Analysis and track test of Honda's 125 DOHC twin production racer
The Honda CR93 125cc production racer was perhaps the nicest racer ever offered for sale to the public. After nearly a decade the droning DOHC twins still make the leaderboard at the TT, yet never figure well in the speed trap hierarchy. The 125 twin is of classic degign with gear-driven double-overhead camshafts and four valves in each cylinder. The whole machine is of a small size that the overworked metaphor of "watch like" precision is justified. Though outclassed by the two-strokes now, the long long races on the Isle of Man still allow the magnificent reliability of this classic motor to win through.
A small batch of CR93s was produced with full road equipment, and these few machines are now to be found in the private collections of rich and enthusiastic Americans. There are a few machines that merit the description "classic" even though they are still active on the tracks today. The list is short but the entries are familiar. The Manx Nortons, the Matchless G50 and AJS 7R, the MZ 204, the Honda CR93 head this list and we intend to obtain track impressions of each and oll of them, treating them as modern racing machines and making no allowances for their elderly design and construction. One severe problem is that of locating such machines in original condition. I have been looking for a Vincent Shadow in original specification and god enough condition to ride fairly quickly for several years - with a complete lack of success. All the "original" machines that one locates swiftly display feet of clay in one modification after another emerges under the stress of verbal examination. Much the same situation exists for these old racing machines, and the number of original specification bikes is severely limited.
The first of these machines to become available was the Honda CR93 owned by Tom loughridge. The offer to try it asose at a chance meeting at the Spondon Enginering stand at the Show at the Horticultural Halls earlier this year. Late in April we met at Silverstone during a Southern Sixty Seven Racing Club practice day, and a fair fleet of two-strokes poured out of Tom's van, together with the little Honda.
Although it is a 125, the machine as a whole is fairly big. It is certainly heavy for a 125, and is long enough for men as large as Tom and myself to fit on to it in comfort. COmfort is no luxury on a long race. The merest ache in an ankle or wrist can become crippling after the first hour or so.
The high, wide and handsome riding position natually means that the CR93 is hardly the smallest of 125s. In fact it must be one of the biggest, and certainly there can be few heavier! A dry weight of 128kg (310lb to the unmetricated) is substantially in excess of that of a TD2 250 Yamaha. Whereas the Yamaha has 250cc and about 44bhp at 10.000rpm, the "little" Honda has around 10.5bhp at 11.500rpm (a figure that i see in the manual and beg leave to doubt after riding the machine, as even the Honda UK datasheet reders to gearing for 13.500). Natually the Honda is no ball of fire in accelleration: however, this does not mean that the top speed is low.
After a long run-in over 100mph is available, and certainly it must be easier to maintain such, a long-legged pace on the Isle of Man due to the smooth power delivery and reasonable power band. This is very much Tom Loughridge's experience: never does his CR93 show up in the first few (or even the first 10) 125cc speed trap figures, but nevertheless he gets on to the leaderboard within a lap. The other virtues required of a heavy 125 are god brakes and good handling. The Honda certainly has the brakes - which work extremely well - but the handling under Isle of Man conditions is not so good. At Silverstone, however, the CR93 handles well and also steers beautifully. The smooth surface of the airfield circuit does not really provoke strange antics, and the sheer weight of the machine tends to help a but in the island. The need to press on a bit plus the IoM roads is the combination that provokes complaint.
When considering long, long races such as these, the shere reliability of the Honda is a considerable asset. Last year this was proved to be true, but since this asticle was first drafted Tom Loughridge has run in the 1972 TT where he was forced to retire after about a lap with the engine, possibly crankshaft troubles. The origins of the CR93 were in the fleet CB92 road machine. This sharply styled little 125 was proved to be capable of over 80mph on the road a decade ago. It is worth remembering that, when fulsome praise is being bestowed on 80mph 125s of today: that old Honda really had speed. The CB92 was - and still is - the basis for quite a few racing 125s, privately converted by using Honda Racing Kits. Contrary to present day politics, once upon a time in the Golden Days Honda (UK) used to bring in Race Kits. SAdly these days are long past, and you can beg in vain for even a knobbly camshaft for your Production Racer these days... unless you go to France: or Sweden: or USA: or Singapore: or...
