I thought about posting a thread like this several times already, got a PM from @Ricco today asking me the differences and if it’s worth. So I share my thoughts here and it would be great to hear more opinions and experiences of people who have ridden both sizes.
OFF-ROAD RIDING
Here is the biggest difference. The 9" have better traction, give more ground clearance and roll easier above holes or other evil obstacles on the way. They feel much safer to me. For example the same gravel path where I rode 30 km/h (18.6 mph) with 8" I can ride 40 km/h (25 mph) or even faster with 9". It feels more stable.
TORQUE
I haven’t tried 9" with 1:5 gearing so someone else please chime in but with 1:6.25 gearing the difference between 8" and 9" is not really big to my surprise. If you increase motor amps a bit it feels about the same.
In general if you want more torque at low RPM without changing the gearing then just increase motor amps. The difference between 80A and 100A on the 6374 is sick and I didn’t expect that much difference in torque. Actually I have the most powerful version with 9" and 6374 220kv motors, I can climb 35° hills from standstill like a boss. Even better with 9" than with 8", a bit less torque but better traction. 6384 are more powerful but with 80A/-40A it’s quite similar to 6374 with 100A/-60A (of course they get hotter and you need to push the throttle further).
CONS
9" are heavier and more expensive than 8"
9" are a bit sluggish, 8" feel more agile when steering compared to 9" but it’s not that big difference. Also I can slide easier with 8".
For off-road riding 9" is the clear winner IMO. For normal or all terrain riding 8" is fine as long as you don’t ride too fast or on heavy terrain. Personally I won’t go back to 8" and really enjoy 9" on my regular MTB. I wish I had switched earlier.
I like both setups.
For example my light white build I couldn’t imagine on 9“.
I didn’t ride 9“ on a dual yet, so I as well can’t really compare the torque loss when going bigger without changing gearing or motor kV.
If you don’t plan to go crazy off-road and not sure what to choose I would start with 8“ wheels but get a flexible drive train.
Means 9“ hubs and in chase of gear drive a maxi gear drive with 1/5 gearing and 190kV motors.
If you don’t like that setup and want to go bigger, it’s easy to swap to 9“ tires. Just need a new set of tires and motor gear to get a higher gearing.
Mainly ride on roads but 9 inch tires really helps with all the potholes and cracks.
I don’t really care for the clearance in fact I actaully prefer to be a a little closer to the ground. Nothing a haero deck and 3dservis hangers can’t fix though, they do a great job lowering the setup.
Weight is really the biggest issue I have with the setup but the whole bulid is already stupid heavy at this point.
I also have a 7inch belt drive trampa build for the times I want something more nimble and lively
9" for the win!
if your hitting trails that is.
my local trails are full of tree roots and stones and i smash straight over them, my mate following on 8’s not so much and i’m always waiting for him. So much so that we’re changing his ride over to 9" tomorrow.
Also you get a much smoother ride on the dirt tracks and pavement.
In general my favourite setup is 9" on the rear and 8" on the front. keeping the 8" on the front makes the board more nimble than when you slam 9" all around. This setup is a great compromise for all type of riding.
8" or even 7" on all wheels definitely is better for pure on road use, but they greatly limits what you can actually do off the road.
Overall though i have to say that the 10" tyres all around really ruined both 9 an 8" for me. Those fookers just go over stuff like it aint there. They sure make the board heavy and clumpsy but shit is it hilarious to surf over whatever the heck you want! I tried to put the smaller tyres back on the board but when out riding i found myself constantly wanting to hit those gnarly forest paths that only them BIG boys can handle. Always in the back of my head "why did i change back to theese peasant rubber infants.
I really need to get that 1:6,25 ratio instead of my current 1:5 while using the 10" though. it currently tops out at 75kph which obviously takes a huge crap on my low RPM torque.
I guess that you can always just keep on going bigger with the wheels, at some point though you have to compensate for those fat wheels by changing gearing. My board like most of our mountainboards is really built for 8" tyres in mind. With 180KV motors and 1:5 gearing in my case. never thought i would be doing real off-roading, not trail riding but actuall real off-roading. I am able to do this quite well with my 2wd board and 10" tyres. Not like Peter’s insanity tractor but still
usually got the system set at 120 motor amps and 80 battery amps.
sometimes i do turn it up to 140 motor amps and 90 battery amps but that makes the motors run quite hot so i try to avoid it.
then higher gearing as you said