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Sunday, May 04, 2003
Bigger Wheel Skates: Testing 125mm Wheels
Background and a Brief History
Here is a picture of my 100-84-100-100m Millennium 12.1 inch setup with an early Miller boot and two Belotti wheels at the rear.
It became clear that I might have to wait quite a while to find my answer unless I took the bull by the horns and did it myself. It was with considerable reluctance that I undertook this task as my work bench is equipment-challenged having neither an extrusion facility nor a CNC mill. Nevertheless, I came up with a method to test the 125mm wheels using a hacksaw, drill and assorted hand tools. I built up a homemade aluminum frame, essentially a U-channel (cut from a 1/8 inch wall aluminum rectangular tube) able to hold 125mm wheels at the front and back and 100mm wheels in all positions.
My test frame has 4 axle holes for 100mm wheels equally spaced along a length of 12 inches which allows a gap of only 1-2mm between wheels. Now by adding another hole at the front and rear about a half inch from the 100mm axle holes I can test a 100-100-100-125 at 12.5 inches long setup and 125-100-100-125 at 13.0 inches long. Note that the 100mm wheel is very close to 4 inches diameter and the 125mm wheel is nearly 5 inches diameter. I was skating with a Millenium 100-84-100-100mm frame on the left and the custom test frame on the right. I tried the custom 100-100-100-100mm setup only long enough to verify that it matched up pretty well against the Millennium 100mm frame and moved right on to 125mm.
The U-channel can contain all the 100mm wheels but the top of the frame had to be cut out for the 125mm wheels to protrude through. The U-channel is level with a height of 105mm above ground. If you want some pitch a heel riser can be added on top of the U-channel at the heel. To keep the heel as low as possible I inserted only a washer at the heel giving 1mm of pitch. The result was reasonable stability and no problem with performance. So for these tests the boot's mounting block sat 105mm above ground in front and 106mm above ground at the rear. For comparison the Millennium had a front elevation of 90mm and 100mm at the rear. So effectively my setup sat about 6mm higher than the Millennium (or a conventional 80mm skate). The stability seems to come from the heel which is only 6mm higher and not the ball or plantar area which was 15mm higher. There was really no stability problem with these dimensions during my tests but improvments are likely needed for longer distance skating. I got 125mm scooter wheels from Skates Away having a choice of 78a Yak wheels or a blue unbranded 85a hardness wheel.. For the first tests I chose the 85a blue wheels. For this first prototype frame I used steel bolts for axles and have made no attempt to cut the weight by carving the aluminum artistically.
My first tests of the 125mm wheel had the boot mounted forward of the position shown in the following pictures but I soon moved it back to the location shown as it improved steering and acceleration. My later tests of the 100-100-100-125mm skate were a huge success in my estimation. I found no noticeable difference in performance between the Millennium 12.1 inch setup and the 100-100-100-125 12.5 inch setup. Both had great manueverability and acceleration and responsive turning. But the 125mm version had the roll advantage and was more comfortable on rough terrain. The quick turning of both skates made it quite easy to double-push and to move into a sprint.
Some comparison between 5x84mm and 4x100mm skates:
Then I plugged into my power loss model for a 5x84 vs a 4x100 and the 4x100mm skate came out 1% lower in rolling loss. But now if you take into account the different hub/tire radius of the Belotti wheel the 4x100 comes out a few % lower in rolling loss. The result seems to say that there will not be a really dramatic difference between the best 5 wheel and 4 wheel skates until wheels get larger than 100mm diameter. But the 4 wheel frame has the powering advantage as it is shorter and turns faster the stroke rate can be higher than a 5x84mm skate.
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