Prius Tire Tuning

The Prius is a gasser too, Bob--or have you forgotten that already?

No Elmo, this is a Prius technical site, not a dating service.

Bob Wilson

hehehehe So Bob *does* lie and make things up in order to try to make himself look better to the ignorant others in the world.

You really should head over to the alt. sex sites. They will welcome you with open flys.

Bob Wilson

What's that I said, Bob?

. . .

Well I'm so glad you asked. We were discussing Prius tire tuning ... matching the right tire side for optimum performance. To summarize:

o too small - the speedometer indicates higher than true which results in slow speeds that reduce drag and improve actual MPG. However, it also leads Prius drivers to being slower than other traffic and inflates the indicated MPG. It is also associated with 'skitterish' handling as it needs more driver attention.

o just right - the speedometer is right on as is the indicated MPG. Slightly larger diameter leads to more straight-line stability.

o oversized - the speedometer indicates lower than true speed which can result in more drag and a loss of true MPG. Also, the indicated MPG is lower. However, straight-line stability and handling is improved.

Thanks for the question, I needed a reason to summarize the thread, Bob Wilson

Did you ever measure how the tire sizes might effect the braking distance?

While I would suspect that the 195/70R14 would provide better traction which would translate to better stability and braking performance - how much better might one expect?

It sounds like that these bigger tires are fooling the ECU (which is still thinking the prius is using the smaller tire size) to allowing for a slightly higher velocity at which the Prius changes from all electric to electric and gas. If I recall correctly that hybrid transition speed where the Prius goes from all electric mode to running both the electric motor and the ICE was to protect the ICE from damage. While extending the top speed for a Super Highway Mode Cycle sound great - I am wondering if there are any risk . While I suspect that Toyota has probably engineered enough tolerance to allow for your slight increase in speed - I would suspect that going signficantly higher may damage the ICE.

Walter Lee

Dc/md/nova metro area hypermiler on training wheels odeo 11800 miles, overall 58.7 mpg last tank 4/19/2011, 598 miles of hilly urban/suburban, 9 gallons e10

. . .

Not yet since I'm trusting the ABS to moderate the braking force. My understanding is the ABS releases braking force when the wheel begins to stop, keeping it in the peak braking force region, and this is independent of the tire diameter.

This is a hard problem as I don't have a good metric nor know of any, the units of stability. Pretty much universal, the automotive magazines and even TireRack use driver impressions. But two approaches come to mind:

We still have a problem with absence of a known impulse that would require a steering adjustment. Perhaps a vertical air foil that oscillates between +5 and -5 angle of attack. Each transition would move a force from one side to the other, a well defined force application, and then the vehicle and steering response could be measured.

I thought about this before the experiment and came up with these states:

acceleration - the amount of torque is engine and transmission limited so I'm expecting slower, maximum acceleration. But I don't do hard accelerations except when benchmarking the car. I just don't have frequent requirements for maximum acceleration and the NHW11 at 13 seconds to reach 60 mph, is more like a truck than a sports car.

hill climb - the torque needed to climb even an 8% grade at 55 mph is well within the capabilities of the existing car. I've done maximum speed hill climbs and topped at 85 mph so I don't see a risk.

regenerative braking - will have a maximum reverse torque and the larger diameter tire may lead to more mechanical braking. However, I drive so as to minimize regenerative braking so no impact is expected.

What we've done is made the last gear, the tires, taller.

I've only been testing these tires on our 2003 Prius. We have a 2010 Prius but while it remains on warranty, I'm keeping the new Prius stock with one exception. I've started a 5K, 15K, 30K, and 60k, transaxle oil change and test plan. I want to measure transaxle wear over time and make sure I know the change interval based upon metrics.

Bob Wilson

Ummmm....no. They use instrumented data to measure braking performance.

It's a simple thing to do, really--and yet you can't/won't do it?

