Impact of Rolling Resistance and Top Speed

[~el~jefe~]

I watch that Engineering Explained and also Auto Expert John guy on youtube a lot. They always factor in friction and rolling resistance to top speed calculations.

Yet, people hate the Primacy OEM Tires. I wonder if these grippy tires people are using are reducing their speed by a notable chunk. And, considering the lower hp lower torque of our cars compared to other 350+ hp cars, i would think the impact of ultra grippy summer tires with terrible rolling resistance would create for a notably slower car when not driving on curvy mountain roads.

I tried for about 10 minutes to look for a forum post that addresses this, but did not find anything clear. I figure it would be good to have one with the title laid out so. Then people could search for it one day. I tend to make most of my forum posts that way so people who are new or using search engines could find a tidbit they are wondering about.

[Lynxis]

I have done some experimentation on this at my local track and talk about rolling resistance slowing you on the straights is blown out of proportion. On the OEM tires, I could hit a top speed of 167kph before needing to brake and on 225/45r17 Hankook RS3s, my top speed is 166kph by speedo. Also factor in the 1.02 increase in tire diameter with the 225 over OEM 215 and that speed is... 169kph.

So my top speed is actually higher? Yes. With a caveat that I can enter the straight at 55kph on OEM tires and and 60kph (61kph corrected for taller tire) on the RS3s.

So, the total acceleration on OEM tire is 112kph over the same distance. On my RS3s, it's 108 (corrected for taller tire) but as much of that can be attributed to the lost torque from the taller tire as increased rolling resistance. Unfortunately, I'm not enough of a math nut to figure out how much impact the torque loss from the taller tire would affect things but it's pretty easy to imagine 4kph difference over that speed. Of course, my trip down the straight is still faster on the RS3 by virtue of entering it at a faster speed to begin with.

The reality is that the difference in resistance is so low compared to the power the wheels are trying to put down that you'd need a runway worth of distance for it to matter and if you are more interested in straight line speed, there are better cars out there for the money.

[Gunman]

Coast down testing would be an easy way to find out.

[ZDan]

Originally Posted by Lynxis

On the OEM tires, I could hit a top speed of 167kph before needing to brake and on 225/45r17 Hankook RS3s, my top speed is 166kph by speedo. Also factor in the 1.02 increase in tire diameter with the 225 over OEM 215 and that speed is... 169kph.

So my top speed is actually higher? Yes. With a caveat that I can enter the straight at 55kph on OEM tires and and 60kph (61kph corrected for taller tire) on the RS3s.

So, the total acceleration on OEM tire is 112kph over the same distance. On my RS3s, it's 108 (corrected for taller tire)

You can't just go by speed differential because acceleration drops dramatically with more speed. It would be more appropriate to go by energy added down the straight. Energy is a function of speed squared (1/2 mass * v^2).
OEM tires: 166^2 - 55^2 = 24,531
RS3 tires: 169^2 - 61^2 = 24,840

Not really any difference in energy added down the straight.

Rolling resistance is a very small factor even for a lowish hp car.

The reality is that the difference in resistance is so low compared to the power the wheels are trying to put down that you'd need a runway worth of distance for it to matter

Exactly!

[JazzleSAURUS]

I think there's some effect, but when you're building a car and selecting tires you do it for the best balance. When was the last time an Autox guy said 'Nah I want skinny tires on here so I can have a higher top speed on the straight?' they are 110% concerned with having more and more and more grip. Tearing your sway bar mounts out of your chassis amounts of grip. (Been there...)

More cornering grip = higher cornering speeds = higher straight line entries to put down whatever you've got for power.

It's a balance, and depending on what you're doing you have to balance those things. Do you have so much grip that you can give some up for better feel, and slightly increased top speeds?

[Turdinator]

The ultimate top speed may be better but general spirited driving doesn't get anywhere near that speed. Even on a track I doubt you get anywhere near top speed.

[Spuds]

Also, oems probably wouldn't last as long at top speed as high performance tires.

Also, oem tires are lighter than most high performance tires, so that has some effect on acceleration.

[ls1ac]

Are you including air drag coefficient which is non linear with speed.
Aero plays a large part in energy required .
A large wing will increase down force and increase drag, lower top speed and higher cornering speed. Tire weight is far less important unless you are looking for tenths or hundredth of a second.

[Ultramaroon]

Originally Posted by ls1ac

Are you including air drag coefficient which is non linear with speed.
Aero plays a large part in energy required .
A large wing will increase down force and increase drag, lower top speed and higher cornering speed. Tire weight is far less important unless you are looking for tenths or hundredth of a second.

You showed up just in time for this party.



yikes

[churchx]

"Top speed" imho won't change a bit, as IIRC it's ECU limited, not with less rolling resistance tires nor taller gearing.
Acceleration slightly might be enhanced.
But as on track you'll loose more speeds on less grippy tires in corners and will have slower exit speeds, it will more then offset gains of low rolling resistance.
For only daily driven car that never sees track, i'd be more interested in fuel economy/road noise/wear though, as grippy track tire may increase fuel usage/be noisy/last shorter mileage. But again in this list i wouldn't have that slight acceleration improvement among choice-maker reasons.

[gtengr]

horsepower lost to rolling resistance = rolling resistance coefficient*speed*weight of car multiplied by 0.002667 to convert the units to hp.

The rr coefficient is a function of tire properties, tire pressure, speed, and driving surface. You'll have to estimate so it's best to use a spreadsheet to play with the variables. From the generic tables I've looked at, I think it's fair to estimate the stock BRZ tire rolling resistance would increase from ~0.010 at 1 mph to ~0.027 at 140 mph.

Using those numbers, on a BRZ with stock tires you'd be looking at ~8 hp lost at 60 mph and ~30 hp at 140 mph on my spreadsheet.

For reference, my spreadsheet puts aero losses between 8-10 hp at 60 mph, and 105 to 132 hp lost at 140 mph depending on what Cd you use. I was looking at Cd ranging from 0.27 to 0.34.

[gtengr]

Originally Posted by churchx

"Top speed" imho won't change a bit, as IIRC it's ECU limited,

Yep, I think it's very close to being drag limited but not quite. I think a minimum 130 hp is being consumed by losses at 140 mph, and a stock BRZ has about 160 rwhp at 6300 rpm (6th gear at 140 mph). The gearing limits a '17 BRZ to 163 mph at 7400 rpm in 6th, which should be possible with anything over 210-250 rwhp.

[Yardjass]

Yep, your air resistance is going to be much more significant, especially as you get up to higher speeds. Keep in mind that a lot of that low rolling resistance has to do with how the tire deforms as it rolls, and not necessarily the tire being super light weight.


Our OEM wheels are not particularly light weight. They aren't heavy either but it is fairly easy to get a lighter wheel/tire combo with a lower rotational moment of inertia from the aftermarket. This is sort of apples to oranges at this point but I question how many people would go through the trouble of getting a high performance tire compound while opting to keep stock wheels.

[churchx]

Yardjass: many. OE wheels are strong enough, not that very heaviest out there. And while new at dealership prices they are expensive, there are many classifieds to get replacements for cheap. Yes, lighter wheels may add a bit performance, but not revolutionary. Tires on the other hand is one of most important bits for performance & handling. So what's wrong using OE wheels with performance tires on track and investing saved money elsewhere?