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Trying To Understand Tire Data: PSI/Load Index

M

MikeT

Member
There have been numerous post all over this community forum focusing on the question of what is the optimal rear tire pressure for a replacement car tire on a Spyder. The resultant answers range from 18-30 PSI. After much research, I decided to go with the Kuhmo Ecsta AST(225/50R15 91H) tire on my RT Limited.
Today I found hard data regarding a 91H rated tire vs. a 76H rated tire from the TRA Standard Load Inflation Table

PSI Load Index
22- 904 lbs vs. 584 lbs
23- 948 lbs vs. 617 lbs
24- 975 lbs vs. 634 lbs
25- 992 lbs vs. 650 lbs
26- 1047 lbs vs. 683 lbs
27- 1069 lbs vs. 697 lbs
28- 1091 lbs vs. 705 lbs
29- 1135 lbs vs. 739 lbs
30- 1179 lbs vs. 772 lbs
31- 1190 lbs vs. 779 lbs
32- 1224 lbs vs. 794 lbs
33- 1268 lbs vs. 827 lbs
34- 1268 lbs vs. 838 lbs
35- 1312 lbs vs. 849 lbs
36- 1356 lbs vs. 882 lbs

I could not find data for <22PSI. The data's end point is 36 PSI because that represents the maximum pressure for the tire by TRA.
To say that this data is an eye opener is an understatement. Using my riding style(2-up 95%) as the example, I've got my Kenda inflated between 28-29 PSI (705-739 lbs load). When I change to the Kumho, based on the data, I could run it @22lbs and have more "strength" in it compared to the nominal pressure for the OEM tire. What I need to know is this the data that I should be looking at?????
Mike
 
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TIRE PRESSURE

Did you happen to check whether or not this Data Table is for Motorcycles ???? .... If it's for cars or pick-ups then it wasn't tested using Motorcycles ( or Spyders ) ...... But you are in the correct area for a Car tire on the rear of any Spyder ..... I like 18 psi ..... ask Peter what He thinks .... Mike :thumbup:
 
MikeT said:
What I need to know is this the data that I should be looking at?????
Mike
Click to expand...
I would say it's interesting to see this data but I wouldn't focus on it. Think of it this way. You want the tire to wear evenly across the tread. A tire that is inflated to, say, 10 psi but carrying no load will sit flat, or almost flat, on the ground. If you add a thousand pounds load on it it's going to squish down and not have a flat tread pattern. So what you want to shoot for is that pressure that carries the load you have on the bike and gives you even wear across the tread. You'll determine that best by more or less trial and error.

18 psi is probably a good starting point. If you look at the data for the 91H load you'll see it varies about 40 pounds/psi. I just graphed it and it's surprisingly linear. I thought it would be exponential, but it's not. Going down to 18 would drop roughly 160 pounds off the 22 psi load number, which is on the order of the 700 pounds load you mention for the 76H tire.

This, IMO, is all the overthinking your question needs!! :thumbup:
 
BLUEKNIGHT911 said:
Did you happen to check whether or not this Data Table is for Motorcycles ???? .... If it's for cars or pick-ups then it wasn't tested using Motorcycles ( or Spyders ) ...... But you are in the correct area for a Car tire on the rear of any Spyder ..... I like 18 psi ..... ask Peter what He thinks .... Mike :thumbup:
Click to expand...

Mike,
Thanks for your comments. The TAR data(multi page report) was developed for cars and trucks. The Load Index data is a universal grading system for ALL tires, motorcycle, auto, or truck(I checked before posting). Looks like I misjudged the community's desire for hard data on tires. The interesting point in the data is that the Load Index does not change equally as you increase, or decrease pressure.
 
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