[CR] Nervex Pro or Wagner crowns

(Example: Framebuilding:Technology)

Date: Tue, 23 Mar 2010 01:41:37 +0000
From: "Norris Lockley" <nlockley73@googlemail.com>
To: classicrendezvous@bikelist.org
Subject: [CR] Nervex Pro or Wagner crowns

As a framebuilder I have , over the years, been taken to task on a number of occasions by customers who wanted me to build the forks to their frames using Nervex Pro fork crowns, and I have advised against their use, prefrring a cast of forged alternative,.

Having built a lot of forks and repaired others brought into the workshop, I am convinced that, strengthwise, the forged or cast crowns come out on top everytime. Wherever possible when using Nervex Pro lugs I would try to use a fairly blank cast crown that offered the opportunity of fretting and filing it to make it look as much like the Pro one as possible..

In my mind it was not so much the time needed to file and smarten up the pressed Nervex ones that put me off them so much as their lack of rigidity and overall strength. Apart from the large voids that existed between the inside edge of the fork blade and the outside edge of the steering column, there were cavities that existed at the top of the reinforcement tangs at the point where the steel was transformed from a the horizontal plane into the vertical one. These voids resembled those that are present in pressed lugs in the angle between the two tube sockets. I am sure that it would be possible to fill those voids but it would almost surely rely on the builder placing large slugs of bronze spelter inside the crowns when assembling the blades and column, and then relying on heating the whole contraption up sufficiently to ensure that the slugs melted..and that the large amount of molten braze material did not flow out.

Some Wagner cast or forged crowns had large areas of solid steel betwen the inside of the blade and the outside of the column ie spacers. One criticism of this type of crown is that it took more heat to get the whole mass to the melting point of the brazing alloy. However many astute builders would drill four 6mm holes very neatly, two in each area, to remove some of the mass without significantly reducing the strength.

The underside of the Wagner crown in Mark's photos shows that the manufacturer has in fact machined two large chamfers that slope from the inner face of the fork blade towards the column. These reduce the amount of heat input needed while still retaining support for the inner face of the blade ( an asset totally lacking in the pressed Nervex Pro crowns) and giving very adequate support to the fork column.

Just a comment on the simple Dubois crown and the Nervex Pro one...I think that they are one and the same stampings and pressings, with the latter's fancy curlicue just cut off in the stamping process.

Norris Lockley

Settle UK