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True Grit

by Tim Bunting

Last year I repaired the moderately worn carborundum roller on a 1969 Solex 3800, building up the profile with Araldite and creating a smooth surface over approximately half the area.  The roller, which had worked well in its worn state, continued to grip the tyre on dry roads.  What with one thing and another, I did not get round to testing the repair in the rain.  Its first experience on wet roads occurred early in a long run last October - slippage was severe.  I decided to make roller slip a thing of the past.

To determine the extent of the problem, other possible causes of slip must be eliminated - brakes, tyre condition and pressure, the roller-to-wheel set up.  On a Solex the weight of the pivoted engine, plus a pair of springs in tension, pull roller to tyre.  Sliding mechanisms take care of lateral rigidity and rebound resistance.  Dirt or lack of lubrication on these sliders can cause excessive friction, hence roller slip.  Every type of roller-driven engine needs to be checked in this department.

Doubt about the Araldite repair had not prevented me from building up some badly worn detached rollers, one of them a deep U in profile, till they resembled marzipan-coated confections.  I came in for the usual head-shaking from the family.

What I now sought was carborundum grit.  My dictionary defines carborundum as "silicon carbide ... carbon + corundum".  Corundum is defined in part as "mineral species of crystallised alumina".  What I was offered - are you awake readers? - was a variety of aluminium oxide grits from the company I tracked down, namely DGS Abrasives Ltd, 189-191 New Road, Portsmouth, PO2 7QU, tel 01705 661481, fax 01705 673476.  Mr Colin Scarrott is the man to speak to.  After a couple of 'misses', the grit he recommended seems to be the very stuff that Solex (and Berini) rollers are made of, the grit being bonded by a resin compound such as ... Araldite.  This grit is type UV, a virgin (ie: not recycled) material, black in appearance and produced in different grades, each passed through several meshes to achieve accuracy in this respect.  Inquiry brought the choice down to 24UV (0.028" dia) and 36UV (0.020" dia).  Half a kilogramme of UV costs £7.23 including p&p and VAT.  (Araldite costs a fortune too.)  A 35mm film container holds about 75g of grit, enough for a roller worn to the bone.  Therefore, I suggest you ask Mr S for a small amount, maybe trying both grades mentioned - I now prefer the finer - or share ½kg with a number of other owners, as gardeners traditionally do with seeds.  Or sell some on.

How do you go about repairing a roller in situ?  Remove the engine from the frame - a practised five minute task with a Solex - and lay it roller upwards on the wife's softest sweater.  Scrape away muck from the surrounds and continue with detergent and water worked hard in all directions over the roller with a small nylon brush.  Then rinse, still brushing, rub down and leave to the air.  De-rust and protect any metal parts.  The roller must be really clean and dry.  A roller detached from the engine is easier to work on but this consideration hardly merits stripping down half an engine.

There may be other adhesives suitable for roller repair, but I stick (sorry) to Araldite, using 'Standard'; 'Rapid' might go off too quickly in this application.  Put a small square of shiny card in an old plate - the plate retains loose grit.  Mix less resin and hardener on the card than you think you will need.  The tool I favour for this and many tasks is the steel stiffener from a car's wiper blade.  Add as much grit as the Araldite - pour back excess grit at once; this is a small scale operation!  Stir until the colour of the Araldite is gone, yet the mixture remains moist - this is crucial.  Then build up the roller towards its original profile, pressing down the mixture with a damp finger or scrap of damp cloth.  Don't wet your finger on your tongue.  Rub in a small quantity of dry grit until the surface is matt.  Roll an in situ roller with a piece of damp wood, applying pressure.  Roll a detached roller on a piece of damp wood, applying pressure.  Scrape off obvious high spots if they occur.  There is no need to hurry.  Make up more mixture and repeat the process as often as necessary, feathering towards the edges.  Check the profile of the roller against a light background until the profile is restored, turning the roller continuously.  Then stand on the other side of an in situ roller; turn a detached roller end-over-end, the object being to check profile from the opposite side.  Clean your fingers whenever you wish - xylene thinners then soap are effective.  Wipe of the rest of the roller and any other parts contaminated by the mixture.  When satisfied, put the work aside to set, preferably in a warm place - not a hot one because even this mixture can creep initially.

Next day, file the repair to check for high spots.  You might try fitting a detached roller to a drill, then offer a file to its revolving surface, as if to a grindstone.  But as one who meets his own ignorance round every corner, I prefer to travel the low-tech route wherever possible.  Now is the time for you and your file to regret any sizeable high spots. Depressions can be filled as before.  Rub the finished roller with a damp cloth.  The roller from a new Solex has a surface that is smooth but not shiny.  Your repair will not be shiny if you have proceeded as above.  Leave the repair two or three days to cure.  Hide the remains of the sweater.

My slipping roller received the treatment described.  Again too impatient to await testing, and ignoring the family head-shaking, I repeated the technique on the previously mentioned badly-worn rollers; first, of course, removing the marzipan coatings with a favourite 6mm chisel, which did it no end of harm.

To date the repair has remained intact.  Much of the Box Hill route in February was ridden in the rain.  The roller slipped no more.  I hope to put up a good mileage before the end of August and shall report back if anything goes wrong.  However, there can't be anything new in all this.  The Belgian 'Solex Appeal' for Jan-Mar '95 carried an ad for roller repairs.  Whatever you do, keep old rollers!  I believe the all-grit ones could be home-made from scratch, given the dimensions.

The idea occurs that all-metal rollers, with their propensity to slip, might benefit from a coating of grit.

Roller drive, especially where rollers are turned at crankshaft speed, has a marvellous simplicity.  Efficiencies are said to be low, but the system works!  I calculate that in the course of one hundred miles a 3800's 42mm dia roller rotates 1,236,293 times; the 38mm dia roller of a German 3800 rotates 1,333,895 times; the standard 19" tyre rotates 89,239 times, a Micron's 12½" tyre rotates 173,589 times.  These awesome figures are without slip.  A roller that does slip will wear faster and so will its tyre; its engine will rev faster and longer and consume more fuel; the rider will pedal harder and longer and arrive unhappy for the experience.

The true grit of the title is what it takes to get the job of slip prevention under way.

First published - April 1997

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