My Heavyweight 500cc Ariel Trials Iron

By Dave Blanchard

I learnt to ride motorcycles when I was about 13 years old, this was made possible because my Dad would let me have a go on his 1933 600cc Panther Redwing combination when a deserted minor road was safely available. But, as a young schoolboy my enthusiasm for motorcycles started many years before that first uneasy, but wonderful experience.

However, this story is about my Pre-65 trials bike that works hard for its keep.... just as any British Bike should. I do not believe in over polishing any of my old bikes, but prefer to substitute those cleaning hours for mechanical maintenance... plus riding. My Girder forked Ariel is not entirely original (unlike Trigger's Broom)....but is more than eighty per cent 1938 500cc single with some Red Hunter parts mixed in. The older and more respectful name for it would be a "Special"... but these days "Bitsa" is the commonly used term. However, "Special" assumes a degree of build skill is incorporated, and it is. Originally made on the production line at Ariel's in Selly Oak...Birmingham. In its later years was converted into a Vintage road racer... some of the setup being assisted by Owen Greenwood of 3 wheeler mini fame. Apparently an engine blow up finished its Vintage road racing days and it was then bought by Ray Blackwell who converted it into what he called his "Arbuthnot Trials Special". Ray had done some useful mods which included a shortened army frame with the engine bottom rails removed, plus standard Army front forks because they are longer and therefore gave more ground clearance... as does the Army frame. A small and beautifully made central oil tank was fitted just above the gearbox, this holds about 2 pints which is quite adequate. The engine has never overheated... even on LDT trials.

Ray also fitted a low compression HT piston and this proved a great help in softening the power delivery. He retained the standard Ariel rear wheel sprocket at 47 teeth but by changing to a 17 tooth engine sprocket and 16 tooth gearbox sprocket, would get the gearing low enough for old bike trials as they were set out in 1982. When Ray's arthritic wrists became too painful to pilot it on the rough it was sold to his good friend Dennis (Jim) Townsend....who being an ex Army man always kept the Ariel absolutely immaculate...unlike me. Jim had great fun riding this bike having several successes in Pre-65 trials events around the South and South Eastern centres. When Jim also gave up competitive off road events I narrowly missed the chance to buy this Old Warhorse. But! Less than one year later it was up for sale again from Alan Ward the latest owner, who lived in Brighton. I did not miss it the second time around!

My first competition on this Ariel was at the VMCC "Festival of 1000 bikes" back in 1994. During the event it was oiling spark plugs but I cured that a few weeks later with new valve guides and piston rings. Screw on canister oil filters are a really good idea so a 2CV one was fitted at the same time. Soon after this the monobloc carb was changed for a concentric, because with this type of carburettor the mixture strength is not affected by angles of lean. Unfortunately early Concentric's, seemed to be made of cream cheese, so scored slides and worn bodies were a natural way of life after just a fortnight or so of trials riding use... air filters are essential! The timing chest mod was also done whilst replacing a camshaft bush. This is where you plug up the oil drain hole back to the crankcase and drill one a bit higher. This allows the camshaft to dip into the raised oil level prolonging its life and giving silent running. Well as silent as an old Ariel can be!

Old Ariel's always steered well off road, but the girder forks took a bit of getting used to on the rough. The harshness of a rigid rear end was no problem to me as I had been competing on a 1949 AJS 18C for a few years previous to this. My next job would be to make and fit a prop stand for the odd occasion when you cannot find a tree to lean it against. The footrests were "old style" and in one solid heavy piece, so modern "knock back" rests were essential. When fitted they gave good grip to my boots, with the welcome addition of fold back safety.

Due to its past Vintage Racing heritage the inlet valve and port on this engine are quite large and the later (ingenious) wide single lobe Red Hunter camshaft is fitted. This is a fairly sporty combination so it did not run as slowly as I would have liked. Therefore to gain an increase in gas speed at very low revolutions I made an aluminium sleeve with a 1 inch bore and fitted this into the inlet tract. This was a great improvement! A 500cc engine generally isn't as good as a 350cc in muddy conditions and wheel spin arrives more easily. Modern trial's rubber goes a long way in controlling this but I would still run at anything down to 3 psi in the rear tyre in extremely slippery conditions. A soft tyre also has the "effect" of giving some rear suspension movement.

