I've been discussing NHW11 tire tuning in a USENET group and wanted to share the results here. The NHW11 is the 2001-03 Prius, the sedan style body that looks like the Ford Focus. Unfortunately, our 2003 Prius always had skitterish handling ... neutral, straight-line stability. But while testing a larger diameter tire to see the milage effects, I discovered the larger diameter tire improves straight-line stability.
Sumitomo makes a line of 14" tires and supports 51 psi, maximum sidewall pressure. Furthermore, Consumer Reports rates them 5/5 rolling resistance and a 4/5 rating for dry braking (note, only 1/5 for ICE braking.) Based upon tire revs per mile and load, here is how the Sumitomo 14" tires stand versus the 44 psi, OE tire:
175/65R14 (OE) - 902 rev/mile, 1102 lbs:
175/65R14 - 919, 1019 lbs, and small of the 14" series, tested
175/70R14 - 886, 1102 lbs, exact match on load, tested more stable
185/60R14 - 926, 1047 lbs, the smallest, untested
185/65R14 - 902, 1124 lbs, closest to an exact match, untested
185/70R14 - 867, 1201 lbs, untested
195/60R14 - 907, 1135 lbs, untested
195/70R14 - 849, 1321 lbs, known to fit, likely the largest, tested, stable
The term "stable" is straight-line stability, the degree the car tracks true. It turns out the local Toyota dealership sells the 175/65R14 as the replacement for the OE tire. I didn't know that it was under sized and under load rated.
On a 750 mile trip from Huntsville to Madison, I drove up on the 175/65R14 with the 175/70R14 on the rear. I swapped the front/rear tires on the drive back and was pleasantly surprised that the car started tracking and not trying to run off the road. So I continued my testing and have 195/70R14 on the front and 175/70R14 on the rear.
Looking at the list, the most interesting testing combination would be:
185/60R14 (926 revs/mi) vs 195/70R14 (849 revs/mi)
This would allow us to compare and contrast the smallest to largest diameter tire. We already know the larger diameter tire improves straight-line handling over the 175/65R14. The larger diameter increases the gyroscopic forces to hold the car in a true line but I reviewed the definition of caster angle:
"The pivot points of the steering are angled such that a line drawn through them intersects the road surface slightly ahead of the contact point of the wheel. The purpose of this is to provide a degree of self-centering for the steering - the wheel casters around so as to trail behind the axis of steering. This makes a car easier to drive and improves its directional stability (reducing its tendency to wander). Excessive caster angle will make the steering heavier and less responsive, although, in racing, large caster angles are used to improve camber gain in cornering. . . ." - Wikipedia
A larger diameter moves the road surface intersection a little further in front of the tire contact patch and this should also increase the effective caster. So in addition to the higher gyroscopic forces, caster is also increased, a two-for-one advantage.
Given the different revs/mile of the Sumitomo 14" tires, there are choices for for different degrees of handling. For those who want maximum stability, 195/70R14. If one wants a faster responding car, the 185/60R14 would be a good choice and would also lower the relative center of gravity, faster turns.
Bob Wilson
Today
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