Increased pressure in lower profile?

[midNight Sun Xpress]

There are different opinions about which tire pressure should be applied when changing to lower sidewalls. I.e. should the pressure be higher in a xxx/35/18 compared to stock given the same diameter? If so, is there any "golden rule" for how much the increase should be when wheelsize is increased by 1"? Or aspect ratio is decreased by 10?

I have heard anything from 0 to 20 % increase.

 

[Zishan]

The general rule of thumb is to follow OEM recommendations that are listed. From there, you can make some adjustments as desired.

With 18/19's, I over inflat my tires a tad to protect the rims a bit. I've read that it doesn't really make a difference, but I do it anyways.

Also, I've noticed a tad more PSI up front helps the steering feel at slower speeds. In other words, my steering feels easier. My car of course has no power steering. But I could be imaging things, as I do often. :biggrin:

 

[angus]

I like to run the highest pressure listed for the given tire, unless you are trying to tailor the handling or get a softer ride.

 

[Track Addict]

You should always stick to the OEM tire pressures, Honda (and all other manufacturers AFAIK) figure out the best tire pressure for handling, braking, performance, comfort and MPG. I am no engineer, so when i am driving on the street i try to make sure my psi matches what is inside my door jam.

on the track is a different story however :biggrin:

 

[Vega$ NSX]

There's not a super strong correlation between tire sidewall height and air pressure. Meaning just because you go with a lower profile tire, doesn't necessarily mean you need to increase tire pressure. Stick with the recommended tire pressure for that specific tire and adjust accordingly for individual preference.

 

[midNight Sun Xpress]

To sum up; there is no general rule of thumb.

I used to apply the OEM tire pressure, but we had a big discussion on the national Honda forum where some "tire guru" recommended 20 % increase when increasing the wheel size with 2" (= decreasing tire height ratio by 15-20).

I run 18" on our Volvo, and they are noisy when a inflate the tire by 10 % or more.

On the NSX I have increased pressure by 10 % on my 16/17 combo, and they feels a tad more slippery on wet roads. I'll try OEM pressure for a while.

What about track days? Should the pressure be decreased?

 

[Vega$ NSX]

Again, changing the profile of the tire does not correlate to air pressure. It's like saying, if I change the paint on my car to a different color, what kind of wax should I use? That is because every car and tire are designed for a specific operating pressure. It is a very complex function of tire wear, rolling resistance, grip, sidewall strength etc.

Think of the OEM tire pressure as a baseline. When you change the pressure in the tire, you are trading one attribute for another. So let's say you are at the drag strip and you need a good launch. You can decrease the pressure in the tire to increase the contact patch and sidewall rotational give to increase traction, with a tradeoff of higher rolling resistance, decreased and uneven treadwear. However, given the circumstances the benefits outweigh the tradeoffs. Conversely, say you are driving across country and will be droning at highway speeds for 500 miles at 65 MPH. Then it's probably a good idea to increase the tire pressure and improve your gas milage because you aren't going to be taking high speed turns and really don't need the traction.

My theory on why someone would tell you to increase the tire pressure on a lower profile tire is because as you decrease the sidewall, you lower the margin of forgiveness on wear along the sidewall. When a car corners very hard, the tire will roll. If the tire does not have enough air pressure the tire can roll hard to where the sidewall can start to see some contact with the pavement. If you roll far enough, you can even roll on to the wheel or even pull the tire off of the wheel. This is very dangerous. On a tire with a large sidewall, you have more play in the tire and a larger distance of forgiveness before you get to that point. A shorter sidewall tire, has less. Take it to the extreme and imagine rubber band-like tires and imagine that car taking a hard turn. How much can the tire give before it actually rides on to the wheel? Not much. By increasing the tire pressure, I suppose the thought is to increase the sidewall stiffness (makes the wheel more rigid) and give yourself a bit of added protection on hard cornering. However, tire manufacturers already know this and stiffen the sidewall to compensate for this attribute. That is why you should stick to the factory recommended pressures because the tire is designed at that given pressure to function correctly. Again, when you deviate from these pressures you are trying to trade off one attribute at the cost of another.

