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Retracnic (Automotive) (OP)
20 May 03 21:02
A colleague and I were debating large diameter rim/low profile tire combinations. I made the statement that the only reason for large diameter rims was to provide sufficient clearances for larger brake components. And that low profile tires were a necessary evil to accomplish this.

He completely disagreed. His belief, is the use of larger rims is for the sole purpose of running low-profile tires. Then he went on to claim alleged performance benefits.

To me it seems that a lower profile tire would provide less straight line traction, and not be as capable of generating high slip angles. The one benefit I can see is that low-profile tires would be less prone to rolling over in a corner, but I’m not sure.

I would appreciate any input in this debate. I leave the ruling in the court of public opinion.

Thanks,

Bryan Carter
Helpful Member!(2)  GregLocock (Automotive)
20 May 03 22:31
Improved steering feel and response, styling, lower rolling resistance, styling, big markup, styling, marketing, sidewall stability under braking, styling, bigger brakes, less camber distortion.

Oh, and styling. It costs about 10 bucks a wheel and less per tyre to move up 1 inch in diameter.  How much more do they go for in the showroom?

On the other hand, less progressive breakaway, easier to damage*, heavier, slightly more expensive.

* not necessarily a bad thing, for us. So long as one replacement OEM wheel is cheaper than 4 or 5 aftermarket ones then most people will go for the OEM one.

Cheers

Greg Locock

Retracnic (Automotive) (OP)
21 May 03 0:28
So I gather that styling may be a factor in choosing low profile tires...

Aesthetics aside, how would you compare the performance characteristics of a normal tire to a low-profile one? It would seem to me, that most forms of motorsports do not make use of tires of very low profile (Touring Cars being notable exception).

On a side note, do you think I could trick people into thinking I live in the UK (or Australia) if I said "dampers and tyres" instead of "shocks and tires"?

Thanks,

BC
Helpful Member!  patprimmer (Publican)
21 May 03 6:59
In Australia, we call them shocks and tyres, I don't know what Greg does, as he is a pom liveing here.

Apart from styling (read imitation of race cars), the pros and cons for big dia rim low profile are as Greg said:-
Pros
   Improved steering feel
   Improved steering response
   Lower rolling resistance
   Improved sidewall stability under braking
   Room for bigger brakes
   Less camber distortion when cornering.
Cons
   Less progressive breakaway
   More susceptable to sidewall damage
   Heavier
   More expensive

I would add
   More susceptable to tram tracking
   Greater steering accuracy
   Harsher ride
   More critical to tyre pressure



Regards
pat

GregLocock (Automotive)
21 May 03 21:43
The ride thing is interesting. High sidewall tyres have a slight tendency to a boulevard shake concern, where the sidewalls flex, and the shocks don't break stiction. this is undamped and can be at customer complaint level, I've had to work on it twice. Low profile tyres don't flex as much so you can damp this out in the shocks more easily. That is, admittedly, a fairly subtle point, but one that I have seen in print as well.

I have a confidential document here showing the progressive effect of changing a tyre from 155/80 R13 to a 175/70 R13 to a 185/60 R14 to a 195/50 R15.

Side force/slip angle generally improves by about 25% say at 4 degrees. subjective ride&comfort rating drops by 20%(hmmf). subjective dry handling improves by 50% (hmmf). Aquaplaning performance halves.

Cheers

Greg Locock

evelrod (Automotive)
23 May 03 20:42
Pat, "imitation of race cars"?  What type of car are you boys racin' down there.  The current rage here in SoCal is the 20 and 22 inch wheels.  Some of the SUV's are even coming factory with them.

Oh Greg, did you mention "styling"? As to the 'profile' my guess is styling because even race tires are usually around 35 series or so and a 20" wheel/tire on a Honda Prelude CAN'T be over a 15 series.  Just a guess. The black part is only about an inch off the rim!!!
Hey, it keeps the ecomomy rolling.

