Subaru Legacy/Outback 2005+

c_hunter

Is that Affinity H-rated?? I would make sure to get V-rated like the OEM tires. These are commonly described as speed ratings, but they are really load/endurance ratings. I think it's good to stick with the proper rating. The only exception would be going to winter tires, which often only come in the lower ratings.

Craig

mwhitt

I know the comparision is strange - some is cosmetic, some money, some functional.

I would look at the 2.5T on the subie - if I could stand the look of the hood with the scoop. I loved scoops when I was 18 but not much of a fan at 48.

why the 2.0T on the audi - money - trying to keep costs below $35K and the 3.? on the A4 throws me over the top of where i want to be.

I also like the VDC and I only see that on the 3.0 limited.

I like the Audi but have an idea I might spend more time in a loaner than my car and nobody has the time for that.

know it is kind of off center, but what the hey..

cptplt

Craig
thought the load rating was the number, 97 etc and the speed rating the letter - H etc.While they are no doubt related you can have a very high load rating and low speed rating as truck tires often do eg the tires on my Winnebago are 109 load and only S speed rated.
SOA have a weird outlook on tires when Impreza TS's get the same V rating as a WRX and Legacy /outbacks were being delivered with H! I suspect it has more to do with their stocks than anything else. No Legacy/OB probably needs more than a T rating for real life use.

c_hunter

They "speed" rate the tires by loading/pressing them against a rotating drum in a lab setting and then ramping up speeds (in 10km/h steps in 10 minute increments) until the tire fails. Then they back off a step to get the rating. What it means is that the tire is rated at a certain speed/load combination. While you might never encounter that "ideal lab speed" in real life, you can encounter the equivalent speed/load combination with a heavily loaded car on a hot day traveling on the highway with bumps. When you back off speed ratings, you are backing off on the capability of the tires to handle the combined speed/load/heat in a dynamic setting. So that's why I think it's important to stick with the proper rating. Honestly, my OB XT can't even go as fast as the 149mph V-rating on the tires, but they did not choose it based on raw speed. They chose it for a certain level of tire endurance under load on a hot day at highway speeds in the real world, which is what matters to us!

Backing off a speed rating doesn't mean a tire will fail, but it will definitely be less capable of handling everyday extremes which can ultimately lead to failure over the long term.

I think that is what's good and bad about speed ratings -- the good thing is that it's an integrated factor taking speed/load/heat into account (the higher the rating the more robust the tire). The bad thing is that most people interpret the rating only in terms of speed, which is misleading. There have been several people here who have backed off on speed rating for various reasons, arguing that they never travel at high speeds. Unfortunately, that is not the only thing to consider.

Craig

c_hunter

I believe the load index is a "static" rating, based on the tire's ability to support a load. So it doesn't necessarily have anything to do with the dynamic capabilities.

When sizing airplane landing gear a long time ago, I was really interested to find out that there are simple relations for static loads on tires. For instance, if you know the tire's pressure P (PSI) and the amount of weight the tire is supporting W (lbs) then you can compute the tire's contact area A (sq inches) as A=W/P. Now obviously that is very simple and makes total sense, but it never occured to me! I always figured that the tire construction, sidewall height, etc... factored into the contact area, but it all comes down to simple physics -- you have a certain amount of weight transferred to the ground over a certain area. The pressure has to satisfy that weight and area distribution. I thought it was pretty cool at the time!

Craig

kens

A=W/P. How simple, yet powerful! The units cancel out perfectly.

Too add to Craig's comments on speed ratings being a dynamic load -- there's a reason why lower profile tires usually are available only in higher speed ratings. With less sidewall to bear the load, tire manufacturers probably want to increase the margin of safety as well. ...Not to mention that owners will typically want to drive harder and faster with these tires.

Ken

ateixeira

On the Audi, the cup holder does not retract anymore, some of the dash lights are out, the climate control switches back and forth to Celsius and Farenheight on its own

Maybe it just wants you to use Celsius because it's made in Europe?

