Originally Posted by EagleRay

Sorry to say that Jim, but I think here you are mistaken. Your assumption absolutely ignores the inner restistance of the tire due to the rigidity of the material and the form of it's build.

Your formula works fine if you have a weight being embeded on a fixed and well known area to caclulate the mechnical pressure w/o considering any counter forces or losses (ideal/linear function). For the case of the air pressure in tires it simply doesn't work as the counter forces implied by the tire itself are completely ignored but significant. These forces make it impossible to apply any kind of simple linear formular!

The general statement that the patch size is depending on the tire pressure and increases when lowering and decreases when increasing the pressure is true though, but it's also common sense to anyone who ever saw flat tire.

Originally Posted by CalBear96

Hi Jim,
I think you are missing the crux of the formula. If I am reading this right then the fact that you are not getting a 20% jump in rear tire pressure means that a cold PSI of 42 is not the correct pressure for your riding parameters. If you were getting a hot reading at 50 then 42 would be right on the money. The conclusion that I draw from all the information you have given is that 42 PSI is too high and that you will likely get the 20% increase if you decrease your starting pressure and test and retest for as many days as you need until you get that magic number. Now, if you reach the minimum cold pressure and you are still unable to get a 20% increase (the nice thing is that the lower your starting figure the fewer PSI you need to increase to reach 20%) then if not outright disproven, you have certainly poked a large hole in his theory. But isn't that the very nature of a theory and the beauty of the scientific method?
If motorcycles behave anything like cars, then the only thing over-inflating tires will do is increase your fuel economy due to lower rolling resistance. Personally in my car, I run a cold PSI of 40 on long freeway trips. For my motorcycle I prefer a larger contact patch and traction to marginally greater fuel economy.
One last thing I question is your assertion that a wider contact patch necessarily decreases the length of contact. That would hold true if the area of the contact patch remained constant, but as far as I know there is nothing that forces that to be so. I would need to do it in practice, but in my mind thinking about letting some air out of my tire, I do not think that as the contact patch widened it would also decrease in length. If anything I would think it would increase slightly. But it is totally possible that it is one of those counter-intuitive physical phenomena. I am not a motorcycle nor tire expert, just a geek. And i truly hope none of this offended; I was honestly hoping to gain clarification as this is a topic that interests me as well.

Cheers!

Originally Posted by JimGnitecki

I believe I have found a rear tire solution that improves the performance of the Breakout dramatically, but I need some advice on tire pressure from a “tire expert” if we have one on the forum. Let me explain.

I have been unhappy with the stock tire provided by HD, the Dunlop Elite 3 Custom Wide tire, for several reasons.

First, this tire is rated by its manufacturer for a maximum pressure of 41 psi, but HD, incredibly enough, recommends 42 psi on the Breakout with this tire. Since the capacity of this 79V (load index) tire is 963 pounds, this HD recommendation is very suspicious beyond the obvious fact that it exceeds the maximum psi specified by the tire manufacturer. The Breakout is NOT a very heavy bike – in fact, outside of the Sportster range of HD bikes, it’s one of the lighter HD models.

Secondly, the tire has an initial tread depth of 12/32 inch. After just 3400 miles of pretty easy riding (I average 42 to 45 mpg ☺ ), I was down to 1.5/32 inch, in the CENTERLINE ONLY area of the tire. A motorcycle tire is considered worn out when it hits 1/16” tread depth, so I have consumed 95% of the 11/32” of “usable” tread in 3400 miles, which means I would have hit the 1/16” limit by 3600 miles. This seems low to me, given that the Breakout is a cruiser, not a sportbike, and I ride it like a cruiser, not a sportbike.

Thirdly, the fact that the wear is concentrated along the “centerline” of the tire, despite the fact that I do mostly winding roads in the Texas Hill Country, reinforces the suspicion that HD’s recommended tire pressure of 42 psi is too high – at least for a solo rider who weighs 235 pounds with safety gear on.

