Honda XR400R and XR600 - Trail and Supermoto Gearing
Note: The following article is the basis of my published article on gearing that appeared in the April 1998 issue of Trail Bike and Enduro Magazine. Issue 33.
WHAT'LL IT DO MISTER?
What'll it do Mister? It's that perennial question.
Gearing. It's all about wheels. In a conventional two or four stroke the piston goes up and down which converts linear motion via the conrod and crankshaft to a rotary one. A small primary gear wheel on the end of the crankshaft meshes with a larger primary gear on the outside of the clutch basket, which is attached to the mainshaft with its corresponding gear ratio meshing with its respective counterpart on the layshaft or countershaft. This outputs to the gearbox sprocket and then via the final drive chain to the rear sprocket and wheel. A simple direct link. Providing the engine has the power/torque to pull a certain rpm (then disregarding variables such as clutch slip, wheel spin and tyre growth) the bike will do what the combination of ratios dictate. No arguments. Forget aerodynamics, weight, mud and wind for now. IF the engine will pull a given rpm then the bike will hit a speed in each ratio directly related to the revs and overall reduction ratio chosen.
The trick then is to decide what you want and where for a given set of conditions? Assuming the manufacturer got the best compromise in the first place (which invariably they do these days) based on their average rider and terrain. There is such a beast. I discovered you weigh 168 lbs and use your bike for its designated use. Of course we are naturally all different, ride in our own style and on differing terrain with differing machines and ability. Trails, Trials, Motocross, Enduro, Raids, Green Lanes, Supermoto, Rally. Take your pick. You may even vary the use of your bike to that for which it was designed. It'll still have though, one of two engine designs.
A two stroke has differing gearing requirements to a four stroke. A general rule of thumb is that a four stroke trail/enduro bike produces a broader spread of power than a two stroke hence the gaps between the gears can be wider. i.e. wide ratios ( you change gear less because the gaps between the gears are wider) whereas a two stroke motocross bike with its more frantic, peaky power will require narrower gaps between the gears and more shifts to stay in its powerband. Having said that a modern two stroke trailie with neat porting, variable timing and variable port heights can be pretty damn torquey.
Assuming we're all not going to leap into our engines and change internal ratios we're simply left with changing our final drive ratio. Or we can change tyre size which affects the overall ratio - marginally I may add (2/3 mph on the same rim size in top gear). To get an idea how changes affect things, let's take an XR400 with standard gearing revving to 8,000 rpm. It's a convenient one because the final drive ratio is 3:1. i.e. for every 3 turns of the 15 tooth gearbox sprocket the back wheel with 45 teeth on the sprocket turns once. If the rolling circumference of the back tyre (say an average part worn 120/80 by 18 trail tyre on a 2.15 ins rim) is 80 ins, then that is how far the rear wheel rolls for 3 revolutions of the layshaft/gearbox sprocket and one of the rear sprocket. Fitting a different profile rear tyre and/or rim width may give a larger rolling circumference hence the bike will travel further in a given time. More speed that is. Fitting a smaller rear sprocket or larger front has the same result. The converse is also true. Bear in mind lowering the gearing narrows the gap between the gears as raising the gearing widens the gap.
An example of a standard European XR 400R is shown below. Note the imaginary 6th gear.
Example 1 - Standard trail XR400
OK so now you want to alter your gearing. What to do? Your bike is geared too tall or high. Simply put a bigger rear sprocket on you say. That's when things get a bit more tricky.
Without getting too technical there are also other factors to take into consideration when altering gearing such as unsprung weight, suspension travel, chainline/driveline angle, swingarm clearance, chain guide position, force in the chain due to altered torque at the gearbox sprocket and such things as pro squat and anti squat. All well and good if you understand such matters yet in the real world one only needs to be mindful of swingarm clearance, whether you want to keep your gearcase saver or whether the rear sprocket is going to snag on rocks. For that reason alone and a combination of the above its best to change sprockets in pairs.
For the dirt we also tend to have differing considerations to say road bikes. So, although 13/39, 14/42, 15/45, 16/48 all give identical final drive ratios (3:1 or put another way taking one tooth off the gearbox sprocket is the same as adding 3 to the rear with the converse being true) we'd choose the 15/45 because of the reasons mentioned above. Naturally if your sprocket sizes don't divide as a whole number ratio you'll need to use next best combinations. Sprockets with half teeth don't exist! Certain folk excepted! Note for most bikes the most practical minimum gearbox sprocket size is 13 teeth, partly due to the above factors and that chains don't like getting forced into much smaller diameters. Trials bikes may be an exception here. Upper limits on front sprocket size are a function of clearance and case savers. Rears are hub sizes and reasons mentioned previously.
How then do you decide what is right for you and your machine's application if your standard gearing and tyre doesn't work for you? There are a number of ways:
1) Ride on your usual terrain with your standard gearing and use good old seat of the pants evaluation. If you are slipping the clutch in first, changing up and your engine dies or your not pulling max revs in top (if required) then the gearing is too high. Crawling in first, maybe not even using first, frantically changing gear and screaming in top, then its too low.
2) Calculate the changes for differing sprocket combinations. For this you can use the formulae at the end of this article, a calculator, spreadsheet or the ready made computer programme we've made available.