Does it change from Miles to KM as well? :P

-juice

kat95

Have any of you considered trading in your Subaru for a more economical car due to rising gas prices? We get alot of snow here and I couldn't even think of giving my car up and going back to a front wheel drive. Who knows.

ateixeira

Compared to FWD cars, they may give up a little, but remember that Subaru specializes in AWD wagons. For many they are seen as an SUV alternative that is more efficient, not less.

So, will I trade my Forester for some less efficient SUV? Not likely.

Same goes for Outback, I doubt those owners would move to a plain FWD car. They might give up their Explorer or Jeep Grand Cherokee, though.

Legacy and Impreza is a different story, those buyers just might opt for something more efficient. Even then, 23/30 is not bad, even the mid-size best of 26/34 is not really significantly better.

We might see fewer turbos and more base engines, perhaps.

Any how, Subaru's bread-and-butter is the Outback, and the Forester is a close second, so both those vehicles actually stand to benefit, if anything.

-juice

zman3

Not even considering it. Gas will have to get much higher than it is for me to dump either my 98 Outback or my 03 Expedition. Both are still fine vehicles. I figure I can spend a lot on gas before it makes sense to eat the depreciation, additional sales tax, etc that come into play when buying a new vehicle. I am guessing that large SUV's especially are falling like rocks in resale value.

Gas mileage may be more of an important factor when I buy my next vehicle, but I would not trade a vehicle I am happy with just to save money on gas.

My $0.02.

Karl

ateixeira

My Forester average 25.1mpg, real-world actual average.

Prius owners report a real-world average of 48mpg, according to one web site.

Let's say you drive 1000 miles per month. The Forester would use 39.8 gallons, while the Prius would use 20.8 gallons. Sounds great so far. You'd save 19 gallons per month. At $3 per or so, that's $57 a month.

But I'd have to spend $22,000 to get a Prius, subtract about $4000 trade-in value, and I'd have an $18,000 loan. Even on a 5 year loan at 6%, we're talking a payment of $348.

There's no economics in that.

Interest alone for the first month will cost you $90. More than the gas you save. So instead of the oil companies getting rich, your bank will get very rich.

And you'd have a smaller vehicle with less cargo space and also have to give up AWD and some acceleration. Get less, pay more.

No thanks. I'll keep waxing Sandy.

If I wanted a FWD compact car, I'd probably get the new Civic. 30/40 mpg on regular fuel with 140hp, and probably about $4000 less than a Prius, which buys enough gas for a few years even if prices do spike up.

-juice

rob_m

Have any of you considered trading in your Subaru for a more economical car due to rising gas prices?

Not in this life! Actually my 05 GT sedan gets the same gas mileage as my 03 Outback wagon - 24 mpg. The GT costs me about an extra $6 a week because it requires premium gas. I have recently adjusted my driving habits, where I am not doing 80 on the highway any more, and rarely kick in the turbo coming out of off-ramps. Rob M.

hektorviktor

Thanks for the info on breaking in the new car. I can't wait to go on a road trip with it!

ssminton

Thanks to everyone for their tire input! I have decided to go with the Nokian WR product. Nokian actually specifies a extra-load tire for both the XT and 3.0R Outbacks so that is what I will get. Winter and poor road traction is really my number one concern. As someone who only spends a couple years in each car, it is not worth the investment of winter/summer tires. The Nokian's tread wear also seems to be better than most V-rated tires. I actually had a nice conversation with a gentleman at Nokian in North Carolina. I had called to better understand the difference between the WR and WR SUV products (SUV is basically the extra-load version). Just based on the quality of the conversation, with a consumer no less, it is reason enough to go with their product. The tires should be in early next week. I will let everyone know how they ride.

