To maintain the lowest deflection resistance of the face of the tire, it is
best to stay between 3/8 to 1/2 inch deflection of the side wall of the
I learn this from master mechanics that work on long haul trucks and the
following method of how to adjust the tires air pressure for the best
You first weigh the front and rear axle of the vehicle. In my case I have
about 2200 lbs on each wheel and tire on the rear and 1200 lbs on each wheel
and tire on the front.
I use Michelin Low Rolling Resistance
tires that are rated for 2200 lbs at
50 psi which are mounted on 17 inch forge aluminum wheels that is also rated
for 2200 lbs.
The face of the tire has 2 plies of polyester, 1 ply of steel and 1 ply of
nylon which keeps the tire face very stiff and rounded. The side way is a
very soft 2 ply polyester which picks up most of the deflection and does not
transfer the to the face like the older tires did.
To adjust the tire air pressure, you first jack the tire off grade and air
it up to the maximum PSI as listed on the side of the tire. Now we than
measure the side wall width by lowing the tire until it just touches the
Measure from the floor to the lower edge of the wheel rim. Lets say this
reads 5 inches. Now we lower the vehicle until the entire weight of the
vehicle is on grade. Lets say it reads 4.5 inches which will be ok for a
0.5 inch side wall deflection.
For my rear tires, it becomes 50 psi at 2200 lbs for a 0.5 inch deflection.
The same 2200 lb load rating at 50 psi tires are also on the front, but I
have to reduce the air pressure to 35 lbs get a 0.5 inch side wall
deflection for 1200 lb load.
This initial adjustments is normally done at about 70F. As it get warmer,
the side wall deflection may decrease to about 3/8 in. which is still ok.
If you have more than 0.5 inch deflection, then the maximum load rating of
the tire is not correct for the vehicle.
Using the correct tires and wheels made a big difference for me. The first
tires that were use were all nylon and steel with a very stiff side wall
which transmitted the side wall deflection to the face of the tire. As the
temperature drop while the EV was setting outside, the face would develop as
flat spot which took about two miles of driving to round them out.
With the older tires at very cold temperature, it would take between 4.5 to
5 ah per mile. With the correct tires at the correct air pressure, this
became 3.8 to 3.9 ah per mile. At 80F it became 3.3 to 3.5 ah per mile or
about 380 wh/m.
----- Original Message -----
From: "Cor van de Water" <email@example.com>
To: "Electric Vehicle Discussion List" <firstname.lastname@example.org>
Sent: Thursday, June 28, 2012 2:03 PM
Subject: [EVDL] What a difference tire pressure makes!
> I was getting more and more concerned about what
> seemed to be a dropping range on my truck, no matter
> what I did the consumption did not want to go down
> much and I could barely maintain 55 on the freeway.
> I had inflated the tires a month ago when I started
> driving this truck but at that time they were warm
> so I had slightly over-inflated them (35 PSI max
> cold pressure so I had put a little over 40 in them
> as Light Truck tires are required by law to allow
> at least 5 PSI over max sidewall cold pressure)
> and when I checked both rear tires now they were
> indeed still close to 40 (I did put a few extra PSI
> in them anyway) but the front tires had lost more,
> one was down close to only 25 PSI, so I pumped all
> of them to around 42 (this EV is never going to
> heat up its tires like an ICE truck that can do
> max speed for hours at a time).
> Now it easily maintains 55 on the freeway except on
> one long overpass and I arrived at work with the
> consumption meter showing 63% SoC while earlier it
> usually showed between 55 and 58% depending on the
> number of times I would have to stop for a light.
> The truck is still taking too much power to drive a
> constant 55, at least to my liking the 200A draw at
> 110V or approx 22kW is still relatively high when
> compared to the 15kW that I would like to see for
> this type of trip. But at least the 400Wh per mile
> is more acceptable than the 500+Wh that I saw earlier
> when running in lower gear (more loss in the automatic)
> and too low tire pressure.
> Eventually I may bring the tire pressure up to 50 PSI
> cold so it is equal to the pressure in my S10
> (which also had tires that had max 35 on the sidewall
> but needed 50 to avoid rolling over the tire thread
> and drive on the sidewall in a quick turn).
> Then I may need to see if I can do something about the
> tranny - both the occasional leak from the fill tube
> as well as putting synthetic oil in to see if that
> will further reduce the losses.
> And of course, avoid the more than 2" overfill that
> my transmission has today. That can't be good for
> efficiency either.
> Cor van de Water
> Chief Scientist
> Proxim Wireless Corporation http://www.proxim.com
> Email: email@example.com Private: http://www.cvandewater.com
> Skype: cor_van_de_water XoIP: +31877841130
> Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203
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