Rod

patprimmer (Publican)
23 May 03 21:43
OK Rod
Exagerated imitation of race cars

Regards
pat

BFTCC (Automotive)
3 Jun 03 0:03
Interesting thread which combines the engineering aspects with the styling, marketing and aftermarket issues.

Certainly agree with all the benefits Greg mentioned, and have plenty of data to back that up. I'd add shorter braking distances to the list, and higher levels of transmitted pattern and road noise.

The less progressive breakaway issue is an interesting one. Certainly a plot of slip angle vs cornering force suggests that. In practice, however, the phenomenon is generally attributed to the higher cornering speed which results from higher levels of driver confidence inspired by the improved response characteristics.

Bryan, you also raise an interesting issue re the validity of comparisons (responded to by Greg). In order to isolate the influence of aspect ratio alone, you need two tyres with all other factors the same - tread compound, construction variables etc etc. Fact is that its nigh on impossible to "correct" the data for these additional variables, as no manufacturer makes such tyres. Then again its probably not important......

Further, tyre manufacturers are increasingly making lower profile tyres tailored for Joe Average replacement market - where life, noise and ride comfort are all important. In these cases many of the benefits of the reduced AR can be sacrificed, and the tyres may not provide superior "performance" cf a high quality, higher aspect ratio tyre. Market forces at work.....

BTW, recent F1 tyre is 55 AR, Indy 40 to 45. Surmise that the reason lower AR's are not preferred is that air volume is required to sustain downforce loads. Of course, this can also be achieved with higher pressures but then you reduce contact area, and and and.......the interesting saga continues !

Comments please ? Cheers
evelrod (Automotive)
3 Jun 03 12:48
I've been out of the 'real' racing loop for a number of years now sticking mostly to 'vintage' but from what I have observed over the years.---Tread patterned tires with AR's of 55 in F-1 and slicks with 40/45 in Indy(?) are mostly 'rules' driven.  In my area of sports car racing the slicks used are generally ~35 ( I do seem to remember Porsche using some tires of less than this some years back ???). No rules specified tire specs in most classes using 'slicks' in SCCA and usually a DOT rated tire of no less than 55 in vintage.  I am not too familiar with the NASCAR rules but they include the mandatory use of steel wheels and safety high pressure inner liners.  From that I would guess the AR to be something like 55/60 but that's just a guess. My point is that these large diameter very low profile tire/wheel combos are a strictly stylist driven "need" and of no tangible benifit aside 'profit'!.  

Rod
BFTCC (Automotive)
3 Jun 03 23:52
Evelrod, there are enormous differences between the forces driving motorsport and those that apply in the OEM & replacement markets. Tyre regs in motorsport are implemented to control cost, availability and competitor equality issues. So even if lower AR's provide performance improvements it doesn't necessarily follow that they'll be used in motorsport. The other issue is the supplier - often reluctant to invest in new moulds and development of new sizes specifically for motorsport unless there is some guaranteed volume and longevity.

In the OEM & replacement markets the customer is king. If there's sufficient demand, manufacturers will meet it. If customers (OEM and replacement) perceive a benefit in ultra low profile tyres, it will create demand. The benefits of ULP tyres are quite readily measured and demonstrated and have been well covered in previous posts to this forum as well as in numerous texts. It is these benefits (as well as the styling that you identify) that are driving the trend to ULP.

Have fun with it !
Slowzuki (Mechanical)
25 Jun 03 16:44
Another point here is the type of suspenion being connected too.  The camber control on some vehicles is good and they can keep low profile tires square to the road.  Good old solid (beam, live, etc) axles do this well.