As far as gas goes... sure the price increase sucks but realize that increases in heating costs are going to be far more painful for everyone. So my monthly gas bills have gone up $50 a month (and I drive a lot)... just wait til winter. Your home heating bill will probably be $100-$300 higher? I am much more concerned about this impact on my wallet.

Vehicle choice is always about compromise. Sure there are cars with less MPG ratings than the Subaru. However, I cannot think of any "performance" car, AWD or not, that has a better rating? Even if you go to a "hybrid" SUV (MPG's around 30), you would need to drive in excess of 50K miles a year to break even on the extra cost. I certainly understand, environmentally why people desire to drive these cars (I considered it myself), but it is not purely an economic decision.

I cannot see myself changing cars, for I do not think that I can increase my MPG without taking a substantial hit in safety, utility, and comfort. I can see, however, the number of trips to my home in Vermont decreasing in frequency. I often head up 3-4 weekends a month (600 miles round trip). I figure at $4/gallon, I am looking at trips to VT about 25%-30% less.

What I have already started doing though is slowing down! My 2005 VDC averages 26MPG on the highway at 62mph vs 22 MPG at 75 mph.

Before people make rash decisions regarding cars and gas costs, etc... Get out the calculator! That extra cup of latte each day will more than cover your increased gas costs. Just something to think about? It is all about choice!

gmanmd

BTW, has the gas mileage improved on the 2006 legacy's (incl. the GT) as I had read elsewhere that mileage improved, but I can't find anything that corroborates this.

stantont

It's interesting how the way we display data affects how we think about it. We think of miles per gallon, but most people have a set number of miles they want/need to do in a week. They don't say "I've got such and so many gallons, now where will I go with it?"

But look what happens if we display gallons per mile instead of miles per gallon. Thus:
10 mpg = 0.10 gallons per mile
11 mpg = 0.0909 gallons per mile
20 mpg = 0.050 gallons per mile
25 mpg = 0.040 gallons per mile
30 mpg = 0.033 gallons per mile
40 mpg = 0.025 gallons per mile

When you look at it that way, there is a bigger difference between 10 mpg and 11 mpg (0.10 - 0.0909 = .0091 gallons per mile difference) than there is between 30 mpg and 40 mpg (0.033 - 0.025 = .008 gallons per mile difference). That is, there is a bigger difference between a Hummer H2 (10 mpg) and a Hummer H3 (11 mpg) than there is between your Subie (30 mpg) and a Jetta diesel (40 mpg, real world).

Interesting, eh?
Stanton

johnnysubaru

Hi Ken,

Was looking for instructions on installing an FM modulator for my '05 Outback 2.5i and went to www.lgt.com as suggested in your post, but it turns out the URL takes me to a German investment firm website. Can you kindly send me the link so I can look at the instructions?

Thanks much, love the posts...

capriracer

Craig,

I'm sorry, but the average contact pressure of a tire is not equal to the inflation pressure.

The average contact pressure is highly dependent on the stiffness of the belt package and the sidewall deflection (sidewall stiffness).

Hope this helps clear up a misconception.

c_hunter

Well, I was skeptical at first too, but it's really true! I am sure there are limits to the simplification at low pressure where the tire structure does carry load (same goes for run-flat tires perhaps at all pressures), but when a regular tire is near it's design range, the internal pressure carries 99.99% of the load and the sidewalls are almost negligible (all they do is act as a pressure vessel for the air, which carries the vertical load). Part of the reason this is true is that tires maintain a near-constant volume over a range of pressures (ie, they're not balloons). If you do a cross section of the tire and make a control volume analysis of it assuming constant volume, it ends up working out that the contact area, internal pressure, and load are all connected. It was so simple that I smacked my forehead when I realized this!

Here's a Boeing tech article on the topic:

http://www.boeing.com/assocproducts/aircompat/faqs/calctirecontactarea.pdf

Again, these refer to static loadings, not sure how dynamics change the picture but it would have to be a small effect. And of course, tread patterns factor in here too, since they may cause the real contact area to be lower than the apparent contact area. But it all has to come out in the wash.

