This topic really is the stuff of a cycling club meet, during the wet
off-season, around a fire in the local public - house!
>From what I know of quite a few French frame-builders, constructeurs,
and bike manufacturers, this overlap factor is one of the most critical
ones that they consider when
designing a bike frame. The maxim that they adopt is to avoid overlap at
all costs, although I admit that on some massed produced bikes, given
the restrictive nature of certain lug angles, the cost of labour in
manipulating them to more suitable angles, some bikes do come out of the
factory with built-in overlap...but always as little as possible.. The
term Chasse Avant ie the front end centre length is all important.
However some manufacturers managed to overcome the restrictive nature of
lugs by adopting bronze-welded construction techniques, thereby allowing
themselves to vary the head and seat angles far more easily.
I was involved with the TVT company around the time when they started to produce TVTs instead of just supplying tubes to LOOK, and was impressed by the measures to which the designers went to produce frames with no overlap...these frames ...are essentially racing machines, remember, and we cannot expect a rider, when descending the Col de Galibier. to have to wonder where he should place his pedals in relation to the front wheel...he clearly has other more important considerations on his mind...
This was the philosophy adopted by the TVT team, as expressed to me by M. Guigneaud, the frame designer who is still, I believe, designing for the TIME company.Such was the company's preoccupation with this aspect of the frame's design..and the rider's safety, that when they came to design the two frames at the small end of the range, the 47 and the 48cms, and it became abundantly obvious that it wasn't possible to build frames of those sizes, with suitably short top-tubes, both 50cms C-C, and also avoid overlap..that the designers took the radical decision to use a 26" wheel at the front instead of a 700c. This decision also produced a frame with a longer head-tube..and a much better balanced handling bike altogether The front end C-to-C was 560mms.Nowhere in the TVT range is there a front end measurement less than 585mms, and that appears on the 49cms frame....and TVTs are reckoned to be something of a bench-mark in handling terms for a road-racing bike. All TVT frames were designed using combinations of just three angles. All head angles were 73, all seat angles up to the 54cms frame were 74.5, and over that were 73.5. The company also offered a custom service. I once ordered a 51cms frame with a 51 cms top-tube. After various attempts to design the frame, the company declined the order claiming that without shallowing off the head angle to an unacceptable angle or increasing the seat angle to a far too steep one in order to retain sufficient clearance at the front end it would not be possible to produce a frame to the dimensions asked for.
However I can't have been the only frame builder on the List who has been approached by customers seeking winter bikes built to the same design as their best road-racing bike. Almost every time that I undertook this task on small frames, particularly Franch and Italian ones,I would make certain desin assumptions but would end upI being amazed at the mixture of angles chosen by these continental builders. I recall 49 and 50cms frames with 75/76 seat angles, merged with 70/71 head angles...but very short top-tubes...but no overlap. I believe, however that Merckx used to build a stock 52cm frame with a 75 head angle..but there again he always used a long raked fork...but managed the mix without the front-end juddering or wobbling at speed.
Van Impe, the pint-sized Belgian ace climber, used a frame with a 50.5cms seat tube, and a 52.5 cms top-tube, the front-end clearance being 585mms...60mms trail..and no overlap. Zootemelk rode a 54.5cms square frame with 595mm front end and 60mm trial..and no overlap..Merckx rode a 59cms with 57cms top-tube, 600mm front end and..60 trail..and no overlap.
Some one once told me that it is possible to obtain a degree in cycle-frame design at some college or university in the States. Is that really the case?
Norris Lockley..Settle UK