Replica Frame

old.wrench
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Re: Replica Frama

#16

Post by old.wrench » Wed Feb 18, 2015 8:12 pm

There can be a very noticeable change in feel and handling with different diameter rims, especially on the front. The most dramatic (gotta have a little drama :) ) change that I've noticed was on a shovelhead that I have with a springer front end. I switched from a steel 3"x16" rim with a 5.00"x16" Avon MK II to a very lightweight Rev-tec aluminum 2.15"x21"rim with an 3.00x21" Avon Speedmaster. Bam!!!, it was like riding a totally different bike!
Right now, on my '54 Panhead, I'm running an in-line springer with a 19"xWM 3 aluminum Akront rim and a 4.00x19" Firestone D.C. tire (which is a tall one) and the handling and feel is very good both around town and on the highway. The way it sits is very nice and level too.

Another thing to consider, aside from frame and front end geometry, is unsprung weight. It's true what is said about un-sprung weight, when you reduce it, handling improves.

Geo.



58bob
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Re: Replica Frama

#17

Post by 58bob » Wed Feb 18, 2015 10:56 pm

Sounds good old.wrench, I always thought it had something to do with rim width.
Maybe I go for an 18 or 19 incher.

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Re: Replica Frama

#18

Post by old.wrench » Thu Feb 19, 2015 1:00 am

58bob,
It's about the outside diameter or the height of the tire when it comes to trying to make a minor adjustment in the stance of the bike and the same thing (tire height) will also have some affect on the handling. I believe one of the points Robbie was making about the "gyroscopic" effect ties in with what I was trying to illustrate with my description of improved handling with a lighter rim and tire combination (less rotating mass). The typical 16" rim and tire is generally pretty hefty; as you go into the bigger diameter rims the tire mass and weight generally decreases because most of the tires we use are actually pretty close to the same height or outside diameter - within maybe 2" or 3" inches at the most, and less than that if you stick to vintage type tires. Since only half of the diameter (or the radius) of the tire affects the way the bike sits, we're talking about a relatively small difference - maybe an inch or so. I find it pretty interesting, and a good topic when we're holed-up with this weather we've been having :) .

Geo.

bandit67
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Re: Replica Frama

#19

Post by bandit67 » Thu Feb 19, 2015 1:30 am

Wow, thanks a bunch,,,great info Rubone. Info like that only comes from years of building and seeing what will really work,,,,,thanks for sharin. Ok, so the offset springer was limited to 46 only.....guess that solves that. Pretty sure mine is an inline. Had thoughts of going with a 500-16 but now prolly not. Will a 18 or 19 work with those stock front fenders? I have experienced speed wobbles but not sure the whipping you guys speak of. Plan is to RIDE this scoot, so do want as good a handler as I can put together.

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Re: Replica Frama

#20

Post by RUBONE » Thu Feb 19, 2015 1:46 am

Ok, so the offset springer was limited to 46 only.....guess that solves that.
No, the Offset STARTED in mid '46 and continued to the end of BT springer use on special order '49models, which ALL shared the same rake as every bike H-D built after that into the Shovel era. As Old Wrench pointed out, the smaller mass of a narrower, lighter wheel diminishes the possibility of the whip effect and the taller overall size changes the trail favorably as I said earlier. With a big, heavy doughnut on the end of the forks, when the gyroscopic effect tries to straighten it out the extra mass causes it to go over-center which makes it overcorrect again the opposite direction, etc, etc, which is the speed wobble effect.
Interestingly, the 16" wheels on 45s were never as much of an issue, no different frames, no different forks. They became very "heavy" steerers with 16's but no wobble issues. Perhaps due to the speed range they operate in but most likely the steering geometry. And the military specified 18s which handled much better..

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