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The Yamaha DT175MX

Mono-Shock for the Masses

Yamaha Pioneered the Mono-Shock rear suspension arrangement in International Scrambling from the middle of the 1973 season. By 1975, the system was offered to the buying public on the YZ 'production' Scramblers, and was on the 'streets'  for 1977 fitted to the DT250 & 400MX models; but it was the DT175, that with a claimed 16.5bhp, only half a horsepower less than the DT250, only 2/3 the weight, and a lot cheaper to both buy and run, that made it available, and showed it's advantages to the fullest, leading off-road motorcycling in the direction it has subsequently gone.

In 1973, Yamaha fielded a proto-type YZ250 scrambler in the world MX Grand-Prix championship, ridden by Dutch rider, Hakan Andersson, who used it to great effect, winning that years title, Yamaha's first MX GP Championship.

Yamaha and Suzuki were competing very strongly for competition dirt bike honours at the time, and lucrative American market sales on the back of them, But Suzuki were the top Dogs in world Championship Scrambles.

Through the 1960's, the four stroke BSA B40 with legendary rider Jeff Smith, and others had, at least in the 'big legue' 500cc championship managed to stave off the insurgence of the two strokes, which has started to dominate the 250 class. Bikes like the British Greeves and Austrian Husqvarna, and Czechoslovakian CZ had started putting their names into the record books, as the high-performance two stroke was evolved from the 'cheap' utiliterian commuters of the 1940's, and grew in capacity from 125 to 150, to 175 and soon full 250's, from many many European makers, including the Spanish Montesa & Bultaco concerns. But, by the late '60's bored even further, the two strokes were starting to challenge the big-boys on their 'Thumpers'.

In the USA, the Japanese were making bigger 'impact' in the world of off-road ring, and it wasn't until 1969, Swede, Olle Petterson put Japanese bike, a Suzuki in a podium position in the 250 World Championship. 1970, Suzuki returned to take the premier position in the 250 championship, and again in 1971, and again in 1972, re-inforcing their 'supremacy' taking 500 titles in 71 and 72.

Both Suzuki and Yamaha were shown the prototype cantilever mono-shock rear suspension system, on Dutch innovators home built machine, and offered the rights to it. Suzuki though, were rather slow to sign contracts, and Yamaha exploited their tardiness to seal a deal, and get on and develop the bike; taking Suzuki's chance of double crown 'hatrick' away from them, snagging the 250 title for 73.

And the company were quick to exploit this success, and take mono-shock technology 'to the road' as fast as they could to cash in; because their range of 'Street-Scramblers' were, rather 'lack-luster'. So, what diod the press think of the DT175MX at launch?

MCM Road Test - Yamaha DT175MX

People who know about trail bikes are buying the new Yamaha DT175MX. Its BT predecessor bore the connotation "Enduro" on the sidepanels. So does this one. Only it means it a lot more seriously.

A complete and utter revamp the DT175MX owes much of its heritage to Yamaha's lightweight YZ Scrambles range. The result is a fabulous off-road mount which makes the novice feel at home on the rough and is good enough to score a gold medal in the Welsh Two Day Trial. London dealer Dave Rayner proved it.

Just looking at the new bike tells you it has purpose in its design. The seat height is tall for a 175. It has to be becouse of the long travel suspension which gives just over seven inches at the forks at 5 3/4 rear wheel travel.

Also immediately noticeable is the ground clerance of a shade under 10 1/2 in.

The fuel tank has a screw-on cap so that it stays on the tank, and rubber tube breather to Prevent Petrol loss.

The forks feature protective gaiters. The rear indicators are flexible. At the front they are mounted wiihin the braced area of the handlebars to obviate crash damage.

The swinging arm carries a chain guide, tensioner and plastic bush at the pivot area to prevent the chain from scoring marks into the metal. Chain adjustment is by snail cams. The front and rear steel rims each have a security bolt to halt tyre creep. The headlight has a chrome guard. The throttle cable is routed skywards to keep it tucked neatly out of the way. The engine, tastefully finished in silk black, has a radially finned head for good heat dissipation with rubber inserts to cut down on noise resonance. The exhaust routes neatly out of the way and the overall appearance is sharp and functional. It looks a winner before you even sit on it.

MCM Road Test - Yamaha DT175MXBut before you get that far a couple of items make one ponder with slightly less enthusiasm.