Fourthly, this Dunlop tire is a very heavy tire. Per Dunlop, it weighs 21.75 pounds. I am very sensitive to tire and wheel weight. Heavy wheels and heavy tires have high moments of inertia. That creates multiple negative symptoms, including resistance to directional changes, a “cement overshoes” effect when accelerating (particularly when coupled with the bike’s ridiculously tall factory gearing), and significantly lowered power readings on any inertia drum type dyno (like the common Dynojet).

Since I have the already very heavy HD “Chrome Turbine” wheels on my Breakout, the combination of heavy wheels and heavy tires is particularly bad. In fact, when I had my bike in for dyno tuning at Mike Lozano’s shop, Mike commented that the combination of Softail engine balancer weight, heavy rear wheel, and heavy rear tire, is hurting both my bike’s dyno readings and its real world performance.

So, I went looking for a new rear tire. Since the Breakout is a cruiser, looking at “race spec” sportbike tires is not only a waste of race spec tire, but in fact, no manufacturer even MAKES a race spec type tire in the 240/40R-18 size. But, several manufacturers make that size of tire in what they refer to as a “sport street” or “custom” tire.

MY personally most important attributes for the new tire were a manufacturer with a history of high quality, light weight, good dry and wet traction, and reasonable tread life. However, despite my actively looking for information on all of these attributes for each tire available in the correct size, the only two attributes that I could DEFINITIVELY measure or verify for each tire, were the manufacturer’s reputation and the weight of the tire.

Since I have had great experiences with Pirelli tires on prior bikes (Ducati Monster and several 9XX series Ducatis), Pirelli satisfied the manufactuer history requirement. And, it turned out that Pirelli also offers the lightest weight tire in this size range: its “Diablo” series tire. So, I ordered the Pirelli Diablo in 240/40ZR-18 size (Pirelli part number 1682600).

Comparing the specs of the Dunlop and the Pirelli was illuminating:

Dunlop Elite 3 Custom Wide
240/40R 18 79V (963 lb @ 41 psi)
Diameter = 25.91”
Weight = 21.75 lb
Initial tread depth = 12/32” (i.e. 3/8” or 0.375”)
$165 at Dennis Kirk

Pirelli Diablo
240/40ZR-18 79V (963 lb @ 41 psi) part no. 1682600
Diameter = 25.75”
Weight = 15.55 lb (confirmed by Amazon shipping weight = 17 lb, and Jim G’s postal scale measured the actual delivered specific tire as 16 lb 0 oz. This variance versus “stated weight” is very typical for tire manufactuers – they all slightly understate their tire weights, and all tires of a given model vary in weight around a target weight)
Rim used by Pirelli for its measurements: 8.5”
Acceptable rim width: 8.0 to 9.0”
Diameter: 654mm = 25.75”
Acceptable tire psi: 36 to 42
Speed rating = 168 mph
Initial tread depth = 5.5mm = 0.2165” = 7/32”
$171 at Dennis Kirk, $170 at Amazon

Note the following major differences:

The weight of the Pirelli is 6.2 lb less (i.e. 29% lighter!) than that of the Dunlop!

The initial tread depth of the Pirelli is only 7/32” versus the Dunlop’s 12/32”.

I am NOT knowledgeable enough about tires to say anything about “what these differences mean”. I am just pointing them out as obvious evidence of entirely different tire design philosophies.

Unlike Dunlop, Pirelli actually provides a recommended ACCEPTABLE psi range for the Diablo, and it is 36 to 42 psi. This means that Pirelli says this tire can work within that overall acceptable rnage, but naturally, within that acceptable range, the VEHICLE manufacturer is supposed to determine and provide the specific psi or range of psi that will result in the tire working properly WITH THAT SPECIFIC VEHICLE AND STATED OVERALL GROSS VEHICLE WEIGHT (which includes the wet weight of the bike, the rider, the passenger, and any luggage or accessories added). Sometimes, if a rider is lucky, the tire manufacturer publishes the recommended psi base don data from the vehicle manufacturer.