It has nothing to do with damaging the ICE, it has everything to do with damaging the transaxle, though. The lubricating oil/fluid pump for the transaxle on NHW11 and NHW20 is driven by a shaft that extends from the planet gear carrier of the PSD, through the center of MG2, to the pump which is in the MG2 side cover just inside the driver's side wheel well (on LH drive cars) Since the PSD planet carrier is driven only by the ICE, any time it is off the trans is essentially unlubricated and relies on the residual oil film in the bearings to keep it from self destructing.

Also for NHW11 and NHW20 there is no such thing as a speed where it transitions from just electric to electric and gas, because the speed for the transition cam be anywhere between 0 and the top speed of 105MPH depending on throttle pedal position. If you hammer the throttle from a dead stop it starts immediately, if you were to put it in drive and roll down a mountain road without touching the pedal it might never burn a single drop.

What does always happen is that at 42MPH indicated the engine crank begins to turn, even if it isn't burning any fuel, AKA the car transitions from "stealth" mode to "warp stealth" mode.

Poor baby, stepped into it again, I see. You are such my bitch.

The ABS prevent the wheels from locking up which helps them achieve maximum braking and avoids loss of control. Quite effective, the tires are working fine and no problem braking. So this is what the NHTSA published in DOT HS 808 206, December 1994:

"The fundamental safety problem addressed by ABS is that few drivers are able to optimize the pressure they apply on the brake pedal, given a sudden emergency situation or unexpectedly slippery surface. When excessive pedal pressure locks the wheels, the vehicle can yaw out of the driver's control (rear-wheel lockup), or go straight ahead, impossible to steer (front-wheel lockup). On most, but not all, road surfaces, a skidding vehicle needs a longer distance to stop than a vehicle with the brakes applied and wheels still rolling. The objective of ABS is to take over the optimization task from the driver. A four-wheel system is intended to keep all the wheels rolling during panic braking, to prevent yawing, to allow steering throughout the emergency and, on many surfaces, to shorten the stopping distance. The combination of efficient stopping and steering is intended to help the driver avoid mobile and fixed obstacles. . . ."

The ABS works just fine, tested, with the larger diameter tires and there is no apparent change in stopping distance. In fact the NHTSA has a number of reports on ABS and stopping distance:

DOT HS 808 875, "NHTSA Light Vehicle Antilock Brake System Research Program Task 4: A Test Track Study of Light Vehicle ABS Performance Over a Broad Range of Surfaces and Maneuvers." which states:

"This study found that for most stopping maneuvers on most surfaces, ABS-assisted full pedall brake applications stops were shorter than those made with the ABS disabled. The one systematic exception was on loose gravel ..." well I don't drive on loose gravel so no problem.

Then there is the SAE 1999-01-1287 "A Comprehensive Light Vehicle Antilock Brake System Test Track Performance Evaluation"

"For most stopping maneuvers, made on most test surfaces, ABS-assisted panic stops were found to be shorter than those made with best effort or full pedal applications with the ABS disabled. . . ."

Except for your lying, fake outrage, blow it out your ass troll. The ABS works perfectly fine and the car braking is as before, no change. "Such a moroon" - Bugs Bunny

Bob Wilson

Hi Daniel,

On May 15, 2:06 pm, "Daniel who wants to know" wrote: . . .

Increasing the tire diameter doesn't change the ratio of ICE ON/OFF as much as it lets the car slip down the road a little further ... roughly in proportion to the decrease in wheel rotations.

The "42MPH indicated" remains the same but now the car is traveling at

If we operated our Prius at the max in all cases, then the tire diameter change would impact the limits. Fortunately, that isn't how I drive. After all, getting 52 MPG in a 2003 Prius is not achieved by running at the vehicle limits.

Bob Wilson

SURE, bob. SURE.

So far so good.

So what does any of that have to do with braking distance?

