Successful tire experiment 2014 Breakout but need tire expert psi advice
#51
08-19-2014, 04:15 PM
I had the Ducati Diavel with the 240/45 ZR17 Pirelli Diablo Rosso II Sport and street tire also. . Awesome awesome tire ,handling was unreal .They stick like glue . I might put Pirelli on my Fatboy soon . The only problem was wear 6000 miles if you were real lucky . 4000 miles and you will lose some performance . Probably should change at that point . Also bike only weighed 436 dry ( 160 HP ) Your bike weighs more so you might get less mileage depending how you ride. I would run 35 to 37 PSI in rear.36.9 is what they recommended . Tire pressure was very important . Great write up Thanks ,you got me thinking too .Love that break out because of that rear tire . Need tires soon too on Fatty ,have Dunlaps now only get 7000 to 8000 miles.
#52
01-14-2015, 11:32 AM
OP
but what about the front tire!!!! My front wears out as fast as the rear. I have beautiful brand new CVO rims for a conversion but I'm waiting for new tires to come out. Your rear looks great but not fronts yet? is is ok to miz tires like that? I would assume it doesn't matter except fro looks? But we need a new front tire for these things. I refuse to keep buying these Dun-rocks. One to maybe one and a half seasons out of them is not enough.
just wondering if you found anything for the front?
but what about the front tire!!!! My front wears out as fast as the rear. I have beautiful brand new CVO rims for a conversion but I'm waiting for new tires to come out. Your rear looks great but not fronts yet? is is ok to miz tires like that? I would assume it doesn't matter except fro looks? But we need a new front tire for these things. I refuse to keep buying these Dun-rocks. One to maybe one and a half seasons out of them is not enough.
just wondering if you found anything for the front?
#53
01-14-2015, 11:37 AM
OP
but what about the front tire!!!! My front wears out as fast as the rear. I have beautiful brand new CVO rims for a conversion but I'm waiting for new tires to come out. Your rear looks great but not fronts yet? is is ok to miz tires like that? I would assume it doesn't matter except fro looks? But we need a new front tire for these things. I refuse to keep buying these Dun-rocks. One to maybe one and a half seasons out of them is not enough.
just wondering if you found anything for the front?
but what about the front tire!!!! My front wears out as fast as the rear. I have beautiful brand new CVO rims for a conversion but I'm waiting for new tires to come out. Your rear looks great but not fronts yet? is is ok to miz tires like that? I would assume it doesn't matter except fro looks? But we need a new front tire for these things. I refuse to keep buying these Dun-rocks. One to maybe one and a half seasons out of them is not enough.
just wondering if you found anything for the front?
120/70-21 Avon Cobra Front 10.55 lb, or
Avon Venom Front 11.X lb
Jim G
#54
01-14-2015, 12:10 PM
first off thanks very much for the reply Jim. But here's the weird thing. I actually called Avon. they told me they do NOT have a tire for our front? He even made me get him the stamped size on the rim(which I didn't even know it was there). I gave him the rim size/width and he told me "they had nothing that would fit". this was last summer.
I wondering if they just came out with this front tire. Sorry to bother you but when did you find the one your talking about?
I wondering if they just came out with this front tire. Sorry to bother you but when did you find the one your talking about?
#55
01-14-2015, 12:21 PM
first off thanks very much for the reply Jim. But here's the weird thing. I actually called Avon. they told me they do NOT have a tire for our front? He even made me get him the stamped size on the rim(which I didn't even know it was there). I gave him the rim size/width and he told me "they had nothing that would fit". this was last summer.
I wondering if they just came out with this front tire. Sorry to bother you but when did you find the one your talking about?
I wondering if they just came out with this front tire. Sorry to bother you but when did you find the one your talking about?
You can use
Rear: Metzeler 880 MARATHON 240/40-18 or 260/40-18
Front: Metzeler 880 MARATHON 120/70-21
#56
01-16-2015, 12:38 AM
I found an article by a writer (Larry Kelly) on Ducati.net. He presents an approach that SEEMS to make some sense.
He basically says that when you run your bike down the road, the tires heat up due to cyclic deformation and deflection, and the heat manifests as higher psi in each tire.
