Prius Tire Tuning

One of the interesting aspects of a Prius is the effect of different tire sizes for performance tuning. Recently someone asked 'what is the
largest tire' and 'what are the effects on mileage' so this is what I shared:
* * * I have experience with Sumitomo:
175/65R14 (919 revs/mile) - what Toyota service centers sell as replacement tires. I got 50,000 miles from first set and only an unrepairable puncture led to their retirement.GPS and mile markers all confirmed the actual was a little generous and the actual speed a little lower. But this was taken as the 'stock tire' from Toyota. This goes a long way to explaining another reason why Prius may be perceived as being slow ... their speedometers are showing a faster than true speed.
175/70R14 (886 revs/mile) - this first pair tested revealed a slight stability improvement on a 750 mile trip to Madison WI and back. I drove up with the smaller 175/65R14 on front and 175/70R14 on the back. At Madison, I swapped front and back so the larger 175/70R14 was on the front and that was when I noticed the slightly improved, straight-line stability. GPS and mile markers show the speed and MPG are less than 1% from true. I really think these are the best tire for the car.
195/70R14 (849 revs/mile) - a pair are now my front tires and the 175/70R14 are the rear. Again, my perception is more stability and certainly it is not so flighty. However, it indicates ~6% slower speed than actual (47 mph indicated is 50 mph true) and lower MPG. However, when I correct for the true MPG, there has been no loss. Best of all, the true hybrid transition speed has gone from 42 mph to just under 45 mph. This means I keep up with traffic and the engine continues to shutdown when it can. <grins>
Here is an image of the spreadsheet showing what I was looking for:
http://hiwaay.net/~bzwilson/prius/pri_tire_100.jpg
(GPS and mile marker testing showed the 175/70TR14 brings the indicated and measured MPG to within 1% of true)
I wanted to find the largest possible tire that could fit on an NHW11 Prius. Of the candidates, the 195/70TR14 was the largest so I bought one and mounted it to see if it would fit in the rear wheel well ... it did. I then drove around looking for speed bumps and dips to make sure it didn't rub ... it worked fine. So I ordered a second one and put both on the front.
* * * Now I have to be careful and use my GPS speedometer or always subtract 6% from the indicated to know my true speed. I also have to adjust my mileage records by 6%. But so far, there has not been a loss of vehicle MPG other than I catch myself driving a little faster now. <grins>
The Prius has a threshold speed, 42 mph for the NHW11, that above that speed, the engine runs all the time. Below that speed, the engine can shutdown. With the larger tires, I can travel 40 mph with traffic and the engine will shutdown whenever it can giving better mileage at this speed. But above 45 mph true, drag rules and the car only gets 52 MPG at 65 mph which still beats the snot out of gassers and diesels:
http://hiwaay.net/~bzwilson/prius/edmunds_020.jpg
Bob Wilson
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The Prius is a gasser too, Bob--or have you forgotten that already?
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wrote:

Bob Wilson
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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.
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wrote:

welcome you with open flys.
Bob Wilson
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What's that I said, Bob?
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wrote:

. . .

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
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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 2010 Toyota Prius III, Blue Ribbon/Dk Grey, OEM floormats Yokohama Avid S33 (psi 48/44) Scangauge II ( AVG, RPM, GPH, SoC) no grill blocking
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 87 oct regular, estimage FE=66 mpg
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. . .

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:
1) monitor the electric steering adjustment voltages over a straight- line course 2) monitor the horizontal accelerations over a straight-line course
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
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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?
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wrote:

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
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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?
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wrote:

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! <GRINS>
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
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On 5/19/2011 8:50 AM, bwilson4web wrote:

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."
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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:
175/65R14 - 919 -1.9%, 1019 lbs 175/70R14 - 886 +1.8%, 1102 lbs 195/70R14 - 849 +5.9%, 1321 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:
185/70R14 - 867 +3.9%, 1201 lbs
If someone is concerned about the 195/70R14, the next size down, 185/70R14 would work too.
BTW, thanks for the reference to "The Tire Society." I wasn't aware of them and they look to be serious people.
Bob Wilson
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On 5/19/2011 1:17 PM, bwilson4web wrote:

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

. . .

