Foam com­po­si­tions and wet bums!

Classic Motorcycle Mechanics - - WORKSHOP -

The nat­u­ral rub­ber (NR) la­tex foam re­moved from the RD350 seat pan in the­ory shouldn’t have been there. Around 1973 the Ja­panese mo­tor­cy­cle in­dus­try moved away from what’s known as NR la­tex to syn­thetic foams. The NR ma­te­rial was ac­tu­ally only around 60% gen­uine rub­ber with the re­main­der made up of var­i­ous or­ganic zinc and potas­sium com­pounds that were en­vi­ron­men­tally big no-nos. The other down­side is that NR based foams have an open cell type struc­ture which, via cap­il­lary ac­tion, al­low water to run all the way through act­ing like a sponge. This is why many older Ja­panese seat bases are rot­ted through. My orig­i­nal seat foam must have been one of the last NR types be­fore the tran­si­tion to polyurethane or PU which is a to­tally syn­thetic ma­te­rial. Pretty much unique in the or­ganic chem­istry world, PUS can be made to do any job from cush­ion and seats through to shoe sole, elec­tri­cal in­su­la­tion and even ar­ti­fi­cial heart valves. For our ap­pli­ca­tion it’s the com­press­ible na­ture of PUS that makes them so ap­peal­ing. By care­ful chem­i­cal ma­nip­u­la­tion vary­ing sorts of foam can be made but it gets even bet­ter. Smart guys like Phil Turner at P&K Seat­ing can in­ject a given grade of PU into a sealed mould care­fully gaug­ing just how much to ‘over in­ject’. By adding pre­cisely the cor­rect vol­ume the size of the air bub­bles is con­trolled which in turn dic­tates the level of com­press­ibil­ity and thereby com­fort. Smart eh? The closed cell struc­ture of Pu-based foams also re­sists water ab­sorp­tion and trans­fer mean­ing you’re not sit­ting on a large wet sponge.

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