Archive-URL: http://search.bikelist.org/getmsg.asp?Filename=framebuilders.10303.0155.eml
Date: Mon, 17 Mar 2003 14:18:23 -0500
From: bill(AT)billbostoncycles.com
Subject: Re: [Frame] Some trail questions

Tony,

I will try to address as many of your questions as possible, but no
guarantees.

As for the '48 Herse, I calculated the maneuverability as 5.5 with AccuTrax,
and that seems reasonable. The stability on the other hand is off my scale
in the unstable range. a -3 for stability would be 62.4 mm rake so it is my
guess that it is around a -4. The Storck on the other hand has a
Maneuverability Rating of 4.5 and a stability of +5. their Caster angles
were 83+ and 79 respectively. Neither are close to the 81 degree caster
angle that I prefer.

I suspect that they were going for the ride comfort produced by the long
rake and the resulting longer wheelbase. Though you did not specify the
wheel size, I used 700 c for my calculations and there were some 650 b
wheels of sufficient section the exceed the diameter that I used for the
calculations. Of course, there will be some effect to the handling produced
by placing a load over the front axel as was common on many randonneur
bikes. This load, depending on the location relative to the axel (fore /
aft) could produce significant changes to the handling characteristics.

I believe that there is more technology available today and that for the
most part, we spend more time thinking about the effects of front end
geometry. I for one, would not build bikes with the geometry specified on
the Herse. But then I would not build a MTB with a suspension fork and 45 mm
of rake and less then a 73.5 degree head angle. I would prefer a suspension
fork with a rake in the range or 50 - 54 mm for better handling with the
72 - 72.5 degree head angles common on MTB's.

By the way, the Stability Index on the two bikes in the last paragraph were
2.6 for the 73 deg. head and 1.6 for the 73.5 degree head.

Hope this helps.

Bill Boston
www.billbostoncycles.com

----- Original Message -----
From: "Tony Rentschler" <w.rentschler(AT)mac.com>
To: <framebuilders(AT)phred.org>
Sent: Monday, March 17, 2003 7:20 AM
Subject: [Frame] Some trail questions


> I've been thinking about trail and steering ever since Tom Anhalt posed
> his question on March 10, and even more so lately because of two
> bicycles featured in the recent editions of Vintage Bicycle Quarterly
> and Cycling Plus.
>
> VBQ has a two-page layout of a 1948 Herse randonneur bike, with a 72.5
> head angle, 67mm fork offset, and 45mm of trail. C++ features a Storck
> road racer, with 73.5 head angle, 33mm offset, and 69mm of trail.
>
> Can anyone say how the handling of these two bikes might differ,
> especially with regard to the significant differences in trail? Also, I
> can't think of any modern touring bikes that would have as large a fork
> offset and short a trail as the Herse, though a 72.5 head angle is
> certainly not uncommon. Why did Herse, and many of the other randonneur
> bike builders, build with relatively large offsets and short trail?
>
> Is the Herse geometry "old fashioned," that is, has it been superseded
> by genuine improvements in handling brought about by a greater
> understanding of frame geometry, or did this range of offset and trail
> just go out of fashion? Would one want to build a bike like this
> nowadays?
>
> Is there any kind of rule or guideline that says fork offset should be
> less than the trail?
>
> What about "wheel flop?" I know that my bike with the 73 degree head
> angle, 43mm offset, and 58mm trail is much harder to keep running in a
> straight line if I'm pushing it by the seat than is my bike with 73.5
> head angle, 43mm offset, and 55mm trail. Is wheel flop a bad thing, or
> is it just a side effect of particular geometry? Is this "slow speed
> instability?" Is there such a thing?
>
> Whew!
>
> Tony Rentschler
> New York, NY
>
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