The trouble with solution 1) of course is that you may buy a lot of expensive sprockets and chains before you get it right, unless of course you can ride other folks bikes like yours with differing combinations for comparison.
Solution 2) is an armchair one and doesn't tell you how it will feel.
3) Using a combination of both works best. However without a computer or calculator the comparisons shown below will give you an idea of what is going on.
I want to change to Supermoto wheels but still retain the standard gearing?
Using a standard geared XR600R and comparing that with standard gearing on 17" wheels and a 150/60 rear tyre with a 4.25"rim .
Example 2 Standard trail XR600.
Example 3 - XR600 Supermoto on standard gearing.
I want to change to Supermoto wheels but raise the top speed?
Using XR600R on 17" wheels and a 150/60 rear tyre with a 4.25"rim with 43T sprocket.
Example 4 - XR600 Supermoto on 14/43 gearing.
Note gearing for the ton may mean the bike won't pull 7,500 rpm so addressing inlet and exhaust to give more power may help it crack it, i.e. more revs. 41T gives exactly 100mph @7,500 rpm. Fuel consumption improved.
Back to altering our gearing for the dirt to help us wheelie over logs, get around nadgery bits and compensate for that 50 lb of mud on the bike at the end of the event. Not to mention that the power on your air cooled four stroke has dropped off due to overheating and inefficient cooling, your knackered and going slower. Or maybe your name is Blezard and your rear sprocket is so worn you jump a few teeth getting instant variable gearing!
On the15/45 (divide 15 into 45 giving 3:1) gearing example for an XR400R, at 1,500 rpm which is just over tickover then in 1st you'll be doing around 5 mph. Flat out in 1st, 27 mph. Lowering gearing to 14/50 (3.57:1) would give us 4 and 23 mph respectively. But now top speed in 5th has dropped to 65 compared to 77 mph with standard gearing. A fair compromise then could be 14/47 (3.36:1) giving around 5 min, 24 max in 1st and 69 mph flat out in top with similar variations through the gears.
What does this look like?
Example 8 - XR400 Trail on 14/47 gearing. Imaginary 6th.
What then of desert rally bikes and Supermotos? As I said earlier as long as the bike will pull the rpm then it'll do what the ratios say. We can help the bike here though by giving it either more power to pull more top rpm or as the rally bikes start to need aerodynamics we can give them fairings to help them get to max revs. Aerodynamics start to matter around 85 mph not to mention that rider fatigue and protection from the elements is also important. We also need to consider the power sapping effect of soft sand. So here a different compromise needs to be reached. Bottom line is broad torque at high revs with tall gearing for speed but still low enough so you don't stall in soft sand at low speed. In the Dakar the bikes can be doing well over a 100 mph on fast stages.
The Supermoto situation may be different again. Tight tracks will dictate your top speed so you may gear for max revs as low as 70 mph in top. This may effectively make 1st gear redundant in the race. It means less gear changes anyhow and you'll be pretty quick getting off the line in 2nd. This sort of thing can be seen in four stroke motocross bikes which may have only 3 or 4 speeds due to the wide spread of torque and slower top speed requirement than say a rally bike. In a road going Supermoto fun bike then your considerations may be different again. Tall gearing can mean high top speeds, better fuel consumption and the ability to cruise around peak torque which is where the engine is most efficient. First gear won't need to be so low either.
I want to fit Supermoto wheels to my XR400 but use standard gearing?
Using a standard geared XR400R on 17" wheels and a 150/60 rear tyre with a 4.25"rim .
Example 5 - XR400 Supermoto on standard gearing. Imaginary 6th.
My XR400 Supermoto still isn't fast enough. What's the smallest rear sprocket I can fit?
Using 15/38 geared XR400R on 17" wheels and a 150/60 rear tyre with a 4.25"rim, 38T is the practical limit. 37 is too close to e.g. the Talon hub for comfort. Some do fit 36 to these hubs but that involves machining. Going up to 16 on front you lose your case saver and the protection that offers.
Example 6 - XR400 Supermoto on 15/38 gearing. Imaginary 6th.
My XR400 Supermoto still won't crack 90. Whats can I do?
Make it rev more with more power and new ignition and/or change the primary ratios. Get tucked down and pull those revs. Get a following wind or steep hill. Note on this gearing your average fuel consumption will be around 48 to 52mpg whereas a standard XR400 on enduro gearing 14/47 will be 32 to 37mpg.
Example 7 - XR400 Supermoto on 15/38 gearing with HRC 24/64 primaries. Imaginary 6th.
So there you have it, various gearing for differing machine types and situations. If you're not the techie type then simply ask around or get counting teeth next time your out and see a bike like your own. It'll save you time and money in sprockets and different length chains. Ask to ride the bike as if it has different gearing to yours, seat of the pants stuff will often tell you what's needed. For you techie types then here's the formula:
Overall reduction ratio is R = pGf.
Where p is the primary reduction ratio. No of teeth on clutch divided by number of teeth on crankshaft gear.
G is the gearbox ratio selected. No of layshaft gear teeth divided by main shaft number of teeth.
f is the final reduction gear. No of teeth on rear sprocket divided by number of teeth on gearbox sprocket.
If n is in rpm and if the rolling radius of the rear tyre is r inches then the speed of the wheel (bike) in mph = nr/168.07 R.
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