Cars with strut + lower a-arm or trailing arm don't do well at all! You need to play with the camber so when the car is loaded in a turn the tire is square to the road. Old vehicles generally have lots of slack in the bushings too (just popping poly bushes in can show how bad some inaccurate some geometries/manufacturers are)

So dropping some 35 series on your vintage racer isn't often the best soln.
tireman9 (Mechanical)
6 Jul 03 11:13
Having driven an SUV with 24" prototype tires I can attest to the potential for no loss in ride comfort( or even an improvement) but many of the benefits of low AR tires.
I believe that on large vehicles such as SUV, the styling is a major factor. The vehicles are getting so large thay just don't "look" right even with P265/70R16.
I guess with the switch away from 70 AR 13" tires on a lot of cars we expect that the tire will take up a certain portion of the profile of a vehicle. Go out and look at the % the tire & wheel is of a car side view vs the % in a large SUV.
lawrencedai (Electrical)
18 Aug 03 12:32
. What are to be taken care of when changing low-profile tyres?
evelrod (Automotive)
18 Aug 03 13:14
When the aluminum "mag wheel" trend (it was a "fad" then) started in the late 50's early 60's it was extremely difficult to find tire shops that would change tires.  They were afraid that they   would damage the wheels---personal experience!  As the machinery has improved and aluminum wheels become commonplace that has changed.  Now we are presented with ULP tires with dias of ~24 inches and AR's of ~15:1 ---my guess, just a guess mind you, is that it will be difficult and to find shops to do the   work at anywhere near a reasonable cost.

Rod

PS---Add in the additional $cost$ of a set of ULP tires such as the ones used on the new Ferrari F-50 @ $4000US/set!
livingwater (Aeronautics)
22 Aug 03 14:02
Retracnic,

Stability should be a benefit.  Not sure why some others are saying bigger rims are heavier, Larger diameter rims with same OD tires (lower AR), should reduce your unsprung weight.  The mass of aluminum used to increase the diameter of the wheel will be considerably less than the mass of tire compound that you're doing away with.  The lower weight will result in a fuel savings in the long run.

We like to run 22 inch rims on our Semi at the Family ranch as opposed to the 20 inch type.  The lower profile tires make a BIG difference when you consider 18 of them at a time.  The only concern I see, is cost.  The bigger rims and tires are quite a bit more expensive right now (for pick-ups and SUV's anyway)

Have a nice day,
Paul.
tireman9 (Mechanical)
22 Aug 03 17:12
Sorry livingwater but with Aluminum being approx 2.5 times heavier than rubber its a little difficult to see how you can end up with less un-sprung weight.

When you increast the rim dia the majority of what you are reducing in the tire is just the sidewall. You have to increase both the spokes and rim portion of the wheel.
JayMaechtlen (Industrial)
13 Sep 03 13:50
" Aquaplaning performance halves."
???
that could be kind of important....
Anyone else have data on this?

Jay Maechtlen

GregLocock (Automotive)
13 Sep 03 19:43
Jay, when R&T do tyre tests don't they include braking in the wet?

I disagree with the 50% figure in that document, by the way, for such a small change in tyre. There again I disagreed with all the other measures as well!

Another factoid I heard recently is that they have started to include a hinge like feature in the sidewall of low profile tyres. This is to reduce the vertical stiffness and so improve the ride etc. Of course it will also decouple the belt from the hub, so the steering stiffness will drop, which negates the only sensible advantage they have over a higher profile tyre.

Anybody read John Miles' column in Vehicle Dynamics International? Read the last sentence!

Cheers

Greg Locock

patprimmer (Publican)
14 Sep 03 23:46
Tireman9

While i agree with your statement, I am about to knitpick a bit about the details. Aluminium is only about twice as heavy as rubber, and that figure is a bit rubbery (excuse the pun).

Cast aluminium is about 2.2 to 2.3, and rubbers are somewhat higher than 1.

There are a number of types of rubbers, as well as various fillers in various quantities. Also tyre sidewalls are a rubber composite with textiles for reinforcement.

The final SG for the composite compound is probably about 1.3

The tyre is probably slightly thicker than an aluminium rim, and the spokes are not disks, so they have less area, but more thickness than the rubber they replace.

