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Here They Are: Scientific pre/post-mod results

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Preferring empirical data to "seat-of-the-pants" analysis, I finally got around to running time trials for the post hop-up configuration of my son's XR70. For anyone who cares....

Test conditions Pre-mod / Post-mod

Air Temp: 54 degrees F / 45 degrees F

Relative Humidity: 88% / 52%

Wind speed & direction: negligible / negligible

Rider Weight: 155lbs / 155lbs

Test Distance: 250 ft / 250 ft (note: allowed for mid-third gear)

Test Surface: Asphalt (flat)

Run profile: Standing still start - straight course (drag strip)

Timing Method: Human (stopwatch)

Bike Configurations: Bone stock VS. Trailbikes HD clutch, XR50 primary driven gear, Trailbikes 88cc kit, BBR big bore exhaust system.

Test Methodology: Five runs in each configuration, high and low times dropped, remaining three averaged for computations.

Test Results (Stock)

Trial:Time(sec)

#1 : 8.22

#2 : 8.15

#3 : 8.15

Average: 8.17 seconds

Calculated Average Speed: 20.85mph

Observed Top Speed at Finish Line (on-board computer): 28mph

Test Results (Modified)

Trial:Time(sec)

#1 : 6.50

#2 : 6.68

#3 : 6.51

Average: 6.56 seconds

Calculated Average Speed: 25.97 mph

Observed Top Speed at Finish Line: 37 mph

Gross change in average time: 1.61 seconds

Percent change (decrease from stock) in average time: 19.7%

Calculated distance interval based on average speed (i.e. how far behind is the stock bike when the modified bike crosses the finish line): 49 feet

Unrestricted distance, top speed trials:

Stock: 43 mph

Modified: 57 mph

(the stocker just couldn't overcome the wind resistance!)

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The only thing I dont really beleive is the top speed. My friend has a xr70 with the TB88 and all done to it as well, with stock gearing, it could just barely muster about 50mph, he weighs almost identical to you as well. The stock top speed looks right on the money pretty much though as well as the rest of the results. Very cool man, I love the scientific way you did everything, never would have thought of that, but kind of wish I had with mine.

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Jetster, did he put on an XR50 primary driven gear? That's the only way I could get the TrailBikes HD clutch kit to work (see my HD clutch kit posts). I went to a smaller (1 tooth) primary drive gear and a larger primary driven gear which, if my thinking is correct, would gear it down slightly maybe making it easier to push through wind resistance. Sound plausible to you? It took A LONG TIME to get up to that speed (I used my local airport's runway!! 😢) and although I wouldn't bank on the accuracy of the computer, I felt like I got a good measurement on the wheel circumference and it sure felt fast!!! 😢

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Oh okay, thats right I forgot about the primary gear. Oh by the way figured Id let ya know how small a world this is. That Primary gear you got came out of my old stock clutch on my 50. Mike asked me if I had it still, said he needed to help out a customer and wanted to get them going pronto. I just peiced it together from your posts that it was you who was in need of the gear. Pretty cool huh lol.

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😢 Man, small world is right! That's great! Well, I'll tell ya, it worked perfect and I appreciate the help 😢. I can't say enough good things about Mike and Trailbikes - they really exceeded my expectations. I wish he'd had the race head/big bore combo when I bought my kit a few weeks ago though - separately the head and BB kit are $540, now he's got a combo for $399!!! What a deal! I've heard the race head makes a huge difference - what's been your experience?

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Did somebody say, "Even FASTER"? I have a feeling I'll be making a few more time trial runs in the near future.... 😢 (Got to watch out for those planes on short final approach!!! 😢😢)

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