As pre-65 trials sections became harder I found I needed more ground clearance and lower gearing. So the 47 tooth rear wheel sprocket was changed for a 52 tooth one. Also I cobbled together a 16 tooth engine sprocket... retaining the cam-type shock absorber...but the primary chain was now kissing the cam boss a tad. That first gear was now too low, it was only fit for extremely slow "twiddly" bits through trees. So it was back on with Ray's original 17 tooth engine sprocket. I also did my part in shortening and raising the rear frame, by cutting bending and welding even more than Ray had... plus making front wheel spindle lugs an inch or so longer and getting work mate Andy Barnes to weld everything in place. The seat was also lowered more... in case I needed to foot through a difficult section. Later on, Andy would also help me to straighten the forged handlebar lugs after I had looped the bike on a difficult steep climb. Aircraft alloy "Renthal" handlebars are renowned for their strength, but not when an over 300 lb Ariel is bolted to them and landing from a height of about six foot or so! New ones have been needed now on two occasions after a "loop the loop"! I have looped it a third time too! But this time the handlebars were saved.... because it landed on its front wheel! So with a buckled rim and another competition looming drastic measures were needed! A wheel and spindle from a 1980s Yamaha DT400 was hastily adapted to fit. Strangest thing of all was the total weight of the "all aluminium" Japanese hub and rim! It was exactly 2 lb heavier than the old Army "all steel" front wheel that the Ariel factory fitted all those years ago! Yes it's true! British iron is lighter than Japanese aluminium!

Other mods along the way included a spacer under the very tired front fork spring, just to lift the front end a little more. An almost obligatory right angled twistgrip was fitted, so that those wayward branches would not snag the throttle cable and give full power when it wasn't needed. One inch was sawn off each handlebar end for clearance, only necessary because over the years mother nature makes trees grow closer together. Another particular and very useful mod was made quite by accident. Most of you will have heard of "stiction"? Well, it's when you have friction dampers fitted like my Ariel has and there is a reluctance to initially move because it has to overcome inertia. I over greased the front fork spindles on one occasion and the slippery stuff got onto the friction discs. This had the "beneficial" effect of reducing the "stiction", but you need to compensate by tightening the friction-adjusting wing-nut a little more to keep control of movement. It has been so successful in improving an ancient type of damping that I will never go back to dry friction discs.

In the hands of Ray Blackwell, Jim Townsend and myself, the Ariel Special has had class wins in most of the major Pre-65 trials throughout the South Eastern and Southern centres, with just the one that I know of in the Eastern centre. That was the "Thumpers Trial" in deep snow, with icicles hanging from trees, plus freezing fog to peer through. Proper trials weather as you can imagine! Plenty of runner up and first class positions also add to the bikes pedigree. On several occasions I have ridden in the championship rounds for modern Long Distance Trials in the South Eastern centre. These had over a hundred entries and could be 120 miles in duration (alternatively, 90 miles if you didn't want to compete for an award). The riders are mainly on modern Enduro and Trail bikes....for comfort and speed. My best position to date is 17th overall, just 2 places shy of a championship point. Whilst queuing for a section in one of these LDT events a chap on a brand new Enduro bike approached me and said, "Wouldn't you like to have a nice new modern bike like I've got"? "Oh yes!" I exclaimed. "But I haven't worn this one out yet"! He looked bemused and did not reply, then strode off to talk to some other more "trendy" and considerably younger riders than me.

Trials bike development is an ongoing thing and there are some improvements I would like to make. The Red Hunter camshaft on my 500 is a bit lively but the Army WNG cam is so tame that you lose about 1500 revolutions at the top end. This is no good for modern Pre-65 trials, so something in between needs to be cobbled together. I have a suspicion that the flywheel balance factor could be improved for low revolutions to give more "plonkability" but don't know yet what it should be. The carburettor is quite worn now but will have to stay, because to keep cream cheese efficient you would need a new one every month or so. Thankfully a slimmer air filter will eventually stop my right knee from being bruised, when I find the time to make one! One major improvement that is slowly ongoing is to replace the Burman CP gearbox with the later GB type. The current CP is an ex Army one with a big jump from first to second. Army Convoy ratios are not ideal for today's events! I have now obtained the very rare trials pinion pairs for my spare GB gearbox. When these are fitted I will have a much more friendly and efficient set of ratios to play with in sections. Of course the fitting of a standard road ratio box with sprockets changed to suit is fine as long as you do not want to do any long distance events. But! I really do need to keep a high top gear for when I am clocking up the miles in between the sections during this type of trial.

The bottom end of the Ariel's engine is also very worn, with a death rattle that sounds like a Dinosaur with Bronchitis. The crankshaft that has enough vertical lift to measure with a wooden rule! The con- rod has a twisting movement in a side to side plane where it shouldn't have any. A worn big end is obvious! When speaking to Ray Blackwell (the originator of this "Arbuthnot trials Special") at the VMCC West Kent International several years ago, I mentioned that it could do with a bottom end overhaul. His reply was... "Don't fix it if it's not completely broke... just keep on riding". He was so right back then...good advice at the time. But I now wisely retard the ignition lever a lot... when flat out in top gear at speeds a shade over 35mph. This softens and quietens the mechanical noises and prevents the con-rod escaping from those ancient Pre-War crankcases.

It has been said before... "A happy marriage is the blissful union of a Motorcycle and its Rider". But consummation and regular conjugal rights are essential!