What is the history of the four stroke displacement rule?

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6/16/2018 1:12 PM

What was the allowable size for a thumper before the 1998 rule change? What size were the old school four strokes from the 60's?

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6/16/2018 8:06 PM

Four strokes in the GPS in the 60s were 500cc. A berg won a GP moto on a TT500 around 1977 or 78.
I forget the year maybe late 80s AMA four stroke limit in 500 class was 550cc I believe. Greg Zitterkopf rode a ATK 560 around 89 or 90 . When the 500 class got ditched I think the 550 limit applied to the 250 class.
Something like that.

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6/17/2018 1:10 AM

When I first started racing in England in 1977 the rules in my organisation were 125 two-stroke and 200 four-stroke. There were no four strokes racing and this, I was told, was a left-over from when British four-strokes needed help against the Japanese bikes.

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6/17/2018 5:28 AM
Edited Date/Time: 6/17/2018 5:29 AM

Cooper ran a 540 in the early 2000's, before the 450cc rule came in. Giving "Airtime" 40 year old Coop a 70hp motorcycle was fun to watch.

http://www.motorcycledaily.com/2002/04/24april02cooperandktm/

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6/17/2018 5:33 AM

^^^ Dowd did as well

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vomiting equals disqualification.

6/17/2018 5:34 AM

I found a '93 rulebook in a box a few years ago and it was 550cc

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6/17/2018 7:59 AM

I remember the first time I rode a 250F (late 2000) and said to myself that this will make the 125 obsof’nlete. Then during the SX season I remember DC on TV saying “we’ll have to see how this double displacement rule works out”.

In regards to Coop on the 70 hp Thumper, Jimmy Button told me that his Yamaha dyno’d at 76 hp and it made it all in a pretty short range. Nearly unrideable he said, but he never stated what the displacement was...wink Seems they were trying a lot of things early on.

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