[a7mgte]

My brother and I are looking into the possibility of custom building a hybrid rock crawler. We have plenty of experience mechanically to fabricate just about anything, but neither of us has any experience when it comes to dealing with electric powered vehicles.

We decided to go electric because the power curve of an electric vehicle makes it ideal for "rock crawling." As the motors are able to deliver full torque from basically 0 rpm.

The area we need help in is what motor and battery configuration is needed to accomplish the build and have it drive as intended. The finished vehicle weight is estimated to be around 800 pounds or so, not including the motor and batteries. It will spend about 80% of it's life traveling at under 10mph, however it should be able to acheive at least 25mph...40+ would be ideal. We would like to keep voltage down around 48 volts, but 96 isnt out of the question. Mostly because we dont want to carry more than 4 cells...8 max to keep weight down. We realize this will limit range signifigantly,so we plan on running an onboard generator to help keep the batteries charged. Since the vehicle will spend most of its time at very low speeds without much of any load on the motor, we were thinking 6500 watts (120v A/C) or so should do it, or is that too much / little? Hooked up so that it starts and stops itself as needed to keep the batteries fully charged. Further, in any given day on the trail the distance traveled is usually between 10 and 30 miles...sometimes more or less. In cases of fewer miles, it typically means we are driving over much larger rocks and difficult terrain at very low speeds requiring lots of low end tourque, and the opposite is true as distance traveled increases.

We would like to couple the motor directly to a transfer case without a transmission at all. We plan on using a gear driven toyota transfer case. We belive that between the available gearing of the T-case a transmission shouldnt be needed as there are alot of options for gear ratios without a tranny. One or two cases stacked together are available options for the build. Each case has a standard high range ratio of 1:1 and 2.28 low range, and one or both cases can easily be re-geared to produce a low range of 4.7:1. The cases are able to be shifted on the fly independant of each other as well. So bewteen the T-case(s) and available axle ratios ranging from about 3.5 - 5.7:1, making the final drive ratio a range from 3.5 to over 120:1. Will this be enough to turn a 33" - 35" tire? Bear in mind that this vehicle will need to propel itself up near vertical inclines at times at speeds as low as possible. (<1mph) Allbeit, these climbs are less than 30 feet long each at max, typically only 5-10.

Also, being that this is going to be a strict off-road only vehicle, it's motor will need to be able to survive continuous operation in very dusty conditons at best. Are there any motors available that are sealed? Occasionally the motor may be submerged if a deep water crossing is needed, although this will be avoided whenever possible. The vehicle will need to be washed though, and the motor will certainly get wet at least doing that.

Essentially, we need help figuring out what size / type of motor would be best for this given its weight and what of the available gearing would be needed given the large tire size. Any help and or ideas would be great.

Thanks.

 

[a7mgte]

My biggest initial concern is the power required from the motor without over powering things and risking constant breakage. A toyota axle built with longfields and ARB lockers is pretty strong, however the longfields like to let go at around 8500 ft/lbs. I believe the locking hub becomes the weak link after that, although I don't have the numbers for their breaking point however they are a relatively quick and painless upgrade should they let go. Although this is also going to be a very light weight rig, and will be turning a 35" tire max, so I think traction may be the bigger issue for me.

In trying to calculate the torque required from the motor, I'm figuring the mechanical advantage like this. Assuming 7000 ft/lbs of torque at the axle as a safe power target to keep from breaking things too quickly, I see it like this. The equation assuming low range on the transfer cases geared at either 2.28 or 4.7 for either or both would be worked out as follows starting at the tire assembly working twards the motor.

(7000/AR/TC2/TC1=Desired motor torque) right? Where AR is axle ratio, and TC1 and 2 are the transfer cases. Which means (7000 / 5.29 / 2.28 / 4.7 = 123.5 ft/lbs.) is needed from the motor to give me about everything the strongest toyota axles on the planet can safely take.
This seems awfully low to me, but then again this defines a condition of being tire bound and having all the power go to that one tire. Realistically there are 4 tires to carry the load...which can spin, so that number can by multiplied by 4, giving me an ideal max motor torque of 500 ft/lbs. correct? This is where it gets tricky for me as 500ft/lbs from an EV motor would surely tie my drive shafts into cute little bow ties and strip the internals right out of my t-cases the first time I stepped on it too much.


With a 4.7 and 2.28 case stacked my choice of gears would go from low low, which would be 10% lower than a stock truck in low range, next would be 25% lower than stock 1st high range, then just lower than stock 2nd high range, and finally the same as 4th high range. I think that these gears would be ok for trail use, given the extremely low RPMs that an EV motor can run at, and I don't think I'd ever use the highest gear. I may go with a 5.71:1 axle ratio though to bring everything down another 10% though. I want to avoid using the transmission if possible because I don't want to deal with it's size and weight, and the cost of coupling the motor to it. Where as to couple the motor to a divorced crawlbox would be quite cheap and easy, as well as allow me to mount the motor a full foot or more higher in the engine compartment to help keep it out of the mud.

The plan is to figure out the drive motor and number / size of the cells, gather the rest of the drivetrain and then build a frameless tube chassis around it all with a 3 or four link suspension system.

I'm in central california. (San Luis Obispo) So I'm definitely a west coast wheeler. My current rig is an 86 4Runner, SAS'd, 35"'s, ARB lockers front and rear with a 7MGE engine under the hood. I was running the 7MGTE for a while, but got sick of listening to it, worrying about it being so hot, and getting in the way of working on stuff, so I swapped it out for it's N/A sibling. I carry the same handle over at yotatech btw.

Putting the batteries on the axle itself would lend itself to superior weight distribution, but I'd be worried about it stressing the knuckle ball on a toy axle to the point of failure or collapse with all that unsprung weight. Can the cells take the abuse of being mounted there as well?