Craig

snowbird

I have a 05 Outback. My moonroof shade, when closed, squeaks when driving. Does anyone else have this problem? Thanks. Jason

taddison

I had a squeaky moonroof shade on my 2005, open or closed, and it was getting worse and worse until it drove me nuts enough to do something about it.

It seemed that the plastic runners on the shade were squeaking against the aluminum rail that they are mounted on. I applied a small amount of "Duralube" spray to to the runners and the noise stopped immediately. Any kind of lightweight lubricant would probably work just as well. Hold a towel up to catch excess spray so it doesn't get on the seats.

Tim.

kens

Sorry about that. We're not supposed to give URLs of "competing" forums here so I was trying to be discrete. Try substituting Legacy GT instead of lgt but without the spaces.

Let me know if you have any problems finding it. Lots of good posts there, but you do need to piece together the information (ie. taking apart the center console vs. wiring the FM modulator). Feel free to ask if you get stuck.

Ken

kens

Tim,

Thanks for the tip. I was starting to notice that too with my moonroof shade -- not really a squeak, but a very faint "tick" as the runners vibrated against the rails. I assumed it was the lubricant drying out over time.

Ken

snowbird

Thanks, Tim, I will try it for sure. Jason

capriracer

Craig,

Again, I'm sorry, but as a tire engineer I have access to data that the average person doesn't have - and the data contradicts the theory.

But let's move this discussion to another forum:

Tires, Tires, Tires

Post the theory there, and by the time you do that, I'll have prepared a reply to disprove it.

Hope this helps.

Folks who are interested can follow the discussion there.

c_hunter

That link didn't work, but no need to debate it -- I am just telling you what we do in the aircraft world, and the assumption certainly holds there. A lot of work has been done regarding tire performance on contaminated runways (we have a national test facility for that where I work -- it's the place that originated rain grooves for runways and roadways) and all the data I have seen clusters around the linear relationship between tire load, pressure, and contact patch. If it's not a perfect relationship, it's at least 90-95% good which tells me the air is carrying the bulk of the load. Now, if (non-run-flat) car tires are very much different, I'd certainly be interested to see some data on that.

The other thing I found interesting is that farmers use the same relationship when sizing tractor tires to mitigate soil compaction when plowing! They actually work the load/pressure/area relationship into their soil compaction tables, and choose tire width/size based on the weight and application.

Craig

kyfdx

Tires, tires, tires

johnnysubaru

Ken -

Ahhh, yes..I'm new to the forum and didn't quite get that one...

Thanks again for the help, I'll look it over. I'm not quite comfortable taking the console apart to rewire things but I'm frustrated as I'm sure most other OB '05 owners are with not being able to hard wire an iPod to their stereo system. You can't even swap out a system from the VDC (which has MP3 capabilities) Do you know of any other resources (website, forums, etc.) that are monitoring this situation to see if it improves?

Thanks much!

poisson

I also have (had) the squeak along the front edge of the sunroof shade. I bought a roll of 3M velcro and attached some of the soft (hook?) portion along the top of the front of the opening (not the shade itself - rather the leading edge of the opening). It works well.

kens

You're quite welcome -- unfortunately, the rules are the rules.

Taking apart the center console is not as hard as it seems. If you give yourself plenty of time and go slowly, there really isn't much too it. You don't even need special tools to do it. Again, lots of documents available at the other website.

In the very worst case that you should badly scratch or break a plastic piece, you probably could easily find a replacement from the dealer parts department for not a whole lot of money.

With regards to a VDC stereo swap -- it's been done. But it's an expensive modification since the VDC headunit alone costs somewhere around $600. I personally didn't think it was worth the price to be able to play MP3s.

LGT.com is probably the best place to keep an eye out for any developments. There's another website, Subaru Outback dot com (remove spaces) that also has similar information.