The engine covers are held by Phillips screws instead of Allen screws usually found on Yamahas. That headlight grille is rather flimsy, the frame welding looks unattractive and the choice of white is rather suspect. Once the plastic guards and sidepanels have been scuffed and scarred dirt seems to get ingrained and white shows it up to its best, or worst, advantage. These are observations, not criticisms.

Sitting on the bike confirms its competition looks. The seat is deep and firm. lt comes with a strap as standard, but is better without it to allow the rider to shift his position without snagging. A bird's eye view reveals that only the black handlebars stick out.

The high ridlng position makes the bike feel more like a 250, or bigger, lt all spells out an invitation to the rough.

The reed valve engine accompanied by a whiff of blue smoke from the spark arrester sounds crisp. First gear engages with a slight clunk and the front wheel hovers slightly if a fistful of throttle accompanies engaging the clutch.

MCM Road Test - Yamaha DT175MXChanging up through the close ratio six speed box seems to need only as many seconds to execute. The suspension feels soft. When the front brake is pulled on not only do the forks dive, the tail end lifts as the De carbon monoshock spring expands.

These are the initial impressions gained from a first encounter with the new model.

The first job with the bike was to put it through its tarmac paces at the test track, with 750 miles on the clock it pulled 72.5mph with lhe rev counter needle just into the red zone which starts at 8,ooorpm.Trying a run sitting upright, because that is how most of these will be ridden, knocked only 2mph off the top end.

Checking the speedometer proved that it was remarkably accurate with a spot on reading at 30mph. At 60mph indicated it was doing 59, and at 70 indicated 68.5. In addition the trip meter can be reset by one-tenth of a mile increments, an important point for enduro riders. The rev counter was checked later in the test and proved to be approximately 200rpm fast throughout the range.

After the test track session the bike was ridden on a 100 mile road route. lt felt as light as puff pastry. Everything about it had a light easily manageable feel. Only the gear. box required more than an automatic reaction.

The ratios could be selected with or without the clutch, but sometimes, firm pressure was needed on the lever to make the selection work.

Obviously for road work the range of the 1 1/2 gallon tank is limited. This ride yielded 67mpg, the best of the test and more than I expected. Later I rode a tankful and not exceeding 55mph and this gave the next best figure of 66. The range to reserve on the road was always over 76 miles.

MCM Road Test - Yamaha DT175MXThe riding position was comfortable though the firm seat did feel a little hard after 30 miles or so. It is obviously designed to cushion the rider when off-road and therefore has to be firm.

Cruising at between 50 & 60mph seemed happiest for the willing two-stroke and surprisingly sixth gear could pull well from 20mph. I had expected the motor to be less flexible.

The power band defined itself between 5,500 & 7,500rpm. Yet below that figure in each gear there was good pulling power which proved to be invaluable when later fighting for grip in muddy off-road situations.

Vibration was apparent though not disturbing. The Yokahama trail tyres on the road felt as good as any other trial tyres and the brakes were progressive and efficient.

Obviously not designed as a two up machine the bike can be used for occasional two-up use. The pillion has to put up with a shortage of seat space and footrests mounted on the swing arm.

MCM Road Test - Yamaha DT175MXThe bike's six volt battery powers its parking lights and indicators. Main and Dip beams are direct but retain constant intensity until the revs are dropped to tickover. Illumination is adequate.

To replenish the automatic lubrication system the left side panel has to be pulled off and a wing nut undone which allows the plastic oil container to swing out. On Yamaha's RS125 road bike the container swings well clear for easy refilling. On the DT it can only be pulled an inch or two from its normal position making it impossible to get every last drop out of the half litre bottles sold on garage forecourts.

To start the bike the cold start button on the side of the 24mm carburettor has to be pulled out. The instant the engine fires the button can be pushed home. The engine always felt that little bit sharper first thing in the morning. Once it was warm the crisp edge seemed to fade slightly, though it was barely perceptible.

Since the bike has such sporting intensions it was decided to enter it in a practice enduro event run by North Cheshire Trials Bike Club. Preparing the bike involved removing various parts such as indicators, rear foot-rests, rev counter and rear view mirror.