My only point is that even though the wheel section is probably heavier than the tyre section it replaces, it is not a simple nor consistent calculation and will vary from case to case, and sometines the result will be a lighter package, but mostly it will be heavier

Regards
pat

tireman9 (Mechanical)
18 Sep 03 13:26
OK here are some hard numbers.
Size            OD  Weight     Load Index
195/55R15    25.3"    35#    84
215/35R18    24.2    42    73

195/60R15    24.2    35    87
205/50R16    24    40    86

205/55R16    24.9    41    89
235/45R17    25.3    46    93
225/40R18    25.1    48    88

235/60R16    27.1    49    99
255/50R17    27.1    53    100

One thing that I didn't see was any ref to the tire's load capability. With some OE applications only having a few% reserve load capacity this can be criticle in some applications.

35 years as Tire Design Eng.
NormPeterson (Structural)
18 Sep 03 21:00
[quote]One thing that I didn't see was any ref to the tire's load capability.[/quote]What is needed (and what I'd also like to see) would be the tables that provide the rated load (lbs) vs the load index and inflation pressure.  I've got some ancient tables of similar info, but they date back to when bias-ply tires (unbelted) were still common.

Norm
NormPeterson (Structural)
18 Sep 03 21:03
One thing that I didn't see was any ref to the tire's load capability.What is needed (and what I'd also like to see) would be the tables that provide the rated load (lbs) vs the load index and inflation pressure.  I've got some ancient tables of similar info, but they date back to when bias-ply tires (unbelted) were still common.

Norm
NormPeterson (Structural)
18 Sep 03 21:08
Evidently this site supports neither [quote][/quote] nor editing of posts already submitted.  Sorry for the double post above and this off-topic explanation.

Norm
GregLocock (Automotive)
18 Sep 03 21:34
I've got current T&RA(Australia) tables, ie in metric. What specifically do you want to know?

Cheers

Greg Locock

NormPeterson (Structural)
19 Sep 03 7:26
While it's not too hard to find the maximum load vs load index, i.e.

LOAD    LOAD
INDEX    (lbs*)
85    1135
86    1168
87    1201
88    1235
89    1279
90    1323
91    1356
92    1389
93    1433
94    1477
95    1521

. . . that table does not provide any data with respect to inflation pressure.  The oldie table that I mentioned above looked something like . . .

Tire..... Inflation pressures (psi)
Size..... ..20.. ..22. ..24. ..26.. ..28. ..30. ..32.. . . .  
8.25-15 1250 1310 1380 1440 1500 1560 1620 . . .

(Note that the periods before and after the inflation pressure figures are to hopefully make the table line up in columns)

The general reason I'd like to have that information is because it's useful to have handy, particularly if you're going to deviate from OE tire fitment or get involved in such discussions.  

The more specific personal reason revolves around my upcoming need to replace the tires on my 2001 Maxima.  Tire size is an issue that's been brought up numerous times by others on the Maxima forum to which I contribute, but so much of the *information* on this topic over there is juvenile, anecdotal, and appearance-oriented that it's all but useless to me.

The short version of the story is that the OE tire is the Bridgestone RE92 in 225/50-17 (I'm ignoring the speed ratings), and the OE wheels are 17 x 7, which I understand to be the minimum width for that tire.  Not only that, but the size availability table at www.tirerack.com indicate that there's a dedicated part number for Nissan in that size (I'm guessing that it's for whatever it took to get a 94 load index in that size).  

The RE92's aren't very good in the wet as the small sipes start to get shallow even though there's adequate tread depth per the usual measuring methods.  So I'm looking for an alternative.  There's little to choose from in that size, and most of the tires available in that size are relatively expensive.  There's plenty of choice in 235/45-17, though that's at least a half inch outside the rim width guidelines.  And there's a wide variety to choose from in 225/45-17, but that size isn't normally associated with LI 94.  So I'd be giving up some load capacity at any given pressure, though I should be able to recover that by running a bit more inflation pressure.  Yes, I'm willing to trade away ride softness to get there.  No, I don't currently have the means to justify scrapping the OE wheels and go aftermarket (besides, I kind of like the OE wheels anyway).

