If you are talking about my skid, the black spoked rear wheels are Yamaha accessory wheels. These are the stock diameter and have replaceable bearings. A great wheel for the money. I did add (1) additional wheel per side right in front of the original rear wheel adjuster and the extensions. (See Avatar Pic) If you use a stock Phazer Mountain or MTX rear bumper, it would be easy to extend the tunnel. I used a Yamaha MTX GYTR accessory rear bumper which is aluminum and was not strong enough with just an aluminum panel closing out the top of the tunnel. I had to make aluminum reinforcement side plates for the GYTR bumper to make it survive. I cracked my first one which was blue. But a stock steel tubular MTX rear bumper should be strong enough to close out the tunnel on top and is a bolt-on for your sled. G.B.