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Tyre Size Vs Rolling Resistance


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#1
Vegatron

Posted 26 December 2013 - 01:33 PM

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I stumbled across this study http://www.energy.ca.....e Testing.pdf

Below is the excerpt of the Rolling Resistance test part. What do you guys think? Bigger tyre = lower LRR? 059.gif

Smithers looked at the effect tire size has on RR. The data is graphed below.



I am not sure where I picked this up, but all these tires are Goodyear Integrity's. That presents a problem in that some of these tires are OE, while most of them are replacement.

What that means is that some of these tires were designed to the same design characterisitcs (traction, treadwear, etc.) and some are individuals designed to the specs selected by a vehicle manufacturer. That means there should be outliers!

Word of Caution:

Folks might be tempted to conclude that increasing the tire size is also going to increase RR

- BUT -

When tires are measured for RR, one of the test conditions is the load - which would be larger for larger tires. But when a tire is applied to a vehicle, the load on the tire would be the same, regardless of what tire size is used. So the RRF (Rolling Resistance Force) needs to be divided by the test load to get RRC (Rolling Resistance Coefficient) - which is shown below.



This is the same data, only expressed as RRC (Rolling Resistance Coefficient).

Notice that the larger the tire (generally), the smaller the RRC. What that means is: If you apply a larger tire to your car (and use the same inflation pressure), then this results in lower RR and therefore better fuel economy.

It occurred to me that while I, as a tire enginner, pay a lot of attention to load carrying capacity, this way of thinking is not the way most folks think of tires. The average guy tends to think in terms of "size", so I redid the regression analysis only using the 3 parameters normally associated with "size": Section Width, Aspect Ratio, and Rim Diameter.

RRC = 0.00246493 - 0.00000208*(Section Width in mm) - 0.00000386*(Aspect Ratio in whole numbers) - 0.00004700*(Rim Diameter in inches)

r2 = 66%

Note that the expression starts off with a constant and all 3 of the parameters subtract from the constant to get the RRC value. This means that going up in any of those dimensions results in a DECREASED value of RRC.

Put another way: Larger is better!

One of the reasons I (and other tire engineers) tend to think of tires in terms of load carrying capacity has just been demonstrated. Every one of those 3 parameters has major implications when load carrying capacities are calculated.

It may seem counter-intuitive to think that a wider tire, with its additional mass in the tread area, could improve fuel efficiency. However, this data says otherwise.

One possible explanation would be that while the width goes up 10mm, the width of the tread (an important dimension for RR) goes up a fraction of that. The net effect is that while the load carrying capacity goes up directly as a result of the increased width, the RR would go up less than that.

Put another way, the width of the tread in a tire is not 100% of the section width and it is common for tires to be designed with tread widths expressed as a percentage of the section width. So assuming the tread width is 70% of the section width (a reasonable assumption), that would mean the tread width increased 7mm while the width of the tire increased 10mm.

So if I want to estmate what using a wider tire would do:

10 mm wider results in a reduction of about 2%. My experience is that you can only go about 20mm wider before you start to interfere with the fenders, frame, and suspension components - and that means that you should be able to get up to a 4% improvement.

If I use a 1" larger diameter rim, that also results in a reduction - about 5%.

If I use a higher aspect ratio, that results in a reduction of about 2%.

Combining these: A common rule of thumb is that if you want to go up 1" in rim diameter, you need to go wider 10mm, and reduce the aspect ratio by 5 (in order to get the same overall diameter) and the result is about a 5% improvement!
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#2
Vegatron

Posted 26 December 2013 - 01:55 PM

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Another excerpt I lifted from http://forums.roadbi...nce-229343.html

This is talking about bicycle tyres... but I wonder if the same physics apply to car tyres. It should right?




MORE ABOUT TIRES
(than you probably ever wanted to know)

The following articles below get a little scientific. The tests were conducted by Michelin, Continental, Schwalbe, and other tire manufacturers.

1. In general, wider tires roll easier and faster than narrow tires at the same pressures. This isn’t true when you consider very wide tires of 30mm or more. For example, assuming the same tire pressure, a 25 mm tire will roll faster (have less rolling resistance) than a 23 mm tire. The reason for this is, although the tires will have very nearly the same contact patch, the narrow tires’ patch will be narrow but long. The wider tire will have a contact patch the same size, but the foot print will be wider and shorter. The foot print that’s longer will resist rolling resistance more than the shorter one. (To see the truth, read the full article below which includes scientific tests by Continental, Michelin, and Schwalb bicycle tire manufacturers.)

