I test drove the 400 followed by the 500 today.
I must say, the 400 is very nice. Responsiveness is great. Steering light and accurate. And the machine a joy to toss around.
All is not perfect though. They went to a one cylinder engine and this thing resonates and vibrates rather poorly at low RPMs. The entire front dash rattles as a result with when combined with the light spring loaded transmission handles means fair bit of racket up front. Putting some rubber gaskets around the handles should quiet down half of the rattles. But something deep insider of the dash still made noise as I held all the handles to keep them from vibrating.
The transmission shift points were very easy to get into normally (much better than other side by sides i have tested). But once or twice I could not shift it without revving the engine a bit, letting it idle, and then shifting -- much like my Kubota BX.
I then drove the 500. The vibration was essentially gone. Unfortunately it was replaced with worse steering that required more effort and harder to navigate, more lethargic acceleration and what seemed to be a stiffer suspension. I found the seats hard on both the 400 and 500 but on the 500 it was especially noticeable as I was tossed around on even small potholes in the parking lot. Engine breaking was better than the 400 and I was pleased to see it not have any of the difficult to control characteristics of my tractor. It simply let the machine coast to idle. As with the 400, I could not shift once without having to do the trick of rev'ing the engine and then idling.
Seems like Kubota has learned some lessons from the 500 that it has applied to the 400. Alas, in order to go down in price they took some shortcuts with the engine. If they had not done that, the 400 would have been almost perfect.
I had just driven the 4010 mule and found it to be so much more comfortable to drive, with power steering and more complaint suspension. Its two cylinder engine is more responsive and quieter than the 400ci. That is my current choice as I keep testing these things
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If you are in the market for an orange UTV to go with your Kubota, and can get past the vibration and noise at low speed, I think the 400 makes an excellent choice.