Why the BMW i3-REx

My next step is to use the plumb-line, string method to measure the actual camber. Then I have a decision to make.

Once I've documented the camber, I'll see if I can get a tire changing station and reverse the existing tires on the rims and remount. I have 11 months on what had been new tires. So I want to try the existing tires, reversed, for another 5-10k miles. I want to confirm the tire wear finally equalizes across the tread. I also need to verify there are no adverse handling characteristics by seeking and driving in wet weather.

I also want to plan and practice, suspension tuning before the next set of tires. My thinking is even-wear from with even temperatures across the treads should maximize efficiency. As long as it doesn't change the straight-line stability, this is where I want to go.

Finally, there is my plan to double the usable gasoline by adding an aux tank interface to the filler tube. Doubling the gasoline range will make a significant improvement in block-to-block times. At 65-70 mph, the BMW i3-REx then matches long-haul trucker speeds, perfect!

Bob Wilson

ps. Fixed the quote. But that does not change my approach, yet.

 

I corrected the quote but there are facts and data we need:

1. What are the actual, loaded camber of both front and rear?
2. What is the 'natural' rest angle of unloaded new tires?

My thinking is I want to start with the unloaded tire tread angle and adjust the vehicle camber so loaded, the tires under load will be flat. Then test the handling and make some hard decisions:

• negative rear camber - this may be required for straight-line stability. If so, I'll flip the tires so the inner tread, the thicker tread, will wear down.
• positive rear camber - again, make sure the thickest tread is on the wear side.

Then I need to do the same for the front tires.

Bob Wilson

 

More reports from Dept of Energy: 2014 BMW i3 REX | Advanced Vehicle Testing Activity

Bob Wilson

 

Ok, here is what happened:

A couple of weeks ago, I went in for a recall and we got this on the drive-in, tire checker:

Now this was not enough to replace the 13k service mile tires. However, I met the technician outside and he said the tires are 'OK' but I was concerned. So I did a tire tread temperature test.

I took a small tripod and mounted a IR thermometer on it and configured the iPhone to record a video of the readings. Then on a cool day, 65F (18C), I drove 10 miles (16 km) at 75 mph (120 kph). I stopped and video recorded the tread temperatures:


-1.0 degrees is the normal camber spec and I had a 4-wheel alignment in December when the motor mount bolt broke.
. . .

The clue is both of the right tires show excessive toe out. I was asked about whether or not I'd hit something and thinking about it, the light came on.

Here is the culprit:

There are two accesses to my neighborhood and this one has those 'split' speed bumps. The gap is designed so firetrucks and ambulances can enter and leave easily. But it temped me to have the 'driver' wheels go through the gap and the passenger side tires took the bump. This would also put a torque on the car that probably moved the rear camber too far negative.

I still need to replicate the tread temperature tests but at least now we have a clue about the problem.

• The nner tread is 2F (1C) warmer than the outer tread - this is the profile of too much camber and/or possibly toe (unlikely)
• 3d tread is noticeably cooler - I run maximum side-wall pressure, 51 psi, and this suggests the 3d tread may be stiff and off loaded by higher pressure on the side treads. This merits investigation.

So this morning, I dropped the car off with instructions to take 0.5 to 1.0 degrees of camber off. So if already at -1.0 degrees, it should come back at 0 to -0.5 degrees. Then I can replicate the tire tread temperature test and see if we are getting something more even.

I'll then start with 51 psi, measure, and then reduce by 5 psi until reaching 35 psi (yes, this is low for the rear tires.) The goal is to see if between changing the rear camber and pressure we can get even heating across all four treads.

 

Yes but I've driven our Prius over the same bumps for years with no effect on alignment. In effect, the BMW i3 appears to be more sensitive to these effects.

Bob Wilson

 

The tread temperature survey will confirm it.

Bob Wilson

 

Certainly the ones with active temperature management. Although much improved, the air-cooled ones still bother me.

Bob Wilson

 

I got a FLIR and took photos after a +10 mile run at 70 mph that I am still learning how to use. These were taken at night in a parking lot lit by yellow, sodium vapor lights. Also, I didn't use my tripod setup to fix the distance and orientation. Taking photos, I notice that some orientations seemed to washout the image. I suspect it was from the parking lot lights. Next time, I'll use an unlit parking spot and the tripod.

• Passenger side front - a 7F temperature difference, the excessive negative camber is a problem.
• Driver side front - shows the flaring that I suspect is from the lights but the negative camber is evident and a problem.

• Driver side rear - looks reasonable but I'd like to retest.
• Passenger side rear - negative camber is excessive.

I've now have a bubble, camber gauge and two lift pucks. Once I get the driveway surveyed for a flat place, I can raise the car; remove the wheels and; check the camber with the bubble gauge. This will give empirical data for the camber adjustments.

Bob Wilson

 

It is called the maximum, cold temperature pressure embossed in the sidewall, typically by the bead.

Bob Wilson