Tires: Tires for an electric S10 pickup, gross axle weight, chevy s10

Question
QUESTION: Barry --

I have converted a 2002 Chevy S10 shortbed pickup to all-electric power. Specifically: I removed the engine, fuel tank, exhaust, etc., and then replaced all that with an electric motor and 26 six-volt batteries.

Before the conversion, my truck weighed 3040 lbs. After we pulled everything out, it weighed 2600 lbs. GVWR is 4200 lbs.

The batteries weigh a total of 1638 lbs, so you can see right away that the truck is overweight. I compensated a little bit by adding leaf springs and new gas shocks... but I'm still over GVWR by about 500 lbs (after adding up the weights of my controller, motor, wiring, etc.).

All these traction batteries are in the bed, so there's a heavy load on the 2WD back end.

No driving in snow, no off-road, and no racing. Freeway driving to and from work. 5-speed manual transmission.

Got any creative tire recommendations for me?

Many thanks for your comments and suggestions.

Cheers,

Steve

ANSWER: Steve,

On every vehicle sold in the US, there is a sticker – commonly called the vehicle placard - that lists the original tire size and the proper pressure for that size.  The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.

Starting with that placard you need to calculate the load carrying capacity with the original tires and then how much more load carrying capacity is needed (by axle) and then recalculate what tire size (or pressure)  will do that.

I have access to the books that have load carrying capacity, so if you'll post back with the original tire size and pressure, and the new GAWR's (Gross Axle Weight Ratings), I'll tell you what tire size, you'll need.  BTW, you have to consider that your S10 also carries other cargo - for example passengers and stuff in the bed of the truck - so to be safe you need to factor something in for that!

---------- FOLLOW-UP ----------

QUESTION: Barry --

Here's what the placard says for my 2002 Chevy S10 Shortbed:

GVWR: 4200 LBS
GAWR FRT: 2500 LBS
GAWR RR: 2300 LBS
PAYLOAD: 1052 LBS
FRONT AND REAR TIRES:  P205/75R15
RIM: 15x7J
PRESSURE FOR BOTH FRONT AND REAR: 35

A couple of additional points and questions:
1. The rear end of the truck is sagging a little bit -- even with the additional leaf spring on each side.

2. Would it help to get Light Truck tires, instead of the Passenger-grade tires?

3. I intend to get Firestone RideRite airbags to level things out.

4. Given the heavy load of all these batteries, would it help to go to larger wheels.

5. Yes, I'm concerned about the additional weight that will be added -- passengers, cargo, etc. When lithium-ion batteries get a little cheaper, I'll buy some to replace my lead-acid pack.

6. You can see my vehicle and read all about it at:
http://www.myelectrictruck.com

Thanks again for your advice --

Steve Kobb

ANSWER: Steve,

Thanks for the information, but before I can answer your questions I need to get some more detail.

The load carrying capacity of tires is important.  The less usage of that commodity, the less likely you'll get a load related tire failure.  This isn't a "safe/unsafe" sort of thing - It's more a "high risk / low risk" sort of thing.

That means that any individual tire has to have some reserve (unused)load capacity.  Normally, I look to the vehicle manufacturer to help supply that part of the puzzle (since I am not a vehicle engineer)

So in this case, at the very least I need to know what the Gross Axle Weights will be AFTER your conversion is complete.  It would be better if this information came by wheel position (LF, RF, LR, and RR), but by axle is the minimum.  (I'm pretty sure the loading will not be even front to rear).

Since you removed the engine and the rest of the drivetrain, this whole thing becomes much more complicated.

While I am going to think about how to approach this problem, I can't emphasize enough that you need to be able to communicate how the weight is being distributed front to rear.

So post back when you have some data along those lines.  If I come up with additional thoughts before that, I'll send an additional response.

---------- FOLLOW-UP ----------

QUESTION: OK, Barry --

Before getting into the numbers, I just want to comment on one point you made:

"Since you removed the engine and the rest of the drivetrain, this whole thing becomes much more complicated."

Actually, I only removed the engine and fuel tank. The rest of the drivetrain that came with the truck is still there -- with the exception of a modified clutch plate. The flywheel was taken out, and that saved about 25 lbs.

Now for the axle weights...