I seriously doubt any aftermarket company would be able to come up with a solution. The problem is that the headunit and climate controls are all integrated onto one circuit board. The next best solution is to hack into the headunit (and some people are trying to develop a board to do that) or to wait and hope if Subaru ever releases a AUX-in ready headunit for our model. Of course, that last option would still require you to spend quite a bit of money.

Ken

capriracer

Thanks, kyfdx!

I made some errors in the posting and after about 3 repostings, the software wouldn't allow me in to make more revisions.

Craig, I was hoping you'd move this discussion to the other forum, but...

It is true that as inflation pressure goes up, the footprint size goes down - and using inflation pressure to get footprint size is a reasonable approximation - but that's what it is - an approximation. We're probably talking within 30%, which is probably good enough for comparisons and rough calculations, but it's hardly accurate - and more importantly it leads to misconceptions.

Like the air is supporting the weight of the vehicle.

If this were true, then tires should never suffer stress related failures. I'm sure Firestone would be glad to hear that.

There's been a lot of work done in Finite Element Model verification and the net result is that the inflation pressure affects the shape of the tire and how it deflects, but doesn't enter into stress equation. It's similar to the "web" portion of an I beam - which mostly connects the 2 load carrying portions of the beam. The "web" is important, but it doesn't really carry the load.

Hope this helps.

snowbird

Thanks for your sugggestion. I will try that, too. Jason

c_hunter

I agree with you 100% that the air chamber and tire vessel act as a single structural unit, and both are needed -- maybe we have been saying the same thing all along. But do a control volume to look at the static loads/pressures in a typical tire cross section and you see some interesting things including the W~P*A relation. I was starting to write up the details on that analysis, with some sketches, then it occured to me that experimental validation might be an easier way to make the point. My Honda S2K weighs 2835 lbs, with a 49% F / 51% R weight distribution and even distribution side to side. That puts 694 lb on each of the front tires (which happen to be smaller than the rears, but that's another story). I drove the car into my garage so the front tire was on some graph paper and gave the steering wheel a teeny wiggle, which left a real nice impression of the tire's contact patch. Counting that out gave a contact area of 21.6 square inches. With those numbers, I get P ~ 694/21.6 = 32 psi. Then I measured the tire pressure with my digital accu-tire gauge, and damned if it didn't read 32psi! Now, the gauge is calibrated to +/- 0.5 psi and my area measurement is probably good for +/- 0.2 square inches, but even with that uncertainty it's darn close.

Anyway, this is what the control volume analysis also predicts, so it makes sense to me. Interestingly, if you draw another control volume that includes the tire sidewall forces, it comes out saying that the sidewall must be in tension, which is kind of cool. But it all makes sense in the context of the tire being a pressure vessel and supporting a load. So I continue to view the tire/air combination as the structural element, but the sidewalls aren't really providing any vertical support by themselves (which is what you see when there is no air in the tire).

By the way, the experiment is very easy to reproduce if you know the weight pressing down on a particular tire, so I would encourage people to make some similar measurements if they're looking for a science project!

Craig

ssminton

Drove a bit during the holiday weekend. Keeping my speeds under 63mph resulted in average mpg of 27.5 in my 2005 Outback VDC. This is almost 5mpg higher than my "normal" driving conditions. Again... slowing down may be something to consider?

capriracer

Good analysis.

But there has been a lot of work done on pressure distribution of the tire footprints, and there are machines which are used to get these distributions (I walk by one every day) - and one of the side benefits is that the footprint can be accurately measured - not to mention the load.

These studies yield that the width of the tread can have a profound impact on the average contact pressure (it has little effect on the length of the footprint). This is one way to conduct a series of thought experiments on the subject.

But more importantly, there has been a lot of data generated in this area. And, yes, sometimes you get the same pressure as the inflation pressure.