You were asked, quite directly: "did you ever measure how the tire sizes might effect the braking distance?" And your answer was, as quoted above: "Not yet since I'm trusting the ABS to moderate the braking force."

You have yet to provide any connection between your willingness or ability to measure braking distance and the fact that ABS is functioning.

Of course, there's a reason for that: there's no connection between the two. You just spouted random words that didn't answer the question at all.

Why was that, Bob? Was it because you don't want to answer the question?

Yesterday, we had one of those stop sign controlled incidents where the car on the right assumed I also had a stop sign but I had a yield, the right of way. So they started to pull in front of me. I mashed the brake and felt the reassuring, ABS buzz as the car stopped with a lot of space to spare. Excellent braking!

Testing done and passed!

The physics remains the same. The tire foot print is the same as all Sumitomo tires are at 51 psi and kept there. They are all T4 style so the tread patterns are identical. Best of all, ABS braking keeps each tire right at edge of maximum braking based upon rpm changes which is independent of diameter and the car remains fully in control. Yesterday's test was just like all earlier, emergency braking incidents, no drama, rapid stopping.

So Elmo, where is your report showing that tire diameter has any effect on stopping distance?

You don't have any, our fool tool.

Bob Wilson

REFERENCE: J.W. Daws, ?Technical Considerations for Plus Sizing,? submitted for presentation at the 2008 International Tire Exhibition and Conference, September 16-18, 2008, Akron, OH

"General Motors Corporation stated in a recent publication [?Plus-sized Problem??, Tire Business, July 19, 2004] that wheels used for their upgrade packages have the ?same mass, same offset, same width, same mounting flange, same tire pressure monitoring requirements, same brake clearance, [and] same dimensional tolerances? as the original equipment wheels. Implicit in this tight specification are the impacts of the plus-sized fitment on anti-lock brake systems, electronic stability control systems, and so on. Obviously, a wheel with the same mass at a larger OD has a larger rotational inertia than the OE wheel. Generally, as wheel diameter increases, the mass of the tire and wheel for equal load capacity generally increases, as does the inertia of the rotating system. This change in unsprung weight may affect the response of the suspension system. The change in rotational inertia may affect the response of systems like antilock braking and electronic stability control. Tire pressure monitoring based on antilock brake sensors may also be affected by changes in tire size and vertical stiffness. At this time, there is a significant lack of public domain data available to assess these effects."

So your idea of "testing" involves nothing more than your own example of "yes it stopped".

Oh, bullshit. You have no data to back you up. Again, you're making things up.

It puts all of your pretty charts and graphs into perspective.

I don't have to have data. You claim that braking distance is unaffected, and I asked you for the data to back up that claim. You refuse to provide such data, to the point where you say that all you need is the seat of your pants.

If you want to make claims you can't back up, expect to be called on that in public.

Well la-dee dah! Like I care.

The tires work, the car works, the physics remain the same. Put up some facts and data.

I have the car and it works ... stops like it did before. You don't and resort to FUD (Fear Uncertainty and Doubt.) So until you bring something to the party, I don't really care.

Bob Wilson

Thanks!

I found the paper and am reading it now. But I'm struck by this comment:

". . . conventional wisdom usually suggests that any difference should be within +/- 3% of the original equipment tire . . ."

Based upon tire revs per mile, here is how the tires rate versus the OEM tire, 902 rev/mile, 1102 lbs:

Per "conventional wisdom," the first two tires straddle the OEM tire. The 195/70R14 is oversized but the rest of the article discusses stability testing. I'll have to read it closely to see what their test articles were doing and compare it to the Sumitomos.

One good thing is the 195/70R14 is the largest tire that can fit on the NHW11 Prius. There is one I skipped over:

If someone is concerned about the 195/70R14, the next size down,

BTW, thanks for the reference to "The Tire Society." I wasn't aware of them and they look to be serious people.

Bob Wilson

There's a lot of potential liability attached to them, hence their seriousness.