He also says that you can tell if you are in the correct psi range or not via a simple formula, whose derivation he does not explain. Basically, he says that if your front and rear tire pressures are in the correct range for your particular combination of bike weight, rider weight, and type of riding, and have ridden the bike far enough to get the tires to equilibrium temperature, the front psi will increase 10% and the rear psi will increase 20%.
Since I started my testing using 36F and 42R, his formula would predict, IF I had chosen the cold psi setting correctly, hot psi of 40F and 50 psi. (These seem high)
What I actually found after 21 miles was 38F (+6%) and 42R (+zero %).
His instructions say that I should lower the tire pressures until I get the 10% increase at the front and the 20% increase at the rear, at equilibrium conditions.
I'm not sure that I can accept rear tire at 50 psi hot as the target, but maybe could accept some percentage increases a bit smaller than 10%F and 20%R.
It DOES seem obvious that the 42psi rear is too high, because it did not increase at ALL even after 21 miles.
Thoughts?
Jim G
He basically says that when you run your bike down the road, the tires heat up due to cyclic deformation and deflection, and the heat manifests as higher psi in each tire.
He also says that you can tell if you are in the correct psi range or not via a simple formula, whose derivation he does not explain. Basically, he says that if your front and rear tire pressures are in the correct range for your particular combination of bike weight, rider weight, and type of riding, and have ridden the bike far enough to get the tires to equilibrium temperature, the front psi will increase 10% and the rear psi will increase 20%.
Since I started my testing using 36F and 42R, his formula would predict, IF I had chosen the cold psi setting correctly, hot psi of 40F and 50 psi. (These seem high)
What I actually found after 21 miles was 38F (+6%) and 42R (+zero %).
His instructions say that I should lower the tire pressures until I get the 10% increase at the front and the 20% increase at the rear, at equilibrium conditions.
I'm not sure that I can accept rear tire at 50 psi hot as the target, but maybe could accept some percentage increases a bit smaller than 10%F and 20%R.
It DOES seem obvious that the 42psi rear is too high, because it did not increase at ALL even after 21 miles.
Thoughts?
Jim G
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!
#57
01-16-2015, 08:24 AM
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!
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!
I identified the Avon front tires fairly recently, within the past 3 months or so.
As for contact patch, what I meant to say is that for any given constant weight of motorcycle and rider, obviously the contact patch size has to be total weight divided by psi. If psi goes up, size of contact patch must decrease. Basic physics.
So, for 1000 lb of bike plus rider, and psi = 40, contact patch size is 1000 lb / 40 lb per sq in = 25 sq in.
For 1000 lb of bike plus rider, and psi = 30, contact patch size = 1000/30 = 33.3 sq in.
Jim G
Last edited by JimGnitecki; 01-16-2015 at 08:29 AM.
#58
01-16-2015, 11:47 AM
#59
01-17-2015, 06:10 AM
I posted a follow-up thread in which I explained how I found the correct pressures to use, and how it delivered a good "hot" psi. I'm sorry I cannot recall the title I applied to the thread. With the Pirelli rear tire I settled on 38.5 to 39 psi. With the stock Dunlop front, I settled on 38 psi. The reduced rear psi also markedly improved the suspension feel.
I identified the Avon front tires fairly recently, within the past 3 months or so.
As for contact patch, what I meant to say is that for any given constant weight of motorcycle and rider, obviously the contact patch size has to be total weight divided by psi. If psi goes up, size of contact patch must decrease. Basic physics.
So, for 1000 lb of bike plus rider, and psi = 40, contact patch size is 1000 lb / 40 lb per sq in = 25 sq in.
For 1000 lb of bike plus rider, and psi = 30, contact patch size = 1000/30 = 33.3 sq in.
Jim G
I identified the Avon front tires fairly recently, within the past 3 months or so.
As for contact patch, what I meant to say is that for any given constant weight of motorcycle and rider, obviously the contact patch size has to be total weight divided by psi. If psi goes up, size of contact patch must decrease. Basic physics.
So, for 1000 lb of bike plus rider, and psi = 40, contact patch size is 1000 lb / 40 lb per sq in = 25 sq in.
For 1000 lb of bike plus rider, and psi = 30, contact patch size = 1000/30 = 33.3 sq in.
Jim G
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.