Also, keep in mind all of this is at the cost of decreasing the traction performance of the tire as well. Hence why high performance cars (like F1, NASCAR) don't use low profile tires. They are more for looks but actually decrease performance.

 

[R13]

Also, keep in mind all of this is at the cost of decreasing the traction performance of the tire as well. Hence why high performance cars (like F1, NASCAR) don't use low profile tires. They are more for looks but actually decrease performance.

That's not entirely true. You can't compare F1 to a street car...that's like comparing a remote control plane to a B2...sure they both fly, but not exactly on the same principles. NASCAR tire sizes are dictated by rules that originated in probably the 1970s...they also still use carburetors.

A shorter, and therefore (in most cases) stiffer sidewall means the tire is more responsive and is less likely to roll onto the side wall. On the other hand, if you get too small of a sidewall, you lose compliance.

Car & Driver did a test a few years ago and found (for the same section-width tire on a VW) that there was a sweet spot in the 17-18" range for tire performance.

Of course, if you know what you are doing and get all scientific with it by taking the effective spring rate of a given tire vs. it's lateral grip, you can tune the rest of the cars suspension set up to work better with a shorter or taller sidewall (hence why Ferrari, Porsche, et. al can get away with 20+" wheels for appearance without sacrificing too much handling).

 

[midNight Sun Xpress]

Thanks Vega$NSX for take the time to write a very good explanation and a great reflection.

I know a little bit about tires and suspension and how they work together, but sometimes you get questions like this to which I could not give a proper answer. Changing the setup, i.e. bigger wheels, will require trade-offs against grip, softness, wear etc. I understand finding the best pressure (the sweet spot if you like) is a function of the tires qualities, the road condition, driving habits, etc. etc.

As for myself, I want a comfy drive which implies a low pressure ( = OEM) and I prefer to have a certain amount of sidewall to get some suspension from the tires as well. Hence no 18/19 combo on my car. :smile:

 

[Vega$ NSX]

That's not entirely true. You can't compare F1 to a street car...that's like comparing a remote control plane to a B2...sure they both fly, but not exactly on the same principles. NASCAR tire sizes are dictated by rules that originated in probably the 1970s...they also still use carburetors.

I was basically referring to people riding on 24" rubber bands. I was trying to point out that the trend towards plus sizing tires (like on luxury cars, VIP style) actually has a negative impact on comfort and performance as well as increased total wheel/tire weight. I'm not suggesting 5" wheels with giant sidewall tires as the ultimate performance setup or that going with more sidewall was better. I'm refering to people who go above and beyond the OEM sizes to get the "low profile" look. I don't really conisder 18" wheels low profile since many OEM wheels are already 18". I'm talking about people who go above and beyond the OEM size, where again, I mention there is a trade off; in this case looks for performance. Again, I was just trying to point out that many people commonly mistake that plus sizing their wheels to get lower profile tires increase performance, when in reality, that is probably not the case. And that the optimum balance of wheel and sidewall size is often not the most aesthetically pleasing.

 

[R13]

I was basically referring to people riding on 24" rubber bands. I was trying to point out that the trend towards plus sizing tires (like on luxury cars, VIP style) actually has a negative impact on comfort and performance as well as increased total wheel/tire weight. I'm not suggesting 5" wheels with giant sidewall tires as the ultimate performance setup or that going with more sidewall was better. I'm refering to people who go above and beyond the OEM sizes to get the "low profile" look. I don't really conisder 18" wheels low profile since many OEM wheels are already 18". I'm talking about people who go above and beyond the OEM size, where again, I mention there is a trade off; in this case looks for performance. Again, I was just trying to point out that many people commonly mistake that plus sizing their wheels to get lower profile tires increase performance, when in reality, that is probably not the case. And that the optimum balance of wheel and sidewall size is often not the most aesthetically pleasing.

Ah, ok, that makes sense. I just read your earlier post in the context of sizes that folks normally fit on an NSX. :smile: