MKII Chassis

The backbone of the bike is a Seeley MKII frame that has been constructed by Roger Titchmarsh;  A frame maker with over 20 years experience, and the only one with the authorization of Colin Seeley to replicate Seeley chassis.  The process of frame construction is identical to that of the original frames from Seeley, with a few minor updates.

Titchmarsh uses the original jigs from the Seeley frame shop.  The jig is made from channel steel which holds the frame components in an inverted orientation and locates all the tubes in the proper position so that they can be brazed together.  However, the entire jig can be rotated to ease the welding process.  This guarantees the geometry of the frame as well as the alignment of the wheels and suspension.

The frame is almost entirely made out of 17 gauge Reynolds 531 tubing with the exception of the head stock and the swing arm pivot mounting tube which are both CFS3 BK.  The end result is a frame that weighs 24lbs (10.9kg), including bearings, bushings, and chain adjusters.  This helps the full bike to come in at just under 300lbs.

The tubes are joined by hand Bronze welding with oxy-acetylene, including a gas-fluxer, and Sifbronze 101 Manganese Bronze filler.  The only exception being a butted and tube dowelled joint at the back on the main loop of the frame which uses a fusion weld and a mild steel filler rod.

The process starts with the head stock.  This is a thick 8 gauge CFS3 BK tube that is machined to accept a type 30205 taper roller bearing at either end for the forks to pivot on.  The head stock is also back bored in the middle section to reduce the tube to 16-18 gauge thickness which further reduces weight.

The main double loop of the frame is 1.125 inch o.d. 17 gauge 0531 tubing.  Each loop is constructed of two halves that are later joined at the back of the loop near the swing arm.  The tubes are CNC mandel bent by Pipecraft in the UK (a modern update to the original process), with minor bends in the front down tube finished off by hand.  The reason for breaking the construction into two halves is that it allows for more flexibility in mating the tubes to the head stock.  Preparing these complex joints by hand is critical to achieve a strong weld at the most stressed part of the frame.  The butt weld at the back is completed first, then the cross bracing, followed by the head stock, are Bronze welded in.  Care must be taken to distribute heat and minimize distortion.

The box section swing arm pivot mounts are made from a sheet of 16 gauge mild steel.  The single sheets are cut to shape and folded over to form a box section.  The box sections are then hand fit with a grinder and hand tools to the back of the main loop of the frame.  The Bronze welding starts off by tacking in the box section to the frame at each end, then welded all the way around on both sides where the box section mates against the frame.  Later on these plates are finished off by cutting a hole using a template and welding in mounting bosses for the actual swing arm pivot.

The front motor mounts are constructed of a bobbin sandwiched between two 2S 515 steel plates.  The main jig is used to locate the front mounts and after they have been fitted they are Bronze welded in place.  A separate jig is then attached to the front mounts and is used to locate the rear mounts which are also Bronze welded in.

The sub-frame is 3/4 inch o.d. 17 gauge 531 tubing (20 gauge may be used on light weight versions).  The seat and suspension mounts are fabricated first before the tubes are located with the jig and Bronze welded in place.  Cross bracing is added later on.

Another jig is used to make the swing arm.  All tubes are 1.375 o.d. 531, 16 gauge for the arms and 12 gauge for the cross tube (lighter gauge may be used on the arms to save weight).  The arms are CNC mandrel bent and milled for the axle.   Mating the arms to the cross tube is done by hand filing, and they are then Bronze welded together.  Additional bracing between the arms and cross tube is made from 16 gauge mild steel.   The box section brace is first constructed by fusion welding 3 plates together, which is then mated to the cross tube and arms, and Bronze welded.  The cross tube is bored to accept bronze bushes, in which hardened sleeves are inserted.  The whole swing arm assembly is then secured in the frame by a pivot mounting tube made from 3/4 inch o.d. 12 gauge CFS3 BK.  Once mounted the lugs for the lower shock mounts are added using a jig attachment.

The frame is removed from the jig and any final cross bracing, as well as mounting brackets/tabs, are added.

Note that there is an order to the construction that must be followed.  The concern is that any distortion due to heat take place before the critical attachment points of the frame are fixed into position thus avoiding any subsequent distortion.

The end result is a hand crafted frame that is really a work of art.  The skill and care taken in the brazing  really stands out.  We have chosen not to paint the frame for this reason, and its one of the best visual features of the motorcycle.  It is a bit more maintenance as the frame needs to be occasionally polished with Scotch Brite and WD40 (AC50 for you UK guys).  Paint would be like an overcoat on Betty Page…

There are more secrets to the fabrication process, but those are hard learned and closely held by the Titchmarsh family.  There is a legacy as Roger’s son Dan, an outstanding engineer in his own right, has taken up the business and is producing racing Honda frames in his own shop using a TIG welding process.

The chassis is topped off with a  Seeley MK2 racing petrol tank (approx 3 gallon) with breather, and 2 Pingel racing petcocks.  Oil tank is Seeley alloy with additional fitting for breather return.  Both of which are hand fabricated by John Woods and distributed by Minovation.

Fairing is a Kirby style 2-piece slimline racing fairing and screen with incorporated swallow-tail bellypan.

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