Since Harley does not offer Pirelli tires on its bikes, I could not get a recommendation from HD. And, while Pirelli does provide recommended actual psi for many different manufacturers’ models of bike, they do NOT include HD, as HD is not one of its target markets.

In the absence of a specific recommendation for psi from either HD or Pirelli, I went with Pirelli’s GENERAL guidance – the 36 to 42 psi range noted above. And, since the SAFEST starting point for tire psi is to go to the high end of the tire manufacturer’s recommended psi range, I started with 42 psi in the Diablo, and the HD recommended 36 psi in the front Dunlop tire.

Here are some critical data points that I would really appreciate getting some feedback on from someone who has really solid experience with determining correct tire pressures:

I started out this morning at 7am, at 73 degrees ambient temperature, with tires “cold” at 36F and 42R.

After 21 miles:
Front tire psi = 38 (+2 psi versus cold)
Rear tire psi = 42 (no gain in psi versus cold!)

Later, in 85 degree ambient temperature (note, this is 12 degrees hotter than when I set out on the ride), after 55 miles since the last stop on the 136 mile ride:
Front tire psi = 40 (+4 psi = 11% gain versus cold)
Rear tire psi = 45 (+3 psi = 7% gain versus cold)

I need someone knowledgeable on proper tire pressure determination to give me some guidance as to whether I should stay at the 42 psi rear, or whether I should reduce it, since the gain after 55 miles in 85 degree heat was only 3 psi or 7%.

That said, here’s the GREAT news on performance:

The bike feels like it has shed a pair of cement overshoes. No kidding. It feels notably more nimble.

In addition, Initiating turns is now FAR easier than it was with the Dunlop tire. I just have to “think” it now, and the bike goes there.

And, the combination of Mike Lozano’s careful dyno tune plus this Pirelli tire, has utterly eliminated the discomfort I used to feel on sharp right turns from a stop sign or red light. Now, the bike makes the turn willingly and precisely, without any manhandling or careful throttle/clutch work required. It’s a complete joy compared to before. It’s so much improved, that I am now considering forgetting about doing the gearing change (“shorter” gearing) that I have been trying to find the right parts to enable.

I stopped by Mike Lozano’s shop during my 136 mile ride today, and walked him through the tire change and the spec differences. His very first comment was that I will surely pick up “several” horsepower on his Dynojet dyno. He says he had a slightly larger weight difference some time back when he changed from a spoke wheel to a carbon fiber wheel on a customer’s Harley, and the difference on the dyno reading was around 6 horsepower. He pointed out that in MY case, with the weight reduction being concentrated 100% in the tire, versus spread over the entire wheel diameter (rim, spokes, and part of the hub), the difference in moment of inertia is massive. We are both looking forward to dynoing my bike again during his next monthly dyno day.

Now I still have no idea how this tire will wear. That’s going to depend to at least some extent on identifying the correct tire pressure to run, since tire psi has so much impact on tread and tire life. That’s why I am asking for some knowledgeable assistance.

But, it seems clear that the PERFORMANCE of the tire has utterly transformed the feel of my Breakout. The performance is so much improved that even if this tire turns out to have WORSE tread life than the already unimpressive Dunlop tread life, it would actually be worth it to me. It’s that much of an improvement.

So, the experiment will continue. I hope some tire expert who reads this posting will step in and provide some guidance on what pressure to run.

In the meantime, here are some photos of this ridiculously wide tire:


Notice in the following photo how the width of the tire makes even a refrigerator look “small”:


Attachment 566114


Notice in the following photo how low the tire profile is (note that you are seeing the the tire bead of the opposite side of the tire “through” the donut hole. The “near side” tire bead is just below the “white spot” on the upper right portion of the tire):


Attachment 566115



Jim G