Hi Norris , I had to build a very small frame for a lady customer whilst I was at Thorn.......this frame was for 26" atb wheels and was designed by Thorn's frame designer......which wasn't me!......its a long story but they already had a...slight pause and cough.... 'frame designer', so my knowledge was not sought...anyway, this frame had a 68 degree head angle with I think a 60mm rake........needless to say the lady brought the frame back, got a refund and I was instructed to destroy the frame!
You mentioned Merckx riding a 74 or even 75 head angle with a rake of 50 to 55mm....this would seem to be a miss-match, as I'm sure you know that the formula between head angle and rake would suggest that a steeper head angle would require a shorter fork rake, and vice versa.....but sometimes the odd variants can work, on my 1950s Rotrax Concours that I virtually rebuilt in the early 90s now has got a 73.5 head angle......with a 30mm rake!........This came about because the original round forks with the lovely faux twin plate crown was made for 27x 1-1/4" wheels, so to reduce the clearance I had to shorten the rake, fitting new dropouts....incidently this is the only remaining bit of work I have left from my days at Bob Jackson's having first altered the frame [which started life as a track frame] in 1973 for when I used to ride the grass track at Roundhay park.
The Rotrax, now built as a 'Audax' type bike with Nuovo Record rides fine, and is a possible choice for the 'Le Eroica' if I ever get round to doing it.
cheers
Kevin Sayles
Otley
West Yorkshire UK
> The Sam Braxton bike recently for sale on Ebay and discussed in some depth
> on the List really did generate some interesting data and theories both
> about Sam's frame in particular and frames in general..
>
> I think that it was generally agreed that Sam preferred shallow
> head-angles,
> particularly for touring bikes, while there was some opinion expressed
> about
> such angles causing steering shimmy and wobble. Evidently Sam often sued
> head angles of between 70 and 71 degrees..
>
> Fotunately I have only ever once experienced bad shimmying..and that was
> on
> a mid-range Bernard Dangre - Starnord ( Paris Sport ) bike that could have
> been categorised as 10sp sports/type Course. The fault was put down to a
> pretty steep head angle and a long abrupt rake, on a 22" frame. Some how I
> started to associate such frame design with bad steering until I read that
> some of Eddy Merckx's top road frames of a similar size had head angles of
> 74 and sometimes 75 with rakes of about 50 to 55mm ; I had never heard of
> a
> Merckx frame suffering from bad steering, but have never ridden one of his
> frames, so I cannot comment.
>
> Recently I have been trying to buy several more ex-Pro rider frames,
> including one that is reckoned to be one of Lucien Van Impe's
> Tour-de-France
> machines, from his days at Lejeune in 1977 More likely than not it is one
> built by Bernard Carre.
>
> Van Impe is quite short, around my size.-one reason why I am particularly
> interested in obtaining this frame - and rode a 51 cm frame measured
> centre-to-centre, with a 52.5 cms top-tube, 41cms rear triangle and 59cms
> front end clearance. These measurements were manipulated by altering
> accepted angles to provide a frame that had the same measurements front to
> back as Luis Ocana's 57.4cms. Gazelle . and only 1.5 cms shorter than
> Gimondi's 57.7cms BIanchi whicj=h boasted a 58.5 cms !!! top-tube...No
> mean
> feat..
>
> The seat angles on these frames were 74.10 for Van Impe, 74.33 for Ocana
> and
> 73.40 for Gimondi...whereas the head angles were respectively *68.4,*
> 73.24
> and 75.5..
>
> As I read these measurements, particularly the head angle on Van Impe's
> frame, I had in my mind's eye, the shallow head angle on the Sam
> Braxton..which was presumably over 70 degrees.
>
> I then remembered attempting to copy a Vitus 979 frame for a customer when
> building a winter frame, and rememberd that these frames had interesting
> angles. However for a 51 cms VITUS the seat tube angle would be 75
> degrees,
> the head angle a respectable 72 degrees, and the top-tube at 53cms just
> half
> a centimetre longer than Van Impe's I cannot remember ever building a
> frame
> with a head angle less than 71 degrees and that was on a touring machine,
> and consequently I am really puzzled about the claims made for Van Impe's
> Lejeune. Would there have been much shimmy on the front fork which had a
> rake of 35mm and a trail of 9.68 cms.?
>
> Has anyone got any good photos of Van Impe riding his late 70s LEJEUNE
> that
> they could post please?
>
> Norris Lockley
>
> Settle UK