Norm
GregLocock (Automotive)
22 Sep 03 21:21
Incidentally, according to Autralian T&RA the 225/50 17 can be fitted to rims 6J through to 8J. 7J is the measuring rim. 670 kg at 250 kPa. (101 kPa=14.7 psi)

At 200 kPA ie 30 psi (guessing what the recommended tyre pressure is) the load capacity is 560 kg

225 45/17 will go on a 7J rim, but is only a 90 load rating, 600 kg at 250 kpa.

For 560 kg capacity you'll need an inflation pressure of 230 kPa on the smaller tyre.


Cheers

Greg Locock

NormPeterson (Structural)
23 Sep 03 20:10
Thanks, Greg.  A 4 psi increase is about what I had been thinking.  

FWIW, the Nissan-recommended pressures are 32F/32R, though I've adjusted them slightly (to 36-ish/29-ish) in the interest of more nimble turn-in (and with the knowledge that a full 5-passenger + trunk load is a rare occurrence).

Any idea where one might get a copy of those tables?

Norm
GregLocock (Automotive)
23 Sep 03 20:21
I've just posted the contact details for T&RA in the FAQ section. I think the book costs about thirty bucks.

Cheers

Greg Locock

tireman9 (Mechanical)
2 Oct 03 12:58
See the formula in the "tire size & inflation thread"

35 yrs Tire Eng. Designed basic rain Firestone for CART. SCCA & IMSA Pro & Am. Set lap records at 6 different road courses in '89-91.

pvision (Automotive)
9 Feb 04 20:06
Regarding tyre sizes;

I have built an online tyre size calculator which has the useful bonus of showing two calculation to compare two different tyre sizes and/or ARs and/or wheel diameters

I'd like to add a third calculator to show similar data (rolling circumference, diameter, etc) for race tyres. Where can I find a tyre size decoder for race tyre sizes such as 240/515-13?

This is a metric description - the imperial equivalent would be 9.5/20.3-13. I'd like to be able to decode both

The tyre calculator is at
http://www.bolide.co.uk/actions/info.taf?page=tyresizec...


Nick Froome
carnage1 (Electrical)
16 Feb 04 1:23
there is one very important aspect that everyone has seemed to forget about large rims and low profile tires
anyone remember laws about ground clearance?
no part of the car can be lower than the bottom edge of the wheel at ride height, so if you want a decent size and still be legaly crusing low (important especially to large oem and aftermarket companies) you have to have low profile tires.
So i think the consumers may be emulating the custom car world, you may not have the skill to do a good lowering job but you can buy fancy wheels.
SusTestEng (Automotive)
27 Apr 04 10:15
One problem I have found with working with low profile and stiff sidewall tires is matching the compliance of the tire and wheel as an assembly.  Because of the rather quick grip decay at the limit, the compliance of the wheel is critical to help this situation.  This application was an AWD car with an extremly high limit capability.  We chose a forged alloy wheel to allow the wheel to bend slightly with the tire's sidewall in order to achieve a more linear grip decay at the limit.  This made the driving near limit more predictable, but almost doubled the cost of the wheel.  
chapper27 (Automotive)
19 Mar 05 20:37
so what is your conclusion.  I think; lower profile, larger wheel,  therefore more unsprung weight,  therefore straight line performance is decreased with low profile tires.
But what about the true effect in a curve?  does the stiffer sidewall compensate for the greater wt?  
jadcock (Civil/Environmental)
3 Jun 05 10:12
One thing I've noticed, and heard in amateur circles, is a larger diameter wheel with a lower profile tire usually equates to poorer acceleration and "spongier" braking, due to more rotating mass.  I suppose the heavier the wheel, the more energy it takes to get it moving (and to stop it).  That large diameter rim displaces the air that you would have had in a higher sidewall tire (if you keep the overall diameter the same), so it's in effect a heavier wheel/tire combo.  Perhaps that increased unsprung weight could wreak havoc in other areas, like being harder on the shocks, etc.  No?

Jason

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