MCM Road Test - Yamaha DT175MXSince the event was run entirely on private land I was also allowed to remove the rear light and number plate. Fitting different tyres was the next job since the Yokahama's knobs did not look as if they would give sufficient grip, particularly iof the going was muddy. Avon agreed to supply a pair of Mudplugger trials tyres via the Kettering Northants depot of the Central Tyre Company. These covers reputedly can be run down to 2psi.

Going up from a 3.50 to a 4.00 x 18 rear tyre showed there was just enough clearance between the tyre and subframe. But for owners who wish to fit scramble tyres I would say that a 4.00 section would definitely foul, though low profile cover may just clear. One point I discovered after the event was that a screw holding the chain guard had cut a small groove in one run of the tyre knobs. A trial run a day before the event had not revealed this. Hacksawing the screw flush with the plastic guard would have been the obviouse solution.

Fine weather greeted the event which turned out to be over 42 miles during which the bike returned 32mpog, the lowest figure of the the test. Only the bottom four ratios were used and becouse the going for the most part was dry the acceleration was used to the full. I once had a pre-monoshock DT250 Yamaha down to 25mpg so 32 does not sound quite so bad in that context.

Using thus bike hard immedietly showed up points which had not materialised on the road. Under heavy braking the front forks could be completely compressed to hit the inside of the mudguard. There was never any metalic clunk to say the forks had actually bottomed. But if the bike were mine I would experiment with different fork oil to give slightly tauter damping.

With the front brake on and weight transfer forward the rear wheel was easy to lock up. The rear brakes on Yamaha dirt bikes have always appeared to have this trait.

MCM Road Test - Yamaha DT175MXIf the engine stalled the motor could quickly be fired again becouse of the primary kick system which allows restarting in any gear. Just as well becouse fiunding neutral could be quite difficult.

Using the gearbox hard for downchanges without the clutch could result in smooth noiseless changes only if the revs were right.

Several times while atempting to change up from second to third the selection baulked and a fresh birst of speed was necessary in second before going into third again.

Having thought the power to be quite fair on the road there seemed to be a definite lack of it in competition. This was highlighted in the ratio jump from second to third. The specification chart shows this to be the biggest ratio span apart from first to second. On the road it was unnoticed. On a modest gradient in the event the difference was too much. Either the motor was pushed right through second to the point where the power dived dramatically, as the bhp graph illustrates, or third gear had to be selected and it was a matter of waiting for the revs to build up.

Handling was first class. The amount of complient suspension gave an excellent ride accompanied by the hiss or air each time the gaitered forks were compressed.

MCM Road Test - Yamaha DT175MXThe rear mono-shock is adjustable to seven possitions. From the word go it felt right for my ten stones of person and riding gear so I never adjusted it. A hevier person would obviousely need more pre-load.

To make adjustments it is necessary to remove the two bolts which hold the seat, adjust the suspension, bolt the seat back on and try it again.

Removingh the lightweight seat, which also forms a section of the rear mudguard, to examine the nitrogen/oil monoshock I found that only slight thumb pressure on the frame was necessary to make the spring move. It was smooth, well damped and most impressive.

The suspension, although on the soft side, always allowed the rider to feel in contact with the ground. When the back wheel did break traction on a corner it drifted in a controlled manner and the bike always felt stable. Gripping the tank with the knees tightened up this stable feeling even more.

A Key factor in the overall feel is the lightness of the machine. I was out of condition for the event yet the bike was so easy to handle and all the controls so light that it was not physically taxing. Only towards the end of the event did the throttle become slightly harder to operate due to tiring hand muscles.

I came away from the event thinking that if I owned such a machine the first thing I would do would be tune it for more power. The rest could be left.

MCM Road Test - Yamaha DT175MXBut for ordinary trail riding there is no need. Later the bike was taken to the Warley Sports Centre in Warley, Essex where the Essex Enduro Club hold events. Thier practice ground was muddy from heavy rain.

The standard tyres had been put back on and were quick to clog with mud. The problem in this situation was grip, not power. As mentioned earlier the soft power below the power band was a definite advantage for finding traction.

Sliding off and stalling in these conditions proved the worth of the flexible indicators and neat profile of the machine. Nothing stuck out and nothing got cought or damaged.

But by now some signs of use were evident. One of the "Enduro 175" transfers had come off a sidepanel and the kickstarter needed to be greased at its swivil point. The chain did not need tightening thanks to the tensioner, though frequent lubrication was obviousely necessary.

