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Author Topic: What changes permitted in A bike (but not in M) give the greatest improvement  (Read 3241 times)

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Offline panic

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I'm still sussing out my chassis, and still have much latitude as to wheelbase, CG, ground clearance, etc. My maximum speed may reach above 150.
Since the basis is a slightly modified H-D frame with no structural members removed, IMHO it qualifies for M (extended swing-arm disqualifies me for P).
How much stability is added by exceeding the max wheelbase in M (is it 68"?) if I don't mind a tougher class? Is there a point where more wheelbase does not help?
I also would like an opinion (I know I have to make the plan known in writing in advance if I expect to do this) as to whether a structural member (not cosmetic, not bodywork) automatically becomes streamlining if its shape is obviously aero - like a strut that's tear-drop in X-section?
Please bear with me - this is not my strong suit!! Any comments welcome.

Offline Dean Los Angeles

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Question #1. Why not stay with  M frame class?

Question #2. If you are going to run A frame class and you have the ability to make changes to an existing frame, then you might just as well start with a clean piece of paper and design with speed in mind. The less air you push the faster you go. What can you do to make this design lower? Starting with an existing frame puts major limits on your thinking.

Fast is a function of horsepower, and that doesn’t matter what the frame class is going to be. Traction is always a problem and there are some frame modification that A frame class might help. Aero drag is where the big difference between classes comes to play.
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Offline panic

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Thanks.
I don't think I can make it any narrower.
The engine height isn't fixed yet (sub-frame for drive train), but to really lower it the primary drive will run up-hill a bit so that the transmission sprocket/swing-arm/axle line is still good. This is preferable to horizontal primary with the angle at the swing-arm pivot pointing up to the axle, yes? It's a monoshock so the suspension rate isn't limited to shock travel from the arm.
I know that simply moving the axle back will reduce traction (i.e., unload the rear wheel) at launch, but it also creates a place for ballast (battery, oil tank, window sash weights). I have less than 100 hp to start, and I think I can safely limit power in low gear to make it easier to ride.
Never having ridden a long wheelbase bike, how much stable is it at 80" than 68"?

racin jason

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Some of the fastest guys(250-260mph) are running wheelbases in the 59-64" range. i have run over 200  with a 55" wheelbase and had no stability issues. going long doesnt mean it will go straight.

Offline Dakin Engineering

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At Maxton, I was making enough torque to do a fair impression of a unicycle.
That got fixed, but I never got the pucker out of the vinyl seat.
When I extended mine, I went to 67". 
Plenty of room for the oil tank and battery, too.

Sam
Turbo Sportsters since '97

Offline Stainless1

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Panic, what event are you planning to run, that will make a difference in what you will want to do.  Bub rules are different than SCTA, Maxton and Texas have a few of the own rules and classes. 
The 68 inch thing leads me to believe you are thinking Bub.  SCTA allows 10% max stretch for M, that's 10% of book spec, not actual WB. 
Build, have fun, be safe, go fast
Stainless
Red Hat 228.039, 2001, 65ci, MSA Bockscar Lakester with a little N20 
MSA Bockscar Lakester #1000 my fastest mile 245 and change, 84 ci turbobusa motor... but Corey's 233 MPH H/BFL record is still 3MPH faster than mine.