Contour Enthusiasts Group Archives Forums Modifications Duratec Performance Acceleration with 18's

Not if the container was overdesigned to hold the pressure in the first place (because its thickness was dictated by road holding parameters instead), which is what I'm not sure about here.


We don't need a higher design pressure in the tire, but the lower profile causes the same overall pressure to produce a higher pressure concentration at each point on the tire.

Do #1 & #2 as they are cheap. If you have never changed the fuel filter, do it also as they are supposed to be chaged every 15k miles, IIRC. It's also cheap. Test drive it. I bet that's all you need. #3, well if you're going to replace it, get a high flow one, but that'd be last on my list to check.

You cannot change the timing.

OK folks, right out of a textbook, here's the lowdown on the inertia deal. The actual formula for an annular ring (imagine the rim without any spokes, since the biggest part farthest from the axle has the greatest effect) is:

I=Mass(Outer radius^2+inner radius^2)/2

This is including the effect of the thickness of the outer part of the rim. So it does go as the square of the sum of the radii. This means that the actual inner and outer diameter of the two wheels is what is important, not the 16" or 18" generalization.

I ran some really rough numbers and if you neglect any weight difference (which sounds close anyway), the inertia is about 30% greater with the 18. Even if the weight is different, it's the fact that the radius is squared that matters. This is also assuming a similar overall construction, but I feel confident in saying that the 18 will always have a higher moment of inertia.

Physics works! :p

I believe this if we were just looking at the wheel alone.

But add in the tire and I'm not sure you can say the same.

If both the 16" and 18" combination have the same radius when the tire is considered, I'm not sure what you say still holds true.

I will agree that if you just consider the wheels w/o tires, given the same mass, you are correct. But you must consider tires as well.

This horse is almost dead now !

TB

Ooops, I didn't notice the second page on this thread, but my last reply still holds.

tboner, to answer the question it looks like you will ask, I called the tire effect negligible because of what you said about the weights being very close and the distribution of weight in a tire. What I mean is that the vast majority of weight in a tire is not sidewall, it's all that rubber and belt that's on the road, and since I'm assuming negligible differences in diameter the overall wheel/tire combo, that weight is in about the same place, 16 or 18.

Ok, about this air pressure/sidewall thing. I can guarantee that the sidewall is only designed to deal with cornering forces, road generated forces, etc. Holding air is not on the list of worries that they are going to have. Have you ever seen high pressure hose? Heck, look at your garden hose, it's dealing with 30 or so psi and it's thin as hell! So 30-50 psi in something like tire rubber ain't no big deal.

Also, get it out of you head that the sidewall holds up the car. The sidewall does NOTHING for holding up the car (other than containing air, which is what DOES hold it up).

tboner, what you said about the air pressure on the contact patch is exactly correct. If you measure it precisely and have the air pressure, you have the exact weight of the car. There was actually a question about this in the Sunday paper (Baltimore Sun) this past week in the ask the science guy section on the funny pages!

Whew, enough posting, I'm hungry... off to lunch

dammit tboner! I was posting the preemptive answer to your question while you were posting it!

Die horse die!!!

Here is a good bar bet question.

If two tires are inflated to the same pressure, but one is 225mm wide and the other is 195mm wide, if you install a set of each on the same car, which puts more rubber on the road?

I bet you 90% of gearheads will say the 225mm tire.

Yet the answer is neither, they will both put the same amount of rubber on the road. However, the shape of the contact patch will differ between the two. The wider tire will put a wider but short patch, while the narrower tire will have a narrower, but longer patch.

Someone use the nice convenient numbers of 6" wide by 4" long vs 4" wide and 6" long.

Fun with physics

I think I'm hanging with the physics discussion pretty good for a EE with an emphaisis on digital design.

TB

No... we're just humoring you

lol


I'm not quite sure if you're referring to just the example I gave using those specific tire attributes or just in general. The tire effect cannot be considered negligible when comparing inertias, if for nothing else, because it contains pressurized air. Even if a 16" and 18" tire have the same outer diameter they will have different volumes of air contained within them.


I agree to a certain extent here. Tires will rupture at a certain pressure, though, and it's not 3-4 times the max pressure suggested by the manufacturer. So I think there would be a point when the sidewall becomes too thin at certain points (particularly because of the expansion of the tire upon pressurization).

Ok, I think I need to reel you engineers back in the realm of cars

... That's where my original complaint comes in... You see I was neck-and neck with the SE up to the top of 3rd gear. Don't you think that when the stock rubber on the SE rotates at that speed, its contact patch diminishes because of centrifugal force acting on the tire, whereas the wide rubber mounted to my 18's still puts its "4in x 6in" of rubber to the asphalt? Factor in the aero drag because of the width and you see why that SE was able to pass me...

Agreed, we should have never been neck-and-neck in the first place I will investigate the plugs/wires/fuel filter on my car for that issue.

How's my thinking sound??