2. Since narrower tires weigh less than wider tires, it’s easier to accelerate them. This difference is very small however, and equates approximately to having a half full water bottle instead of a full one on your bike.

3. Inflating your tires so that are rock hard, or the maximum air pressure listed on the sidewall will make you slower. The reason for this is when your tires are rock hard your bike bounces up and down a lot over surface irregularities. It takes LOTS of energy to lift your bike up and down. That energy is subtracted from your forward motion. At lower pressures your tire deforms over road irregularities. This also uses energy, but far less than lifting the whole bike up and down. The energy lost is energy that would/could have been used for forward motion.
4. The fact (and it is a fact) that lower inflation pressures are faster is only true up to a point. When the tire pressure becomes low enough to increase the tires’ foot print on the road, the wider tire becomes slower.

5. Wider tires and lower air pressure gives not only more speed, but more comfort due to the shock absorbing characteristics of the wider, softer tire.

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#3
Vegatron

Posted 26 December 2013 - 02:03 PM

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so here's my quack conclusion 059.gif

The bicycle fella was comparing 23mm vs 25mm width tyres. Long story short, wider tyre = better RR.

So 2mm/23mm = 8.7% wider, say we apply this on a car tyre width of 195mm. 195 increase by 8.7% = 212mm. Since there's no 212 car tyres, we take the next nearest 215mm width car tyres. Conlusion,.... increasing tyre width from 195 to 215 would yield better rolling resistance. Betul atau salah? teu35.gif

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#4
vr2turbo

Posted 27 December 2013 - 11:48 AM

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QUOTE (Vegatron @ Dec 26 2013, 02:03 PM) <{POST_SNAPBACK}>
so here's my quack conclusion 059.gif

The bicycle fella was comparing 23mm vs 25mm width tyres. Long story short, wider tyre = better RR.

So 2mm/23mm = 8.7% wider, say we apply this on a car tyre width of 195mm. 195 increase by 8.7% = 212mm. Since there's no 212 car tyres, we take the next nearest 215mm width car tyres. Conlusion,.... increasing tyre width from 195 to 215 would yield better rolling resistance. Betul atau salah? teu35.gif

To me is salah.
Why do hybrid and smaller vehicles all give narrower tyres in the first place?... smile_tongue.gif
Anyway to me wider tyres more contact surface more resistance mah! More over the savings of lower resistance tyres are very very little only. To me tyres are safety items, so why sacrifice safety for a mere little savings on FC

#5
Vegatron

Posted 27 December 2013 - 12:29 PM

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QUOTE (vr2turbo @ Dec 27 2013, 11:48 AM) <{POST_SNAPBACK}>
To me is salah.
Why do hybrid and smaller vehicles all give narrower tyres in the first place?... smile_tongue.gif
Anyway to me wider tyres more contact surface more resistance mah! More over the savings of lower resistance tyres are very very little only. To me tyres are safety items, so why sacrifice safety for a mere little savings on FC


Cozzz... the hybrid R&D fellas are not tyre experts? hahaa!

forget FC means you saying go wide? smile.gif
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#6
521329

Posted 27 December 2013 - 01:00 PM

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QUOTE (vr2turbo @ Dec 27 2013, 11:48 AM) <{POST_SNAPBACK}>
To me is salah.
Why do hybrid and smaller vehicles all give narrower tyres in the first place?... smile_tongue.gif
Anyway to me wider tyres more contact surface more resistance mah! More over the savings of lower resistance tyres are very very little only. To me tyres are safety items, so why sacrifice safety for a mere little savings on FC


Dear all,
This is my humble experience" My drive OEM tyres are 195/55/16, later changed to 205/50/16 and then the present set is 205/55/16. By comparison, with the second set 205/50/16 size, l feel that the acceleration is marginally slower (with stiff cushion and bumpy drive) and then quickly changed, with the present 205/55/16 set, the acceleration worsens further (but with more comfort and stability-my priority) This clearly indicates the law of physic and mechanic-wider areas of contact results in more friction and eventually more resistance and more stability but more fuel consumption.

Actually l plan to downgrade to 205/55/15 tyres for my drive and later shift over the used 205/55/15 tyres to my wife's WIRA (OEM set tyres is 185/60/14) because this will give more resistance and more stability to the WIRA. But brother vr2turbo does not quite support this arrangement due to too big increase in the diameter of the tyres. Instead our shifu suggest to change to 195/55/15 for both the cars. This is my dilemma as it is time to change the tyres before Chinese New Year.

