Mark944na86

I was curious about how much of the total mass of a wheel was rim and how much was tyre.

For an example, I checked out 17" Boxster twists, 7" front and 8.5" rear.

According to wheelweights.net, these weigh 20.3lbs and 22.7lbs respectively.

The max recommended size tyre for these would be 225/45R17 and 255/40R17 respectively.

Here is a table for weights (in lbs) for different sizes of 17" Yokohama AVS ES100:

205/40/17 20.89
205/45/17 21.56
205/50/17 23.29
215/40/17 21.8
215/45/17 23.1
215/50/17 24.22
225/45/17 24.36
235/40/17 24.79
235/45/17 26.34
245/40/17 25.75
245/45/17 25.75
255/40/17 27.35

So putting these tyres onto garden-variety cast Boxster 17" rims, the total wheel weights would be 20.3+24.36=44.66 (front) and 22.7+27.35=50.05 (rear).

In both cases, a bit over 54% of the total wheel mass is tyre, and slightly less than 46% is rim.

Further, since the mass of the rim is closer to the rotational centre than the mass of the tyre, a pound of mass in the tyre has a more significant effect on acceleration and deceleration than a pound of mass in the rim.

So: why does everyone seem to care so much about the weight of rims, but there is never (or hardly ever) any discussion on the weight of tyres? Note, for example, from the table above that dropping a size in width from 255/40R17 to 245/40R17 saves about 1.4lb per wheel, which would be worth more than an equivalent saving in mass for each rim. How much (more) would you expect to pay for rims that would provide the equivalent effective rotational mass saving?

Hmmm...

How much more energy does it take to spin a pound of tyre up to a given rational speed compared to a pound of rim material?

Quick 'n' dirty back of the envelope calc for a 25" wheel on a 17" rim:

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...

ninefiveone

You're right but as you'll quickly see... People on this board generally don't care about rim weight so getting them to care about tire weight is even harder.

That said, two observations:
- Wheel/rim weight has a lot more noticeable effect on ride/handling then on acceleration in my experience
- I look at lighter (but similar performance) tire options in the same size rather then going down a size. Michelins tend to be heavier, continentals a little lighter, etc.

theedge

You list the weights for one tire, but how different is every tire of the same width, sidewall, and tread design in regards to weight? I would bet that all high performance summer tires of 255/40/17 are going to be around the same weight. There might be very little to choose from, and even less if you value some wet performance or other tire features.

Mark944na86

Even if all 255/40R17 (say) from different manufacturers were of similar weight, you still have quite a range of choice of what width tyre to put on your rims.

My interest in the question came about because I was looking for some track rims, and started looking at the discussion around what were considered "light" and "heavy" rims... and what the (often considerable) price differences people were expected to pay for a few pounds of weight difference per rim.

People are typically looking to put the widest possible tyres on their cars for track applications, presumably because the belief is "wider = faster".

But is it always? I suppose it is plausible if you have HP to burn. But I have an S2 with a modest HP output; I suspect 295 rears are not going to be doing me any favours compared to 255 at the rear. And perhaps the 255s aren't doing me any favours compared to 245, 235, even 225? There is a trade-off involved in terms of traction vs acceleration, but where the optimal trade-off point is is not obvious. Empirical question.

JohnKoaWood

There are trade offs to be made no matter what...

Rolling resistance vs tread contact pad
Grip vs weight
Acceleration vs cornering

it is very much reliant on the driver, the car, and personal preference... much like so many other things on our cars... I like standing out in a crowd... being different!

Me, I like the largest lightest rim I can fit the smallest lightest tire on while maintaining the tread pad I want to give me the best mix for me and my car..

Many have said my car is NOT street worth, nor would they want to drive it on a daily basis, but when I am driving it I feel happy knowing I have the car setup the way I want it...

but I also recognise everyone has different definitions of what they want their car to do...

FWIW, my 951 wears 295/30/18s and 255/35/18s rear and front on MY02 Carrera replicas (slightly more rim weight per wheel, but at 1/5th the price of the OEM wheels...

I did weigh them, but lost my notes, but can say they are signifigantly less weight per corner than the 17 inch boxter wheels I had the same size (outside circumference is within .5 inches) rubber mounted on... I love the setup, and it is perfect for me on my car...

In the end, set your car up how you like it, or follow the popular trends and hope for broad appeal on resale!

ninefiveone

Plenty to choose from. in 255/40/17:

Pirelli Pzero Roso - 22lbs
Conti Extreme Contact - 23lbs
Michelin PS2 -24lbs
Kumho XS - 25lbs
Dunlop Star Spec - 28lbs

6 lb weight difference from heaviest to lightest.

Mark944na86

LOL, maybe I'm missing something, but I can't see how it would be possible to make your corners any _heavier_ than they are now...

18" replicas with 295 and 255 rubber? Good God. The squirrels must be nervous around your neighbourhood.