Hope this helps.

ateixeira

Very interesting exercise, Stanton.

Basically you get diminishing returns. If you already own something that gets 25mpg or better, there isn't a whole lot to gain, certainly not enough to offset buying a new car or truck.

-juice

locke2c

Craig, now inflate to 40psi and re-measure, or deflate to 20psi.

I am not an engineer of any type yet I know that the tire's structure bears weight. Someone used the runflat example earlier-- clearly we can see that under minimal pressure, that type of tire is still bearing the weight of the vehicle due to its structure. How difficult is it to imagine a tire that is *nearly* that same type of structure? A summer performance tire has extremely rigid contruction.

~Colin

c_hunter

I do need to adjust pressure since it has been getting cooler, so I'll make 1-2 more measurements tonight at different pressures.

I have seen flat summer tires and they are not really supporting weight in a structural sense -- they are acting like a floppy rubber spacer between the rim and the road. Run-flats are a different ball game because they have reinforced structure that can function without air. I do not claim the trend holds for these tires when they get out of the inflatable structure range, just as it does not hold for regular tires that go flat!

The other thing to keep in mind is that the sidewalls are in tension in a properly inflated/loaded tire under no load. When you load the tire up (W) statically, the tension (T) goes approximately like:

T ~ P*A - W

When the applied load (W) exceeds P*A, that tension goes negative and the sidewall is actually in compression. That would be a case where the sidewall is bearing some of the vertical load.

Craig

locke2c

c_hunter

I just believe Newton's law and physical evidence more than things people say on the web!

locke2c

so you're supremely confident that tire construction is not materially affecting the numbers that you would (have, actually) predicted with the ideal gas law, eh. I find that interesting considering that you readily acknowledge that a runflat exists outside the law.

~c

kens

Over the weekend, my rear gate latch mechanism suddenly failed. When I tried to close the rear gate, I noticed that the latch wouldn't catch. Some close inspection showed that the plastic body surrounding the latch had cracked and some of the vinyl sleeve on the latch had peeled back. No symptoms until that point and I've never forced my gate closed.

Anyway, I brought my LGT wagon into the dealer today to have the latch replaced and received a base 05 OB 4EAT as a loaner.

Just a few observations:
- Strangely, the base OB didn't feel as "heavy" as the previous OBXT loaner I drove. I'm not sure if it's the tires or just my own perception, but it seemed to be a little lighter on it's toes.

- Oh boy do I miss the power. I've been spoiled by my 2.5 turbo. The NA 2.5 engine was working quite hard to keep me at speed on some twisty mountain highways. Leaving the 4EAT in Sport mode only seemed to help a little. I'd love to try the new 2006 base engine next time.

- Again, a subjective measure, but there seemed to be more engine noise coming into the cabin at similar RPMS vs. the turbo. With the NA 2.5, you know when it's starting to climb past 4000 RPM -- brought back memories of my 98 Forester S!

- Not surprisingly, the 4EAT Sportshift downshifts more abruptly due to having one less cog. The 5EAT is quite a bit smoother.

- I noticed this with the OBXT also, but the OB seem to have a more touchy brake pedal action. My first few stops were a bit jerky as the amount of force needed to engage the brakes was clearly less than on my LGT. My LGT brakes seemed more progressive, although I can see drivers like my wife favoring the OB brake feel.

- Like with the OBXT, I thought the rear suspension was a bit underdamped. The rear wobble when coming around a corner with a bump is a bit unsettling.

Ken

ateixeira

Yeah, the ratio spread is greater on the tranny. All Subies should get a 5 speed auto ASAP.

-juice

c_hunter

Believe it or not, it's just an application of Newton's Law (sum of forces = 0 for a static object) that cuts through the tire and sums up vertical forces from loads and pressures. The tire material doesn't matter in this simple analysis (it would if you converted the forces and loads into internal stresses and wanted to know the deformation or mechanics of the materials, but that's going deeper than we need). This is the same analysis you'd use to figure out the basic loads/stresses in an inflatable structure, or for that matter, a pressurized tank/pipe carrying a load or supporting its own weight.

