Monday, March 24, 2008


1924 'Model 5' vs. 1925 'Longstroke'

Since my 1925 Sunbeam Longstroke (Model 6) arrived two weeks ago, I've been curious to compare its character to that of James Johnson's 1924 Model 5. They're both sidevalvers from the mid-20's, with very similar running gear and mechanical configurations, from the same esteemed manufacturer; how different could they be?

The Longstroke was developed from Alec Bennett's 1922 TT-winning (at 58.31mph) machine, and was initially known as the 'Model 6'. The 'Longstroke' name was added for 1925, to what would have been the 'Sports' model in that year, but a 'TT Replica' in 1923. How quickly things changed in those critical years between 1923-25, where the Longstroke dropped in esteem from TT Replica, to a 'Sports' model in just 2 years. Sunbeam added an overhead-valve machine to its line in 1924, the Model 9 (and variants), which sounded the death knell to the sidevalve as a racing machine. Surprisingly, even with the real advantages of the ohv engine, racers continued to develop the sidevalve for racing at events other than the Isle of Man TT; Brooklands, European races, trials, hillclimbs, etc. In fact, although Bennett's win in '22 was the last for a sidevalver at the Island, they continued to be successful for many years in private hands. Take for example A.L. Loweth's record of 94mph on a Norton 16H at Brooklands, in 1934! Supposedly ten years after the model had become obsolete for speed work. Food for thought. I admit my own bias in thinking sidevalve machines couldn't be sporting, and would never satisfy a speed merchant such as myself. Gradually, while investigating Sunbeam and Norton racing history, I came to respect the humble flathead.

James purchased his '24 Model 5 from British Only Austria about two years ago, and has spent considerable time in his workshop, making the 84 year old Sunbeam reliable. Now he feels fully confident in its mechanical soundness; several long rides (including one 800 miler!) have borne out his conviction that his Sunbeam can be ridden as the maker intended. The biggest jobs he's had to tackle were rewinding the magneto and replacing a broken steering stem; otherwise it's been a matter of getting all the details functioning smoothly (cables lubed and adjusted, clutch working properly, brakes working, etc), which is really what 'sorting it out' means. It takes time to do those hundred small jobs in your off hours. That his bike runs so well is a testament to James' persistence.

By comparison, the Longstroke has just started down the road to 'sorted'. Noted in a previous blog are my efforts to replace hoses and taps, get the clutch and carb working normally, and make footrests. The bike's oiling is very curious for a total-loss setup, as there is no breather on the crankcase, but there IS an oil drain from the crankcase back to the oil pump - a semi-recirculating loop. The excess oil seems to be burned off, as the bike smokes a bit, even though the oil pump feed is turned well down.

I haven't found a top speed yet, but I would estimate in the high 70mph range. That's going some for a bike which has very little braking power; the front drum is essentially useless (both 'Beams can be pushed forward with it fully squeezed), and the back brake is only OK. James has relined his brakes, and suggests the rear should lock the wheel. Suspension movement from the Druid forks is minimal, and the springing is very stiff. But, for all that, it's a cracker! As it weighs only about 240lbs, it accelerates smartly, with strong engine pulses. The engine definitely has a long stroke at 105.5mm(x77mm), but it revs fairly freely, and thrives on higher rpm than might seem likely - it has plonk at low rpm, but there is a power surge at around 3500 rpm at which the engine smooths out, and she really starts to fly. The Longstroke engine feels slightly skittish and revvy, and surprisingly high strung for a 20's bike.

The handling is very stable at speed, although when stationary, the whole bike seems very wobbly. In first gear, the front end seems to 'fall into' corners, but as speed increases (I've seen around 60mph so far), cornering feels intuitive and takes less effort. The handlebars are brazed in place and very low, with no adjustment possible, and you must lean over the bike to reach the 'bars. Clearly, you mold yourself to this motorcycle, not the other way around.

The Model 5 has a completely different character; it's a true gentleman's machine, with a comfortable riding position and mellow traits. With footboards and high, pulled-back handlebars, you are seated in the classic British 'L' riding position. Where the gear selector on the Longstroke is stiff, the Model 5 shifts softly and easily (especially as the clutch releases fully). The power band is consistent and gradual, building speed with less drama than the Longstroke, yet never feeling sluggish, just mannerly. The engine is almost 'square' at 85x88mm, but the heavy flywheels keep it from feeling like a short-stroke! One might think it retrograde to add 20mm to the stroke for a racing machine, but as they won the TT with this new long-stroke engine, they knew what they were doing.

The handling on the '24 feels consistently smooth, with no change in feel from low to high speed; I wonder if the riding position has something to do with this? On the Longstroke, my weight - which is only 50lbs less than the motorcycle - is much further forward, shifting the bike's center of gravity towards the front wheel. The Druid forks have softer springs, giving a more comfortable ride. The engines have a slightly different head/barrel casting (seen in the photos), and I would surmise that the Longstroke manages a higher compression ratio (6:1?) than the Model 5 (5:1?). Carb size is the same on both, with a choke of 1". The earlier machine came fully equipped with acetylene lights front and rear (which work!), and a 'little oil bath' rear chaincase, a fully valanced front mudguard, a wider rear mudguard, and a luggage rack. James' bike is probably 20lbs heavier than mine, but I'm probably 10lbs heavier than James, so the weight difference is a wash.

At the end of our test ride (shootout!! - my nod to modern motorcycle publishing), I rolled out my 1928 TT90 Sunbeam for James to try, for a REAL contrast. The 3 years between my Longstroke and the '90' are a lightyear in performance- with the later bike feeling, as James noted, 'planted' and stable, with about twice the power of the earlier bike, and a four-speed gearbox to boot. 'We are probably the only people in North America to ride three Vintage Sunbeams in a day', said James, and he's probably right.

[Sadly, only one year later, James was killed riding his Sunbeam Model 5, after an under-inflated beaded-edge tire came off the rim on a sharp pothole, and he struck his head while wearing the 'pudding basin' helmet shown.  Read more here]


The Creeper said...

Great comparo, Paul! Except... where are the 1/4 miles times and the dyno results?! :-)

Anonymous said...

Its Saturday May3rd 2008, I am just about to get my 1926 M5 (could be a 6 ) for the first time since September last year. Ready for the 2008 VMCC relay rally tomorrow. Next booked official outing is the Banbury run - 600 'bikes this year , I'm #175 (I think !)
Great website , thanks for sharing, Alan