But you are talking about street -- that's just a matter of taste, as you say. I'm talking track -- that's physics...

Mark944na86

Interesting. According to the calculation above, that's like a difference of over 12lbs in the rims!

I notice the Dunlop tyre is the outlier, though. All the others are within 3lbs of each other...

JohnKoaWood

The tires, while WAY wider than what they replaced, have less than half the sidewall height... the tires are CLOSE to the same weights as the rubber they replaced, but with MUCH lighter wheels.... the new setup has NO sidwall flex the old setup had (reason I changed) , and MUCH more grip...

street vs track... HA, I run -2.5 deg front camber, even rear camber (until the rear suspension mods are finished..) , 300lb springs in the front (considering moving UP to 400 when I go coil over TB delete in the rear...)will all solid bushings and a welded in strut tower brace... I am confident I can drive TO the track, flog it ON the track, and drive home safe and sound...

Like I said, my setup isn't for everyone.. I like it how it is, but it does need a fine tuning... but I have other pokers in the fire right now..

FWIW, my 951 setup for street use, is MORE agressive than more than a few track cars I have seen in person, on the track...

And yes squirels are nervous, as are other animals, small children, and basicly anything more than about 4 inches off the pavement!

Before the fenders were rolled and car was lowered another 1.5 inches or so...


and the rear...


Note the sidewall, or lack there of... I like this wheel setup the most on this car, on my N/A I have the 16 inch PDs that came on the 951 when I bought it, and those wheels / tires are working OK on teh N/A... enough secondary spring to the tire to not be to harsh, while allowing the car to be enjoyed from time to time without TONS of sidewall deflection... I am actually considering moving to the 17s I have just to see if I like them on this car more than I liked them on the 951...

and my mini montage of wheels I have had on my 951, you can see the progression of larger rim, wider tire, shorter sidewall.

Mark944na86

I think you'll find that relatively little of the tyre weight is in the sidewall. Have a look at the table above.

205/40/17 20.89
[...]
245/40/17 25.75

Both the tyres above have the same sidewalls, they only differ in width.

By increasing the width 40mm, the weight has increased nearly 5lbs. By increasing the width another 50mm, you would be increasing the weight by another 6lbs again!

In the above tyre, a 295/40/17 would weigh 31.8 lbs...

So no, it's not plausible to replace a narrower tyre with a much wider one and make up the difference in weight with sidewall height.

The Porsche 996 18x8 cast wheels come in at 20lbs each, and the 18x10 weigh 23.4 lbs each.

The Boxster 17x7 are 20.3 lbs and the 17x8.5 are 22.7 lbs.

And your replicas are going to be heavier than the OEM weights, of course...

But is it faster?

Not trying to give you a hard time (well, maybe just a bit), but this is precisely the sort of perception I was addressing in the first place...

JDS968

Kind of surprised at this...and not terribly happy! I'm sure I saved some weight moving down to the narrower 225s...but maybe not as much as I thought?

Strange that there's so much weight in a tire reputed to be the hottest thing this side of A048s...

V2Rocket

a 205/40 would have a sidewall height of 82 mm, whereas a 245/40 would have a 98mm...

double that for overall sidewall

ninefiveone

I find that weight doesn't correlate well to performance in tires. In the end, picking tires is always a balance of performance, cost, and (for some) weight.

Ironically for me, I ended up going with the dunlops because they were the best balance of performance and cost for my type of track usage, and I've got lightweight CCW's to mitigate how heavy they are. Rule of thumb... If you're tire/wheel combo comes in <50lbs you're in good shape. Less is better but you're reaching the point of significantly more cost. With my setup I'm at 45lbs to have 255/40/17's at all four corners.

JohnKoaWood - 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.

JohnKoaWood

Some times it is just best to let some things go...

IIRC the rim / tire combo was within a lb or 2, but the 18s with BFG rubber were lighter than the boxter twists with Falken.... I fell in love with the new wheel setup when I picked them up...

Also FWIW, the fronts are 8 inch wide, teh rears are 9.5 (OEM MY02 came in 8.5 and 10 inch widths IIRC, but there were also other widths available, and still are for up to $2500 a rim..) so mine are somewhat narrower rim, but still heavier than the OEM wheel... but not by much...

I plan to track the car after this itteration is completed... but for now she is resting on stands, waiting for a direction on the block.... and rear suspension upgrade...

JohnKoaWood

Also worth mention, while I am having difficulty digging up the rated weight of the BFG tires I am running, or what they weigh in the sizes indicated on the origional post, I did find 295/30/19 on e-bay listed at 25lbs each... take it for what it is...

if you can dig up weights on them.. please post..

BFG G-Force T/A KDW 2.. Y speed rated, plenty of load rating for us...

OH and I found weights...

255/40/17 27lbs..

If you are directly targetting a track setup, buy the R spec, and shave them... reduce weight, increased grip, etc.. etc.. etc..