MCM Road Test - Yamaha DT175MXWhen the quickly detachable rear wheel had to come out for tyre changes it was a matter of undoing the brake rod wingnut, loosening the spindle and removing two verticle pins at the ends of the swinging arm. With the chain taken off teh sproket the wheel complete with spindle could be withdrawn. The worst problem is balencing the bike which does not have a mainstand.

When refitting the wheel putting the split pins back into the vertical pins is a fiddle.

As bonus concessions for road use the bike features a helmet lock and the front brake operates the brake light. The instruments feature a neutral warning light and red oil warning light which also comes on when the engine is in neutral. There is still a generouse supply of oil when the light begins to flicker. The tool kit is partly covered by the right panel. Getting the tools back inside thioer small plastic compartment is almost impossible.

The frame design is based on the 1977 YZ125 moto crosser. For riders who want to experiment more than with the monosocks seven preload positions the heavier spring from the DT250 and 400 models will fit.

CDI Ignition is employed to fire the offset combustion chamber. The head is fixed to the barrel by six studs while the barel is held to tyhe crank case by four studs. Again this method has been proved on the YZ models and is said to prevent head deformation.

An interesting point is that the left engine side case and the oil pump cover are made of plastic and not aluminium to save even more weight.

The method of clutch adjustment and gear selection is copied from last years YZ125. There are a host of other smaller improvements over the previouse DT model such as an aluminium top fork yoke instead of steel, bigger capacity battery and a leaf ring on the throttle to make it smoother.

It all adds up to a small white charger with enough enthusiasm to charm you away from the street to explore our ancient roads, your riding ability and its perfection.

PERFORMANCE & SPECIFICATION

MCM Road Test - Yamaha DT175MXENGINE
Single cylinder seven port air-cooled two-stroke with reed valve induction and 24mm Mikuni slide carburettor. Capacitor discharge ignition powered by flywheel magneto. Battery: 6v 6Ah. Headlight bulb 35/35W
Displacement...............................................................................................171cc
bore & stroke.........................................................................................66x50mm
compression ratio..........................................................................................6.8:1
claimed output..........................................................................15bhp at 7,000rpm

TRANSMISSION
gear primary drive to six speed gearbox via wet multiplate clutch. Final Drive by chain.
Primary reduction.........................................................................................3.227
final reduction..............................................................................................3.267
gearbox ratio's: ..................................3.500, 2.214, 1.556, 1.190. 0.957 and 0.800

CHASSIS
Single down tube frame with cantilever rear suspension featuring seven adjustments for spring pre-load.
Front tyre............. .................................................................2.75 x 21 Yokahama
rear tyre ................................................................................3.50 x 18 Yokahama
wheel base..................................................................................................53.1in
Overall length..............................................................................................81.9in
overall width...............................................................................................33.7in
castor.........................................................................................................60deg
trail................................................................................................................5in
claimed dry weight........................................................................................216lb
fuel tank capacity.........................................................................................1.5gal
oil tank capacity........................................................................................1.8pints

PARTS PRICED inc VAT..................................................................................
handlebar......................................................................................................5.46
front mudguard.............................................................................................7.57
speedometer cable.........................................................................................2.82
exhaust system complete..............................................................................34.34
piston...........................................................................................................9.09
ring set.........................................................................................................4.94
list price....................................................................................................570.00
delivery.......................................................................................................10.80
warranty..................................................6 months or 10,000 miles, parts & labour.
Importer..................................................Mitsui Ltd, Oakcroft Road, Chesington Trading Estate, Chessington, Surrey, KT9 1SA

MAX SPEED ss 1/4-mile MAX SPEED IN GEARS (Computed at indicated 8,000rpm red line)
 72.5 17.3 first 14 second 25.5 third 37.5 forth 48.5 fifth 59 sixth 72
FUEL CONSUMPTION RANGE TO RESERVE OIL USED    BRAKES        from 30mph      
Best 67 Worst 32 Average 55  76-86 road use only 405 miles per litre 28ft

 HOW IT COMPARES

Model Price Max Speed Av mpg SS1/4 bhp
Yamaha DT175MX 570 72.5 56 17.3 12.4
Kawasaki KL250 699 80 63 18 16.6
CZ175 292 69 66 20 11
Harley Davidson SX125 577 64.5 63 20.4 11.2
Honda CB200 575 83 68 18.8 -

 

 

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