Tamiya Hornet: Grasshopper rear pivot conversion

One of the main differences between the Hornet and the Grasshopper is that the latter uses a fixed pivot in the front of the rear drivetrain while the Hornet uses a full-floating rear axle.

The entire rear drivetrain is one big, heavy unit.  It’s got the motor, wheels and tires, differential and rear axle on there as one big chunk of metal and plastic.

But the biggest issue is that it’s 500 grams of unsprung weight!  This means every time the rear wheels hit a bump, the entire half kilogram weight is knocked around.  Force = mass x acceleration, so that’s a lot of force being applied to the buggy.

The entire buggy only weighs about 1200 grams (with battery), so you’re talking 40% of the entire mass of the buggy is getting knocked around and disrupting the entire chassis.  What you have, is a case of the tail wagging the dog. When the tail weighs as much as the dog, there’s not a lot you can do to isolate the tail’s movement from the dog.  So what ends up happening is the rear drivetrain unit acts on its own, and there’s not a whole lot the chassis can do to dampen the jostling.

One of the “improvements” Tamiya made for the Hornet, was to implement a free-floating rear end, so that this whole drivetrain can move in any direction.  This is supposed to give it more suspension action, and I suppose it might give slightly better traction in loose conditions, but overall it makes the rear end very bouncy and very erratic.  Here’s a clip showing the completely free-floating rear end that can turn in basically any direction.

http://vid1238.photobucket.com/albums/ff497/captainr22/58A4F2EB-EC3E-4595-9EE1-2EDEAD2403C5.mp4

The original Grasshopper uses a fixed pivot point, so that any movement is limited to the up-and-down direction with the shocks.  It isolates the rear a bit more and makes it more controlled.  Personally, I think this is a superior design.  So I modified my Hornet with the Grasshopper pivot points, Tamiya part number 0005069 (Grasshopper parts tree “B”).  It’s an easy modification, you just remove the sliding rod and put on the Grasshopper mounts on each side.

http://vid1238.photobucket.com/albums/ff497/captainr22/DCE700B3-28F4-4637-8B58-79DA671589B3.mp4

This keeps the rear end a little more predictable and not hop around so much.  The downside is that there is absolutely no rear roll at all, but I find the straight-line suspension to work much better and the rear end more stable.  The car wanders less and is overall a more reliably handling buggy.