Tire width impact on fuel economy / MPG

[freebeard]

As that ratio increases (as you would get fitting a narrower, lower volume tire in an existing wheel housing), drag goes up too, about ten counts for every doubling of the ratio. And lift coefficient increases 5x as fast, 50 counts for every doubling. But, it's unclear what effect a smaller volume, narrow tire that still fills the area of the wheel opening would have...

That's interesting. Citation? I'm thinking of those lowriders with their whitewalls on Astros. Vs your stanced Beemer.

As for airflow exposure, spats are back.

[Vman455]

Aerodynamics of Road Vehicles, pp. 184-185 (4th ed.)

[freebeard]

I didn't expect to find Session-3-1-Jeff-Howell-Summary-of-university-dindings-Wheel-Wheelarch-drag online today, and I wasn't disappointed. Er...

Jeff Howell
Tata Motors European Technical Centre
WS12 Aerodynamic Performance
Contents

Drag breakdown
Wheel/wheelarch drag component
Moving ground wind tunnels
Cranfield model rig
Test data
Comments
Conclusions

I did find these two identical links, both display poorly (prolly JavaScript):

https://vdocuments.mx/documents/the-...-and-wind.html
https://vdocuments.mx/documents/the-...-and-wind.html

Both point to The Aerodynamic Characteristics of Automobile Wheels - CFD Prediction and Wind Tunnel Experiment but I haven't tried downloading yet.

[oldtamiyaphile]

Yup, there's a reason why the pros tend to ride the widest tyres that will clear their frames, and why the 19-20mm tyres of old are now seen as indoor only tyres (they still work on smooth wood boards @200psi). Just a few years ago bike frames were only designed to clear 23mm tyres. Pretty much all frames are designed for 25mm today and there are a lot of 28mm frames out there.

Most Pro teams are now running 25mm, the extra 2mm of width (over the old school 23mm's) had a massive impact on RR, jumping up another 3mm doesn't have as much impact on RR (and at an ever greater aero and weight penalty).

For now the second widest tyre is seen as ideal for the Pro Peleton.

Originally Posted by Continental Tyres

Remember that wide tyres of the same type and with same air pressure run better and more lightly than narrow tyres.

That's translated from German, but I think what they're saying is clear.

https://roadcyclinguk.com/gear/using...ad-bike.html/3

etc, etc, etc. Use a search engine, all the data says wider tyres= lower RR. Bicycle data is easier to find than car data, as cyclists are more interested in saving a few watts than car enthusiasts.

BUT

That's not to be confused with 'better' economy. I've personally switched from Fiat's OE 195's to OE 175's and their lower weight & better aero had a massive benefit, even if the RR was unlikely to be any better. The switch also meant going from heavy open faced alloys to light weight steelies with hup caps. That's a lot of variables. RR got worse, but everything else got better, making it easy to assume that everything got better. Traction also got worse so again it's easy to think RR got better.

On older cars, say 90's back, the bodies widest point was at the height of the plastic rub strips (roughly the top of the guards). On my Proton with 205's, the top of the tyre is easily within the guard, however, just a few inches lower the tread is already exposed to the airflow. Newer cars do a much better job at enveloping the tyre, so there's potentially less aero gain from going to a narrow tyre.

Would I fit wider tyres to improve economy? No. I doubt the RR gains would ever offset the other losses.

[California98Civic]

Of course, bike tires are maybe not fully comparable to cars, but I love the chart and I take your point. I have a 23mm Gatorskin on my back wheel right now, but that is the widest tire that fits in the frame (1990s Nashbar design).

The link Capriracer posted a page or so back had this chart buried in it. You'll notice that generally smaller tire size is associated with lower RR:



james

[oldtamiyaphile]

Originally Posted by capriracer

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.

Bigger tyre = lower RR.

Car or bike, doesn't matter.

The whole reason rubber tyres have a high RR in the first place is deflection (hysteresis losses). A bigger tyre deflects less for a given pressure/ load, which means lower hysteresis and lower RR.

Imagine putting a car tyre on your bike. You'd have virtually zero deflection, virtually zero hysterisis and extremely low RR. Doesn't mean you'd have a bike that's fast or easy to pedal though.

[California98Civic]

Originally Posted by oldtamiyaphile

Bigger tyre = lower RR. Car or bike, doesn't matter. ...

Ok. I think I get it now. I did not know the difference between rr force and the coefficient. Learn something every day. But, don't hate me, even the graph you posted is a little ambiguous. While the general trend of larger tire, less rolling resistance is clear, it is not without noise. For example your chart suggests that I IMPROVED my rolling resistance by going DOWN in size from a 185/65-R14 to a 175/70-R13. That is what I actually did and possibly went from a 12.0 Crr to an 11.5 Crr. Unless, again, I don't get it.

So that settles the general picture for rolling resistance, but the thread title is about width and FE and everyokne seems to agree there are lots of variables and that even if a given wider tire provides better RR other factors like weight and aerodynamics might overwhelm that benefit and produce lower FE anyway.

[westygo]

Yep, and in this case, I am going to a 7% larger tire, so it is changing gearing too. Rolling resistance, aero factors, rotating mass, gearing, pumping losses..anything I am missing on the list of variables?
I guess the only way to know is stick with the one variable; use a wider tire with a lower profile ratio to keep the diameter the same, same tire model, same rims, and do some a-b-a testing.

[Ecky]

Something else to consider - while a larger tire may have lower RR at a given pressure, the pressure ratings for tires of different sizes are different.

I have a set of Schwalbe Big Ben tires on my velomobile and I remember reading they were supposed to have lower RR at a given pressure, but they're also rated to be used at dramatically lower pressures. I pumped them up and still have greater suspension effect, I'm unsure about the RR difference, but I also notice the handling is affected - they're a lot heavier.

[CapriRacer]

Originally Posted by Ecky

Something else to consider - while a larger tire may have lower RR at a given pressure, the pressure ratings for tires of different sizes are different. ....... [

Ah ..... not exactly. All standard load passenger car tires (the most common type) have a rated pressure of 35 psi, except for those tires who use the metric system, then it's 2.5 bar (250 kPa, ~36 psi).

Please note: This is different than the max usage pressure which is sometimes written on the sidewall (and the other times what is written there is the rated pressure.)

This is different than bicycle tires because bicycle tires assume a single rider and a relatively narrow range of weight, while a car might have many passengers and a wide range of weight.