Frame Tubing and Shimmy

[Jim G]

Carrying over a thread from the Longleaf blog posting, Adam A.
mentioned:

"Didn’t the G2/G3 P/R get a slimmer downtube? Using a stouter downtube
and slimmer top tube is interesting."

Here's the details, from http://sports.groups.yahoo.com/group/KOG/message/14005

++++++++++
> Is there a summary anywhere of the differences between the first and
> second batches of P/R's?

The down tube changed from 31.8 x 0.6 x 0.9 to 28.6 x 0.6 x 0.9.
The top tube changed from 28.6 x 0.6 x 0.9 to 28.6 x 0.5 x 0.8.
++++++++++

I'd just like to point out that this is how Jitensha seems to mitigate
shimmy in low-trail Ebisu randonneur framesets. Regardless of size,
Ebisu frames seem to always use a 1-inch TT, but Hiroshi seems to
change the DT diameter according to the frame size: Under 59cm, he
uses a standard 28.6mm tube; over that, he steps up a size (to
31.8mm?)

I know a guy who has owned a 59cm G1 P/R, and currently owns 2 58cm
Ebisus, a 58cm BDB Pelican, and most recently got a Toei. All of
these bikes are the same size +/- 1cm, and most are built similarly
(rando style with front rack and bag). One of his Ebisus is 650B,
while all the other bikes are 700C. His 650B Ebisu is made with
slightly stouter tubing than his 700C version, and both are the same
size. He mentioned that, when he ordered his bikes from Jitensha, he
insisted that Hiroshi use standard diameter tubing. According to this
person, his 700C Ebisu shimmies VERY slightly, while his 650B bike
does not.

In short, I often wonder if the P/R would benefit from a 1-inch TT and/
or a 31.8mm down tube. At least in the larger sizes (59cm+).


-Jim G

[rcnute]

I just picked up the 59 P/R on ebay from the "mistake" batch with the
OS downtube. This is going to be a very low-trail, front-loading,
heavy-duty city bike. I'm curious to compare it to my 56 G3 P/Rs
(which don't have high-speed shimmy but do shimmy at low speed with
front loads). I really do love how those bikes feel and I think the
skinnier downtube has something to do with it.

Ryan

On Nov 25, 2:50 pm, Jim G <yoj...@gmail.com> wrote:
> Carrying over a thread from the Longleaf blog posting, Adam A.
> mentioned:
>
> "Didn’t the G2/G3 P/R get a slimmer downtube? Using a stouter downtube
> and slimmer top tube is interesting."
>

> Here's the details, fromhttp://sports.groups.yahoo.com/group/KOG/message/14005