I had scanned the SAE and was not impressed. I've had a chance now to look at some of the titles of the Tire Society earlier articles and this looks to be a good source. Certainly better than the 'hot rod' and 'racer' articles although they helped too. I've already done some work using IR temperature probes to measure tire alignment and identify problems. I'm also doing 4-wheel alignment, toe and camber. It looks like some of these papers have the detailed tire models I've been seeking.
Bob Wilson
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. . .

I've had a chance to read the article throughly and it is about the practice of increasing the size of the wheel and installing low profile tires. So right off the bat, the density of the wheel/tire changes as there is more metal and less air. In contrast, I'm using the exact same alloy wheels the car came with, no additional metal.
So let's go over the quoted elements and their impact:
"Obviously, a wheel with the same mass at a larger OD has a larger rotational inertia than the OE wheel." Absolutely and this is an improvement in Prius handling. The steering even with perfect, four- wheel alignment is a best neutral stability. However, the higher rotation inertia of the larger tire improve steering stability. I observed this on a trip to Madison Wisconsin from Huntsville.
I drove up with the Toyota replacement, the 175/65R14, on the front and the experimental 175/70R14 on the rear. That Sunday I swapped front and rear and highway steering stability was improved to the point of making the drive home easier. In contrast on the drive up, I had to keep constantly on high alert to keep the car between the lines. On the way back, the car tracked true.
As for the other effects, they used a lot of "may" and ". . . there is a significant lack of public domain data available to assess these effects." After all, they aren't 'Elmo' who acts as if asking a question can substitute for research and experimentation. These are serious folks to don't ask silly questions as much as proposing where they and serious people will go to investigate.
Now this paper does discuss one risk and the experimental vehicle is a tall, SUV body which are notorious for that risk. In contrast, the Prius takes corners like they are glued to the road ... they have a lot of margin. I know the larger tires have not impacted my ability to take a sharp turn.
The one effect, a larger moment of inertia, has been found and it is exactly what the Prius highway steering needs. As for the one risk in the paper, the Prius was already superior and my driving has yet to find a problem. The Prius is so good at taking a turn that it can be uncomfortable and scary to those who are not prepared.
I'm still reading the papers and having a great time. I feel like I've walked into a University library and that has been one of my first loves.
Bob Wilson
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On 5/20/2011 1:31 AM, bwilson4web wrote:

Glad to contribute. I have seen prior mention of steering/hunting and tracking/wandering issues, the attempted cure for which are aftermarket strut and chassis stiffening braces. Increasing rotational inertia via tire sizing and profile could be a more direct as well as less radical solution.
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. . .

I've seen some references to changing the caster but these were not single part changes. To do it right, I would need a sophisticated suspension alignment tool along with a lift. I could do it but the cost, labor and tools makes soon evaporates the fun.In contrast, changing tires is relatively easy and there are a lot of articles in the racing community about tire effects and tuning during a race.
I'm waiting for a strut to fail before tackling suspension changes. I have looked for variable stiffness shocks but they are unGodly expensive parts. It looks like installing shims could shift the caster offset, a bit. However, the articles I've read indicate we would also need to change some of the linkages.
I've read a number of reports about how adding a stiffening plate improved NHW20 handling. But we have the earlier NHW11, a sedan-style, shorter, stiffer body, and the ZVW30 which has been built stiffer. I've not seen reports that 'neutral steering' was a problem with the NHW20 and the ZVW30 handles more like our NHW11 with larger diameter tires ... which they are. The NHW11 uses stock 14" rims and the ZVW30 15" rims.
BTW, I was looking for something else in my archives and found this photo showing the tires I've tested:
http://hiwaay.net/~bzwilson/prius/Sumito_T4_020.jpg
Bob Wilson
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