# Unsprung weight of hub motors



## eou_edu (Apr 27, 2011)

After a long hiatus I've come back here, quarantined and pondering my next build: An off road jeep.



Let's talk about unsprung weight. I've read 1000 threads on hub motors that all come to the same conclusion: hub motors have too much unsprung weight for electric vehicle. But one question I've never been able to answer by anybody is who's standards is this based on?



I believe unsprung weight of hub motors causes too much of a rough ride for large scale car manufacturers to appeal to average consumers. But this is the DIY forum. We are here to take a closer look at things that better fit our needs. 



I see some major advantages to HUB motors in a jeep. If you look at a fair comparison of unsprung weight in a jeep of a gas engine vs HUB motor built for off road purposes I see more unsprung weight coming from a gas motor. Mostly people lift and put bigger tires on a jeep to get more ground clearance. If you have a HUB motor you would not need nearly the same size tires to get even close to the amount of ground clearance. For every inch you go up in tire size equals roughly a 5 lb increase in weight. A hub motor with 30" tires vs gas jeep with 35" tires weighs 25 lbs less just in tires and still have better ground clearance. With some alloy wheels and disc brakes to replace rear drum buys another 20 lbs. At this point I only see an advantage in unsprung weight of a hub motor. 



I realize there's so other issues with HUB motors but right now I'm just looking at the one that's mentioned the most often.


----------



## brian_ (Feb 7, 2017)

When the ground clearance concern is the final drive housing (with the differential in it), the solution which does not require huge tire diameter is portal axles. Since few vehicles use portal axles, getting ground clearance without very large diameter tires is apparently not a common concern. One reason that portal axles are not more widely used is that the required gear set in the hub assembly increases unsprung mass... which is a concern.

Large tire diameter is not just for ground clearance. Larger tires handle uneven ground better.

There are vehicles for which the high unsprung mass of hub motors is widely considered acceptable. These are generally vehicles for which handling and ride comfort are not big concerns (e.g. heavy buses and off-highway trucks), low-speed vehicles (those off-highway trucks again), and vehicles with relatively low power (and so small motor mass compared to the vehicle mass). A low-speed off-road vehicle (so, rock crawling rather than desert racing) might be a viable application.


----------



## eou_edu (Apr 27, 2011)

There's a lot of reasons to have bigger tires in an off road situations, there's a lot of reasons to stay small. I personally prefer as small as I can get away with. But it's really up to the individual, their driving style, and the terrain they are driving on. For me the biggest reason for bigger tires is increased ground clearance. Portal axles is one solution. But you are right it adds a lot of unsprung weight, overall weight, complexities, and inability to fix on the trail with an in stock part at the nearest part store........ You might be lucky to get the part in a month. 



A unimog rear end is around 600 lbs. A samurai rear end is around 150. A stock samurai overall weight is around 2100 lbs and the engine is 60 horsepower. It wouldn't take much motor to move a samurai around at least as good as the stock engine.


----------



## jwiger (Oct 18, 2014)

I have a lot of experience with humvee suspension. They are a dual A arm portal design. The disc brakes are mounted in-board near the differential all you have at the wheels is the ball joints, and the gear hub (close to a 2:1 reduction at the hub). This means half shafts, brakes, and differential all have a mechanical advantage. The brake rotors are just 265mm (10.5in) and can strain your neck!!! 

One of these days I would love to convert a humvee to a hybrid and just bolt 3phase motors straight to the differential inputs. Then replace the big diesel with a Prius engine/generator.

Sent from my BBD100-1 using Tapatalk


----------



## motas (Apr 21, 2017)

This thread caught my attention as I'm a massive Jeeper! And still fully intend to convert mine to electric.

I can see why people are so drawn to hub motors, and I wish they were a better solution than they are, the simplicity and clearance would be amazing.

One big problem that hasn't been mentioned is gearing. Even with the the highest reduction that will fit in the differentials, I still can't get an electric motor to its ideal gearing. Doing this within the confines of a wheel, and without adding even more unsprung mass would be extremely impressive, especially in off road vehicles which have small rims, and large diameter tyres. On top of this you'd also lose low range ability.

You also have the issue of durability. Using my Jeep off road would go through wheel bearings very frequently. Having the motor as unsprung introduces a lot of vibration and forces which is going to wear part of the motor out, and be much more expensive than chucking a new wheel bearing on every few months.


----------



## fotajoye (Nov 28, 2011)

One must be concerned with maintaining the critical clearance between the rotor and the stator in electric motors; In hub motors the bearings that maintain this clearance also serve as the wheel bearings. Off Roading kills wheel bearings. This may be one more reason Hub Motors are not common.


----------

