leimkuhl@uiucdcsp.Uiuc.ARPA (07/17/85)
I have an opportunity to buy a custom Bob Jackson at a very
reasonable price, but in trying to take advantage of it, I've
run into the tremendous obstacle presented by my limited knowledge
of frame design.
I'm going to leave most of the details to the experts at JRJ
cycles, but there are several critical factors that are determined
by my riding style, and which I want to know a little more about.
I am not completely ignorant of frame-building--I've read the
Custom Bicycle (highly recommended, by the way), and I've been
riding a good deal these past few years, picking up bike shop
gossip and bicycle data, but I have some questions that I cannot
get reliable answers to locally. Since the bike will be used
for racing, only those with racing experience need respond.
First, the bike will be used for amateur time trials, road
racing, and an occasional century, as well as daily training rides.
I am 6' 1.5" tall, with a 37.5" inseam, so I need a big frame. My
present Holdsworth only barely fits me because the Avocet seat post
I'm using gives an unusually large gap at the top, below the saddle.
The distance from the top of the saddle to top of seat cluster
is 8.25". The Holdsworth measures 24.5" (center of BB to top of
seat cluster). I am concerned that another seatpost/saddle combo
won't allow enough clearance to top of saddle. I am also not able
to raise my handlebars higher than 2" below my saddle under the
present situation (which doesn't trouble me too much).
For those of you who care, the distance from the top tube to my crotch,
barefoot, is about 3.5".
*******QUESTION #1: Should I move up to a 25" frame? 25.5"
(Note, the Fit Kit is just way out of line on the big sizes--it
suggests a 66-67cm frame for me, that's 26.5-27" and that's
mammoth)
I am ordering the top-of-the-line Jackson, which is usually built
of thin-walled 531 Professional SL (.7,.5,.7mm). While I weigh just
155 lbs., I spend enough time out of the saddle to warrant a fairly
stiff bike. I do not want the bracket flex I've heard about on
Austro-Daimler Ultralights and Ultimas (also 531SL). Most
suggestions so far have been to use regular 531 DB (.8,.6,.8) in
the downtube and chainstays. Will this be sufficient? If the
more flexible tubing is used in the seat tube, won't we have the
problem of the seat tube flexing on sprints causing the chain to hit
the front derailleur? Or is there really any significant lost
energy involved there. The bike need not be Team Miyata Stiff, so
I would like to use the lighter stuff where possible.
*******Question #2: Should I use all 531SL, all 531DB or a mixture?
If a mixture, where should I use the thick stuff?
The bike should be "no hands" stable, but still maneuverable enough
to turn corners "with the pack." I do not want another sports bike
like my Holdsworth, with its elongated wheelbase and chainstays, nor
do I care for the harsh and twitchy ride of a criterium racer. And
I'd like to be able to ride an occasional century without too much
discomfort. Various road-racing bike geometries exist: Austro-Daimler
uses parallel 74's with a long top tube, Bianchi uses parallel 73s,
Schwinn's new Peloton has 73 seat, 74 head on the big size, Team Miyatas
and Lotus Supremes have 74 seat, 73 head. For my needs a wheelbase
of about 39-39.5" is probably about right, so the various triathlon
bikes with long wheelbase aren't feasible. Remember, this is a BIG
bike, so the angles tend to be a little more relaxed--I probably couldn't
spin on a bike with a 75 degree seat angle, because the saddle won't
go back far enough to put my knee over the pedal axle at the center of
a stroke.
*******Question #3: What angles fit my bill of a stable yet spry
racing frame? (Bearing in mind the 39-39.5"
wheelbase requirement.)
Finally, what is the effect of a slight change in fork rake on bike
handling, tracking, and comfort. I have my choice of rakes, and, of
course, material.
*******Question #4: What kind of fork should I get? 531SL or DB?
Raked at 1", 1.25", 1.5", 1.75", or 2"?
Obviously, any partial answers from those who know to any of the
questions will be very helpful. As I've said, Bob Jackson will have
the last word, but I'd like to steer them in the right direction.
PLEASE RESPOND BY EMAIL
(uiucdcs!leimkuhl)
Ben Leimkuhler
schwager@uiucdcs.Uiuc.ARPA (07/18/85)
Dear Concerned cyclist: It is the opinion of this rider that you move forward with your original plans. The designers at the Bob Jackson will be able to give you the best fit if you explain to them the needs you have and the results you want. Signed: Another concerned cyclist
thielges@uiucdcsb.Uiuc.ARPA (07/22/85)
You (Ben) state that 26.5 inches sounds like too large of a frame. It sounds to me like the size that would fit you well. I'm 6' 2.5" and my 27" frame fits me perfectly. I'll never buy a smaller frame now. Give large frames a chance. Bart Thielges