Results of testing front toe-in vs energy usage

The weather has been very stable the last few days so I decided to test the influence of front toe on energy consumption. To my surprise there is a very real and rather large effect when changing toe.

Car: P85+ on the factory Michelin PS2 tires (21").

Drive cycle: 7 mile commute, typical suburban drive. Speeds of 40-50mph. Ambient temps 65-80 dgrs (morning/afternoon. Temps stable day-to-day).

Started the test by driving several days with front toe set at 0.07 dgr IN. Energy consumption: 310-320 wh/mile.

Changed front toe to 0.30 dgr IN (max toe-in according to spec). Energy consumption: 340-350 wh/mile.

Changed back to 0.07 IN and consumption is now back to 310-320 wh/mile.

That's a solid 10% difference in range. Next I will re-check rear toe and see if there's some additional efficiency to be found.

The engineer in me is finding this to be a great deal of fun.

To do it accurately I use 4 jack stands, 2 on each side of the car. I run a string between each pair so I have a string along each side of the car at the height of the wheel center.

Each string needs to be close to the rim without touching and exactly parallel to each other. Can't run them at the same distance from the rim front and rear as the track differs. Have to measure between the strings. Once I have that set up, I measure the distance from the string to the front and read edge of the rim. Then take the diff and divide by the distance front-rear of the rim. Do an Atangent ("reverse" tangent) on the number and I get the angle.

True. So far I haven't noticed any lack of high speed stability though. I know BMW likes to dial in quite a bit of rear wheel toe-in for autobahn use. As everything, it's all a compromise. I'll take the extra 20 miles of range over a bit of high speed stability, though.

I've done more experimentation on rear toe. It absolutely hates any toe-OUT. Efficiency nose-dives. What surprised me was that even rather high toe-IN doesn't hurt it. I had it set at 0.4 dgr IN and still managed 299 Wh/mile city driving (not bad for a P85+ on the 21s). Now have it at 0.15 IN and will see if this is the sweet spot.

I'm not quite done yet, but I'm fairly sure that it wants more toe-IN rear than front. The front seems happy at zero toe to about 0.15 IN, but doesn't want much more than that. The rear needs at least 0.1 IN and is fine up to 0.4 IN.

Finding a road where I can do that would be hard. Although traffic is not heavy outside rush hour, there's always traffic here. But the traffic lights are all synchronized so I have an 8 mile loop I test on. I always hit the exact same lights so it is very consistent.

Btw, I set front toe at zero (instead of 0.15 IN) and there is a slight improvement. I think I'll leave the front at zero. I like the way it turns in with zero toe. Feels better.

Kind of spooky how consistently wrong both our cars were. I also found 0.8 dgr toe-OUT in the rear after my service center aligned it. Did you have your SC align it or was the 0.8 dgr from the factory?

I did find a "shortcut" for setting up my toe measurement. From the string to the rear center caps I use 45 mm, in the front 64 mm. Doing this the strings are exactly parallel.

Kevin: You did place the string 19 mm further out from the front hub, right? Otherwise the string is not parallel.

Also, be sure to double check after driving several miles. I found mine changed a bit after the suspension settled.