The CR93 was never a converted road machine. It was, indeed, based on the CB92, but racing development was carried out by Honda (Japan) and a surprising number of CR93s were produced - even a few in full road trim, with lights, stands and other road legal fitments.
The bare technical details of the CR93 are as follows:
- Length: 2 meter
- Width: 0,6 meter
- Wheelbase: 1,3 meter
- Ground clearance: 0,15 meter
- Dry weight: 128kg
- Bore: 43mm
- Stroke: 43mm
- Displacement: 124,8cc
- Compression ratio: 10,2:1
- Carburettors: Keihin RO20, 22P6
- Fuel capacity: 101 litres
- Oil sump: 1,1 litres
- Clutch: Dry, multiplate
- Gears: 5
- Gear ratio: 1st-1:2, 2nd-1:429, 3rd-1:227, 4th-1:083, 5th-1:0?
- Tyres: 2,50x18" front, 2.75x18" rear
- Ignition: Energy transfer
The preparations for running the CR93 are spelt out with great care in a data sheet prepared by Alf Briggs at Honda (UK). Als has for many years kept the CR93 and CR110 racers on the track by his valiant efforts in obtainting and issuing spares from a stock that is kept with (but not of), the rest of Honda's central stocks in the UK. This service is not just extended to those riders in the UK: his customers are worldwide. Suprisingly, after so many years there are stilll enough spares to keep several CRs running, and running well. Honda did indeed produce one of their excellent spares list for the machine, and with typical thoroughness even megaphone caps and machine stands have their own illustration and part numbers. The datasheet takes up where the factory left off, and embodies much of the experience gained by riders running CR93s in the UK. Remember Rod Scyvier's British Championship on a CR93?
Warming up the CR93 is done on soft plugs and is a fairly slow process: the howl of the megaphone is a delightful change from the two-stroke, and seems positively mellow after the head-splitting scream of massed two-strokes, to which we have become painfully accustomed in recent years. (Surely some clever engineer could find a way of getting all that wasted energy into the engine and onto the back wheel - and give quiet again?) Once the engine is warmed, the oil is changed and hard plugs put in. NGK C9H (warm) and 13/14H (hot) used to be suggested. In addition to this process, it is recommended that the oil be warmed before starting the machine up. Like most Hondas, the CR93 has a centrifugal oil filter and this must be cleaned out each time the oil is changed. The R20 grade is usual, with R30 for hotter times.
A compact and elegant device that does it job superbly
The standard CR93 change has been altered on Loughridge's machine to give the UK lefthand braking/righthand gearchange, in opposition to standard Japanese practice. The rear suspension units are now Girlings rather than the (unadjustable) Japanese units originally fitted. I got aboard after a briefing on rpm limits and gearchange pattern, and painfully pulled away in first gear. The gearchange was reasonably light, and had a short enough throw to satisfy my personal desires in that direction. The clutch is fry - fortunately, as one must slip it fairly severely to get of the mark. Once one was moving, the engine was quite prepared to try to pull you along at low rpm, but really anything below 7500rpm was a waste of time, as it took so long to build back up again to the power band. As the revs, rose there was no really sharp power delivery change, it just seemed to pick up and smoothly accelerate away as 8-9000 was reached. The engine is said to deliver full power at 11.500rpm, but i was asked to keep it "down to 12", which seems rather at variance with this. The power was showing no signs od tailing off much at that figure, and indeed that data sheet mentioned earlier recommends a red line of 13.500 - and adds that 14.500 is the absolute limit and "this will prove expensive if often used". I don't doubt it. The brakes were deightful: the twin-sided, four-shoe layout is the original "works" pattern, and is singularly effective. The bite is a but sudden, and their power is such that after braking for Becketts Hairpin on the first occasion is proved necessary to accelerate afain before reaching the corner itself; Down the straight, or should one say "up"at Silverston? - the straight is actually uphill for most of its lenght- the CR93 travelled rapidly and smoothly. The bulky fairing and long riding position made it possible for me to get well tucked in, and the controls all fell to hand without any contortions: it is obvious that Tom Loughridge has set the Honda up to fit him as he is a litle larher than I am, and certainly bigger than most 125 riders.
