I do recommend the curved shafts as they are much easier to adjust for different sized horses as well as balance the vehicle.
I'll give an example here. This pony is 40" tall and is using the same cart I have except she has the lighter wheels and she has straight shafts. The problem she contacted me with was the ends of her shafts bounce when her pony trots. This is a balance issue and is caused by the fact that the front of the cart is not level because in order for the shafts to reach the shaft loops she has to have a bit of an angle. I suggested lowering the shaft loops and moving her seat forward, but her seat is all the way forward. She did lower the shaft loops all the way but the bouncing didn't stop.
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In the photo below I have my Kingston cart hitched to my 41 1/2" pony. So he is actually taller than her pony. BUT I have the curved shafts so they keep the front of the cart more level and then curve up to the shafts. This allows me to use this same cart with my mini mare who is 37" tall and my pony who is 41 1/2" tall It would fit a little bigger pony as well and would adjust for a mini as small as 34" tall. The curved shafts are the reason I can use this cart with the different sized ponies.
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Hopefully that helps!