Positive handling aspects of Hymotion Pack -

My wife and I used to be pretty active in club motorsports and spent maybe 10 days a year driving on one of California's several race tracks. My wife has even been an instructor at times. We have driven many different cars, mostly Porsches though, but also Mini Coopers and Honda S2000s. Cars like the S2000 and the Boxster are very neutral handling cars with close to 50/50 weight distribution. This is advantageous in the corners, particularly for less experienced drivers as the car literally pivots around a central axis and provides very real feedback to the driver. As opposed to our normal track car, a 1971 Porsche 911, which has a 3X/6X (maybe 35/65??) front to rear wegiht distribution. This makes the car very tail happy - to the point that if you let of the gas in a turn you are likely to spin out backwards (ask me how I know). Back in the early days of the 911 club racers would add lead weights in the front trunk (boot) to try to get a more neutral balance.

Now for the Prius - after having the Hymotion pack installed with upgraded shocks and struts to handle the extra weight, I noticed the car handled much better than before. Certainly better shocks will help, particulalry in handling the extra rear weight, but the car feels much more neutral in its handling. From what I can find online, a stock Prius weighs 2900 pounds and has a 59/41 weight distribution (which is why you put more pressure on the front tires). The Hymotion Pack is mounted literally right on top of the rear axle, it adds maybe 200 pounds (after subtracting out the weight lost from the spare tires and other stuff that is removed) exactly where you would want to add weight for better handling.

So now your curb weight is 3100, of which approximately 1700 is on the front wheels (.59 x 2900). That leaves 1400 over the rear, giving you 45% of the weight in the back, or a total weight distribution of 55/45. This is a very nice weight distribution - almost the same as the Porsche Boxster, only in reverse - 46/54. It is definitely a noticeable improvement - lighter more precise steering and very crisp turn in to corners.

As an aside, Toyota built a high performance Prius a few years back and one of the things they did was move the hybrid battery back to sit over the rear axle for better weight distribution - CanadianDriver: Toyota Prius GT concept joins Goodwood Festival of Speed

 

Wouldn't surprise me -

I have a Suzuki Burgman 650 (which is a very large motor scooter) - and added weight in the back - either a passenger or cargo - really improves the planted feeling of the bike.

I think the fact that the battery sits low in the car also helps.

One other positive of the Hymotion is that while the pack has a charge, the car in effect has more power at all times since you constantly have the "turbo stealth" mode available. In fact, I am having my tires changed to better handling tires because, believe it or not, when I turn onto Highway 1 to begin my daily commute, I often spin the tires and trigger the traction control - not a good thing when trying to pul out onto a 50 MPH highway.

 

I went down to a one psi difference - but probaBly not a big deal either way. When I got the car back from the Hymotion installer, they had set the pressure even all around - and that felt fine as well.