 mag/cup phone dial w/ a 245/45/16 Hoosier R6 tire = 39.7 lbs
A 10x18 BBS RE wheel with a 285/30/18 Hoosier R6 tire = 45.5 lbs.
I also measured the actual worn tread width, the 245/16 was 9.5" wide, the 285/18 was 10.5" wide. Hoosier specs show the 245/45/16 is 24.7" diameter, the 285/18 is 24.9".
So with 18s, I gain 1" actual tread width for an additional 5.8 lbs. Not certain that the 16s would be much slower; slightly better acceleration, slightly less grip. Since the 16s are $288 cheaper than a set of 18s, the economics are tempting me to give the 16s a go at the BIR Club Race this wknd.
An 8x16" stock alum phone dial is a couple pounds heavier than the mag wheels. And the tread width of a 245/16 on an 8" rim is 9.25". So the comparison with the 285/18 is then approx 1.25" of additional tread width for an approx. 3 lb weight gain. Here the 18" setup would probably be somewhat faster than the 16s.
The sidewall flex is significantly stiffer with the 18s. Hard to say if that amounts to an improvement in cornering speed, or just a different feel.
A 10x18 BBS RE wheel with a 285/30/18 Hoosier R6 tire = 45.5 lbs.
I also measured the actual worn tread width, the 245/16 was 9.5" wide, the 285/18 was 10.5" wide. Hoosier specs show the 245/45/16 is 24.7" diameter, the 285/18 is 24.9".
So with 18s, I gain 1" actual tread width for an additional 5.8 lbs. Not certain that the 16s would be much slower; slightly better acceleration, slightly less grip. Since the 16s are $288 cheaper than a set of 18s, the economics are tempting me to give the 16s a go at the BIR Club Race this wknd.
An 8x16" stock alum phone dial is a couple pounds heavier than the mag wheels. And the tread width of a 245/16 on an 8" rim is 9.25". So the comparison with the 285/18 is then approx 1.25" of additional tread width for an approx. 3 lb weight gain. Here the 18" setup would probably be somewhat faster than the 16s.
The sidewall flex is significantly stiffer with the 18s. Hard to say if that amounts to an improvement in cornering speed, or just a different feel.
07-30-2010, 07:45 AM
#21
Three Wheelin'
Model the tyre as thin cylinder with radius 12.5".
Model the rim as a thin cylinder with radius 8.5"
Ratio of the moment of inertia for tyre I_t to the moment of inertia for the rim I_r is (12.5/8.5)^2, or just over 2.16.
So, using this simplified model, a pound of spinning rubber in a tyre has over twice the rotational energy of a pound of spinning metal in a rim.
Which means that a 1.4lb saving in tyre mass would be equivalent to more than 2.8lbs of rim mass.
Hmmmm...
Actually it would be closer to say (12.3/7)^2 in reality as the centre of rotational mass would be in from the rim and tyre - so closer to a factor of 3. The R6 tyres are quite light and should help reduce acceleration times.
07-30-2010, 09:02 AM
#22
Burning Brakes
Jim,
I have played for a long time between 16's and 18's. I have done well with both sizes and really can't show a consistent clear advantage with 18's.
I even played with 15's at the rear, but just can't find the right offset, with late offset/ABS.
Right now I have a tire sponsor so have pretty much turned over to 18's. Once I have used up all my 16's I won't be buying any more.
Without the sponsorship, I would switch back to 16's. Without sponsorship, and if I didn't have ABS, I would even consider changing the rear hubs and running 15's.
I have played for a long time between 16's and 18's. I have done well with both sizes and really can't show a consistent clear advantage with 18's.
I even played with 15's at the rear, but just can't find the right offset, with late offset/ABS.
Right now I have a tire sponsor so have pretty much turned over to 18's. Once I have used up all my 16's I won't be buying any more.
Without the sponsorship, I would switch back to 16's. Without sponsorship, and if I didn't have ABS, I would even consider changing the rear hubs and running 15's.
07-30-2010, 09:21 AM
#23
Rennlist Member
Very good to know. I have not run 16s on the car in battle for several years, and it was setup differently back then, so I have no direct comparison. I got caught up in the trend and moved to 18s 4-5 years ago. But the cost of tires and the cheap availability of 16" wheels (other than the mag phone dials...) has me, and others like Rothman and Bryant, rethinking that.