That equation I gave before shows that the combination of the sidewall force and P*A is what offsets the weight. It just so happens in my experience that the vertical sidewall force is really small on a loaded tire compared to P*A, so P*A is pretty much effectively carrying the load. Now, the tire is still required to make the inflatable structure that contains the P*A, so it's not like the air alone is carrying the load. In fact, the tire vessel is containing all that pressure and is still under a tremendous amount of stress. So tire construction does matter, it's just not a direct contributor to the vertical load for the most part.

On a run-flat, take away the P*A contribution and the sidewall (or other internal structure of the tire -- some have internal bracing/ribs) carries the load. In order for the tire to really have that capability, I bet it's also carrying some load even when pressurized. If the run-flat tire can support a load without pressurization, it's not a straight-up inflatable structure, but rather, a hybrid structure. So I don't think it will follow the same trend.

Craig

locke2c

so it sounds more like you're assuming that passenger car tires are similar to aircraft landing gear tires?

erics6

I think an interesting question is... If you need AWD, what are your best MPG choices, given that most AWD/4WD's are heavier trucks.

2005 Approximate HP & EPA MPG Estimates (stick shift)
Subaru Outback 168hp 23/28
Subaru Forester 168hp 23/30
Toyota Matrix AWD (auto only) 123hp 26/32
Honda CRV 160hp 21/26
Toyota RAV4 161hp 22/27

For me, the Legacy/Outback is the only choice for space, reasonable power and fuel efficiency. My Miata gets real world 28-29 MPG around town, but I can't sleep in the trunk, go off road, or pick up my latest IKEA furniture find. ;-)

Eric

luck11

Its been a while since visited this forum.

My 05 OB XTL will be a year old in late Sept (approx 7.5 K miles). In Aug, both headlights burned out within a two week period. They were covered under warranty, but still a hassle to bring into the dealership for only a bulb.

I had my 2000 OB for almost 4.5 years, and never once had to change a headlight bulb.

Some others have posted about headlights burning out. What's up with the 05s? Is this a known problem?

Cheers.
Jay

ssminton

You are not alone. I replaced both headlights in July/August. My 05 VDC has 31K mi and is one year old. I found this odd too, for I have never had to replace headlights before.

c_hunter

Correct, and that may be a stretch admittedly. Tires on airplanes, and the ones we have tested for the space shuttle, look a lot like donuts in terms of aspect ratio. Of course modern car tires have a lot lower aspect ratio.

I just looked at 22, 33, 31.5, and 42 psi in the driver's front tire of my S2K, with and without my butt in the seat. I got two nice linear P*A trend lines with the 33-42 psi data, but the 22 psi data did not lie on the curves. The contact patch was a little tougher to measure at that low pressure, so that could be one source of the discrepancy. But, I am betting at that pressure, the tire sidewall is taking up a portion of the load as the tire starts to flatten out, and the sidewalls go into compression (again go back to the T ~ P*A - W eqn). The measurement suggests that about 100 lbs out of the 700 lbs is supported by the sidewalls at 22 psi. By the time pressure drops to zero, of course all 700 lbs would be on the sidewalls.

So it looks like the linear P*A ~ W trend works in a range of pressures where the tire is properly inflated, but is preceded by a nonlinear range when the tire is underinflated. I imagine there could be another nonlinear range on the high end as well.

BTW, these are Bridgestone Potenza RE050 summer tires, 215/45-17 on the front (rears are 245/40-17).

Craig

capriracer

I'm wondering if we are over complicating the issue, so let me try this:

If the air is what holds the vehicle up (and not the tire), then we can all identify where the air pressure is pushing down on the road surface - the footprint. But where is the air pressure pushing UP (against the vehicle)?