I did some more digging and decided try the surplus car seat motors, at 9.50 each + SH.
Something to keep in mind: Since you are planning on carrying a load, it is likely that these motors do -not- have the bearings necessary (radial bearings) to carry the load, so you can't just put wheels on them and expect them to last for any length of time. Instead, mount your wheels on shafts, and mount the shafts to your platform using some kind of bearing - pillow blocks with ball bearings or bushings will be fine. Then attach the motor's shaft to the shaft of the wheel (use a piece of rubber hose and a couple of hose clamps or something). That way, the bearings (instead of the motor) will carry the load of the robot.
Will try just as you mentioned, differential for steering. Thought of stepped motor for steering but don't want
to complicate it further, keeping simple as possible, simple aluminum tube frame.
Differential steering is definitely easier to work with, vs Ackermann.
The motor details: Draws 1.3A no load, stalls at approx 9.5A and spins 2,500 rpms.
Does that mean it needs 9.5A to stop the motor?!?!?
Stall of a motor occurs when:
1) The motor is under a complete and full load such that the shaft no longer turns. Generally gears in a gear motor break long before this, though.
2) It can also occur (very briefly) upon startup of a motor - the current will "shoot up" to the stall current before settling back down.
3) If can also occur if you switch the motor rapidly from "forward" to "reverse" (which isn't good for a motor or the mechanics of a gear box - so don't do it).
The result of #2 (and 3) is that you need to get an h-bridge that can handle these conditions (plus 15-25% extra - never design to the margin, always allow for some overage). So - you would want an h-bridge that can handle about 12 amps per motor. That's not a huge h-bridge, but it isn't small, either.
I'm planning to use the L298 arduino shield. It says it's rated 2A per side or parallel for higher power.
Won't work for that motor - so forget about it.
Some quick questions maybe you can answer before I get the book.
1. motor says 1.3A no load, so how much current could it increase with load, is only way to know by building it up?
my concern is if it draws like 5A or more, then that would blow out the L298.
I've already answered this: At startup, the motor will draw about 9.5 amps (rated stall condition).
2. any info on how to current limit the L298 for safety? the datasheet mentions adding current sense resistors but does
not show any equation on how to calculate the resistor value.
The L298 already has built-in current limiting (thermal overload protection). But you likely can't use the L298; I wouldn't recommend it for this motor, anyhow.
4. Lead acid battery best for this high current application?
As noted, cost for amperage is nice for this type of battery, but they are heavy. The next best (they'll cost more, but weigh less) would be NiMH packs (commonly used in the R/C hobby world). Next would be LiPo packs (again, RC world - very light, but very expensive, and must be watched while charging; they have a nasty habit of exploding/bursting into flames when overcharged).
Interestingly, there are companies that make "drop in replacement" LiFePo4 batteries for many of the common SLA/Gel-Cell lead-acid batteries commonly used for alarms and UPS systems. They are very lightweight, but very expensive. Something to consider, though!