The frame is a heavy spine with the engine used as the lower member, unlike the CB92's pressed frame.
Handling through the fast Woodcote corner was reassuring, but there was some hint of pitching. The normal fork oil is 170cc of SAE5 Schokol (Castrol), but I am not sure what was in the forks on this occasion. Perhaps it could have been a but harder with advantage. The "ride" was comfortable too, so it is probable that both and read were set up softly. Like any small capacity racing machine car was necassary to keep the engine on the power band, but the sheer punch that the 125 Yamahas now deliver was quite absent. This general lack of power by present standards was demonstrated by the lap times around 1m29s-32s for the Honda compared with wht low 20s for the Sondel Yamaha under similar conditions. It is likely that the full use of the available rev. range would have cut a bit off the Honda's times, but the Sondel Yam was niot used to its rev. limits either. There was no doubt in my mind which machine I would rather ride for a very long race. The CR93 remains one of the rare machines; the mechanical elegance of this small precision jewel is very appealing. The cost of necessary spares id therefore far from as desireable, for just the some reasons.
A great deal of careful precision work is needed to set up a CR93 correctly. The tappet adjustment is carried out by stoning the cam followers "absolutlely square", to quote the sheet of words, and if the tappet clearance is excessive the contact face must either be built up with hard facing weld or the tappets replaced. The clearances are 6 thou with a tolerance of only plus or minus 1,5 thou, so this shows the care required. The piston ring gap is a minimum of 4 thou, and the piston clearance should not exceed 0.1 at the thrust face. Surprisingly there are three rings on the piston. The valve gear and the double overhead camshaft head are the central features of the machines. Drive is taken to the camshafts by a train of gears, and it is therefore necessary to adjust free clearances by using a range of different thickness od head gasket: more time, care, and checking for preperation.
A backlash of 0,6mm must be provided for the camshaft drive gears. The four valves per cylinder layout allows good breathing, but certainly multiplies the time required to get all set up correctly by a notable factor. Valve springs must be replaced if the free length has dropped below 32,4mm (outer) and 28mm (inner). Four vavles per cylinder, two cylinders, and two valve springs per valve indicated a lot of work is needed. The valves themselves should be changed after either season or a TT. Ignition is directly via the generator, and the usual camshaft-driven pair of points are platinum faced. Head tighning torque is 14-16lb/ft. It is interesting to note that an automatic advance/retard unit is fitted to this racing machine. The carburettors take a range od main jets, and 80, 82, 85, 88, 90 are the list of Honda specified settings. A very narrow rance, and indeed Honda riders have been known to het intermediate value made up for even finer mixture control. Pilot jet setting is 40, and the float height is 23mm. The latter is measured with inverted carburettor via the distance between top of the float and the float bowl setting.
The whole of the engine is littered with needle rollers and ball arcs for evert conveivable bearing. The camshafts run in ball races, the camdrive gears run on races, the crank mains and big ends are roller bearings, the gearbox runs on toller bearings throughout. An interesting ommision is the clutch, which does NOT have a ball race for the clutch throwout plate!
The CR93 has much in common with the Honda Juno, in that it is a complex, compact and elegant device that does its designed job superbly at the expense of reuiring expert and considerate treatment. A real classic of the track, and if only Japanese labour costs were lower we could hope to see CR versions of the more recent macines with a similar degree of refinement. Unfortunately, thise days seem to have gone, and the special conditions that allowed Honda to consider the production of the four-stroke Production Racers of this degree of refinement have now gone. We must content outselves with nostalgic looks. Our grateful thanks to Tom Loughridge for provinding us with the opportunity. M.R.W.
Source: Motorcycle Sport, October 1971, Pages 374, 375 and 376
#1 cam1240 2014-07-15 16:47
I have a CR93 and it is setup with only Battery and points. I need timing spec`s, as :firing and what degree btdc.
#2 pbvangeelen 2014-07-19 22:45
Unfortunately, I do not seem to have any documentation about this model yet. Although not everything has been uploaded yet, I also cannot find this model in my offline manual database.