Thanks.

Regards.



#7
Vegatron

Posted 27 December 2013 - 03:17 PM

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QUOTE (521329 @ Dec 27 2013, 01:00 PM) <{POST_SNAPBACK}>
Dear all,
This is my humble experience" My drive OEM tyres are 195/55/16, later changed to 205/50/16 and then the present set is 205/55/16. By comparison, with the second set 205/50/16 size, l feel that the acceleration is marginally slower (with stiff cushion and bumpy drive) and then quickly changed, with the present 205/55/16 set, the acceleration worsens further (but with more comfort and stability-my priority) This clearly indicates the law of physic and mechanic-wider areas of contact results in more friction and eventually more resistance and more stability but more fuel consumption.

Actually l plan to downgrade to 205/55/15 tyres for my drive and later shift over the used 205/55/15 tyres to my wife's WIRA (OEM set tyres is 185/60/14) because this will give more resistance and more stability to the WIRA. But brother vr2turbo does not quite support this arrangement due to too big increase in the diameter of the tyres. Instead our shifu suggest to change to 195/55/15 for both the cars. This is my dilemma as it is time to change the tyres before Chinese New Year.

Thanks.

Regards.


You are changing too many different diameters. But let's have a look at all of them:

195/60/16 = 620.9mm diameter, OEM diameter.
205/50/16 = 611.4mm diameter, 9.5mm smaller than OEM or 1.5% smaller.
205/55/16 = 631.9mm diameter, 11mm bigger than OEM, or 1.8% bigger.
205/55/15 = 606.5mm diameter, 14.4mm smaller than OEM, or 2.3% smaller.

From here, it looks to me like all the different sizes you tried are all withing +/-3%. However, if your ride is under powered, you will feel some difference in acceleration with the bigger sizes. For smaller size, your acceleration should be easier.

Sifu recommend 195/55/15 = 595.5mm, 25.4mm smaller than OEM, or 4.1% smaller. Why ar bro vr? ...

---------------------------------------------------------------------------------------------------------------------------

OK back to the rolling resistance thing. This is from the bicycle fellas again... not sure if it's the same with car tyres. I think it should be the same... as you can see below, narrower tyres contact patch is narrow but long. On the right, wider tyre contact patch is wider but at the same time it is shorter. Long contact patch gives you more rolling resistance than short because you roll forward on the long patch. So if you have wider tyres, the contact patch is wide but short, so when you roll forward the contact patch you roll on is shorter... thus get lower rolling resistance.

This is my purpose of starting this discussion.


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#8
Vegatron

Posted 27 December 2013 - 04:54 PM

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Ahh! I found a picture to illustrate what I'm trying to say, this is car tyre

Narrow tyre has narrow but long contact patch. Wide tyre has wide but short contact patch.


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#9
vr2turbo

Posted 28 December 2013 - 08:31 AM

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QUOTE (Vegatron @ Dec 27 2013, 03:17 PM) <{POST_SNAPBACK}>
You are changing too many different diameters. But let's have a look at all of them:

195/60/16 = 620.9mm diameter, OEM diameter.
205/50/16 = 611.4mm diameter, 9.5mm smaller than OEM or 1.5% smaller.
205/55/16 = 631.9mm diameter, 11mm bigger than OEM, or 1.8% bigger.
205/55/15 = 606.5mm diameter, 14.4mm smaller than OEM, or 2.3% smaller.

From here, it looks to me like all the different sizes you tried are all withing +/-3%. However, if your ride is under powered, you will feel some difference in acceleration with the bigger sizes. For smaller size, your acceleration should be easier.

Sifu recommend 195/55/15 = 595.5mm, 25.4mm smaller than OEM, or 4.1% smaller. Why ar bro vr? ...

If I can remember he wants to downsize his car so he can interchange the tyres with the wife's wira, so if he really wants to do it, I would rather his car tyres smaller rather then his wife's car over. But overall I recommended not to do it and keep OE size to each individual car

#10
vr2turbo

Posted 28 December 2013 - 08:33 AM

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QUOTE (Vegatron @ Dec 27 2013, 04:54 PM) <{POST_SNAPBACK}>
Ahh! I found a picture to illustrate what I'm trying to say, this is car tyre

Narrow tyre has narrow but long contact patch. Wide tyre has wide but short contact patch.


As far as I know wider tyres wider side way contact patch, but for longer contact patch the larger diameter tyres will have longer contact patch then smaller tyres