I estimate the total weight on the Rear Axle to be at 2050 lbs. This includes:
Battery Box
26 Batteries
Charger and Transformer
Power cabling
S10 bed itself

This estimate does not include the springs because I don't know how much those weigh and didn't know if they should be added in. As I mentioned earlier, I did supplement the springs with an additional leaf on each side.


I estimate the total weight on the Front Axle to be at 400 lbs. This includes:
1 battery for the low voltage system (lights, horn, etc.)
Traction motor
Vacuum motor and switch for the braking system
Controller and contactors
180vdc motor for power steering and A/C
Fasteners, cable, etc.
A/C compressor
Power-steering pump & hoses
Welded cross-member

In other words, I added back roughly the same amount of weight that I had removed by pulling the engine.

Note that I have not added in the weight of driver and/or passengers.

While most of the weight is on the back-end, side-to-side distribution is roughly even (at least until the 185 lb driver gets in!).

I will not be hauling any additional weight in this truck, other than an occasional bag of groceries. Also, I'm removing the spare tire. Since the truck will only be used to go back and forth to work and to the supermarket, Triple-A will have to get me home if there's a break-down.

So, that's the way it stands now, Barry. I hope this additional info is what you needed. If not, ask away!

Steve Kobb

Answer
Steve,

I’m going to make a number of assumption and they ought to be obvious as I go along.  The biggest one I am going to make is that the vehicle weight and the distribution of that weight didn’t change – except for the battery pack which is 1,638#.

So what we have is a vehicle that normally doesn’t have any load in it, but now is going to be carrying not only the rated payload of 1,052#, but also 586# in addition.  That’s all the time. In essence this becomes part of the vehicle, instead of an occasional load like the vehicle was designed for.  This means the tires (as well as the axle, springs, frame, etc.) are going to be more highly stressed, but more importantly, more highly stressed for longer periods of time.  This gets us into the “fatigue” arena – which is where tires live.  Anything we can do to reduce the weight is going to be helpful and postpone the fatigue failure.

Notice I said “postpone”.  While it is normally thought that you can improve the fatigue resistance of – say – a bolt, by making it larger, what is really happening is that we are merely increasing the number of cycles before that bolt fails.  In the case of tires, 40,000 miles is about 60 million cycles (revolutions), which puts the part way out in the part of the fatigue curve where a small amount of increased stress is going to rapidly decrease the cycles to failure.  So there is a very real risk, but the good news is that tires “telegraph” this early enough that you can pick up the signs of an impending failure if you pay attention.

OK, so if we assume that the battery pack is centered (weight wise) on the rear tires, then we need tires with a larger load capacity – and AT LEAST 293# more – AND that the battery pack renders the bed of the pickup unusable space wise.  So we don’t have to factor in any additional load.   

I looked up a 2002 S-10 and found it came with 7” rims.  This gives us plenty of options before we have to change them.

So the original tire (P205/75R15) had a load carrying capacity of 1598# at 35 psi, which is reduced by a factor of 1.1 because it is on a pickup, which means its usable load capacity is 1453# - or 2906# for the axle.  Notice that the GAWR (Gross Axle Weight Rating) of the original truck was 2300#, which means General Motors designed in a reserve capacity of about 600#.  This is good and we should try to maintain that if we can (This is one of the lessons from the Ford / Firestone situation a few years back).

A P265/75R15 is the largest tire before we would have to change rims.  That tire increases the load capacity by 792# per tire (1584# per axle).  This makes me feel more comfortable about the increased load – for practical purposes this puts us back in the original load capacity/reserve load situation.

So that’s what I would recommend.  This may create a space problem in the wheelhousing, but if you confine the new tire size to the rear, this is manageable.

BTW here are the specs for the 2 tire sizes:

P205/75R15:
OD = 27.13”
Section Width = 7.99”

P265/75R15
OD = 30.67”
Section Width = 10.51”

This effectively increases the rolling diameter by about 13%.  I don’t know how this would affect the motor, but you should consider the rear axle ratio as part of your deliberations.

I would also suggest:

1)   That you weigh the truck after the modifications to make sure all the estimates were correct – in particular that the original estimate that 100% of the payload was calculated as centered in the bed of the truck.  2300# - 1052# = 1248# would be the original rear axle weight, and the new weight would be 1248# + 1638# = 2886#.  You should be concerned if you are too far off that value.
2)   That you try to move as much weight forward as you can.

Good Luck and let me know how this turns out!