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Time Lapse quad build TRX450ER with Petey

Gearing your Quad for the job.

You will hear a lot of people say "Gearing down gives you more bottom end and gearing up gives you more top end".

This is not correct.

Most standard quads are sold with an appropriate gearing for their general design purposes. If you are doing something different that is other than what the OEM considered the general user would, then you may want to change your gearing to suit. Or if you have modified your machine, ie. Increased the engine output, then too, you might want to change your gearing.

If you ride most OEM quads full throttle in a straight line on a hard-pack flat surface, they will probably only just run out of legs and start bouncing off the rev-limiter. Some seem to hit it quite early and could be geared up from stock if you want to a bit go faster. Others top out at right near the rev limiter anyway, and probably wouldn't go much faster even if they had the gears to do it.

Say you are doing 115kmh and just hitting the rev-limiter in top gear and you think your trusty steed might be capable of more. Two things happen if you decide to gear up.

One, your engine is now pushing through a taller gearing and you are multiplying your torque to the real axle by a smaller number (final drive ratio) before it gets to the ground. On this point, it is worth considering that you will now need some extra power just to do the same speed you were before.

Two, if your bike does have the extra power you thought, you will push a higher top speed before you hit the same factory rev limiter. In this situation you might go up on the gearing again. If you do it is most likely you will get to a point where you are not hitting the rev limiter any more and most likely losing out on top speed.

So why is the above statement untrue?

Most quad racers that race on dirt will take off in 2nd gear because this gives them their fastest launch even though it is obviously a higher or taller gear. Once you are moving you simply need to be in the best gear to put the most power to the ground at what ever speed you are doing. It does not matter if this gear is called 1st 2nd 3rd 4th 5th or even 6th (if you have one).

Some owners of quads with six gears say I was doing 120kmh in 5th! And I still had another gear to go... imagine how fast it would go it 6th? Truth is... most often slower' because the gearing is too tall. Most modern cars will go faster in 4th than they will in 5th, because 5th is too high for max speed. A classic case of where gearing up made the car go slower (in that gear). They keep it up there because it is a cruising gear and some emissions are measured in grams per 100km, so less engine revs keeps that down too. So there are reasons to gear high, like cruising and economy, but as a performance thing you really need to have a proper look at how often you actually go that fast.

But anyway, let's get back to quads... If you have done the big 3' mods or any other modification that increases your power... Chances are that gearing higher than standard might be appropriate.

How high?

If your engine mods mean you are making 10% more power than the factory ever did... then a 10% gear up might be a good place to start. You should consider that gearing up will increase the gaps between gears (wide ratio) and will reduce your ideal gear ratio choices for different hills and corners etc. So you might be revving in 3rd or chugging in 4th etc, where a lower geared quad will have smaller gaps between gears, and you won't have this problem as much. Gearing down lower than standard will reduce top end because you will most likely bounce of the rev limiter well before your top speed and run out of gears, but you might be significantly quicker around an MX track with better gear selection for the job (close ratio).

I have geared speedway quads up to a theoretical top speed in 5th of 170kmh when the bike would only be raced at 88kmh.
In this example, I wanted a higher 1st gear to take off in, less gear changes and run the track in 3rd (so 3rd was effectively my top gear).
But as one of the fastest bikes on a speedway track, it would not pull top gear in a straight line down the beach.
In this case, gearing up made the bike significantly faster for the job.

I should add that the quad could still chew up most other trail quads in 4th gear when they were in 5th or 6th down the beach, because top speed is dependent on the actual gearing and not the number written in front of the gear.

There are other examples where MX racers are doing the opposite and taking off in 3rd gear, for max acceleration to get the holeshot, and sometimes they do.
They do not gear down for more bottom end, and in fact they never use their lower gears on the track.
They might gear their quad down, but select a higher number gear to go faster... in their application.

As far as the above statement is concerned, when I'm playing chains and sprockets...

I gear my DS450 down to race MX with a top speed of 110kmh and gear it up to race speedway at only 88kmh.
In both situations track times and therefore acceleration is improved.

So it is much more fair to say that changing your gearing does not effect your acceleration.
Only acceleration in top gear... but then you could just change back a gear and accelerate even harder if you wanted to.

Only your engine power and how you use it effects your acceleration.

Final Drive Front Sprocket Teeth
Ratio Chart 10 11 12 13 14 15 16 17
Rear Sprocket Teeth 30 3.00 2.73 2.50 2.31 2.14 2.00 1.88 1.76
31 3.10 2.82 2.58 2.38 2.21 2.07 1.94 1.82
32 3.20 2.91 2.67 2.46 2.29 2.13 2.00 1.88
33 3.30 3.00 2.75 2.54 2.36 2.20 2.06 1.94
34 3.40 3.09 2.83 2.62 2.43 2.27 2.13 2.00
35 3.50 3.18 2.92 2.69 2.50 2.33 2.19 2.06
36 3.60 3.27 3.00 2.77 2.57 2.40 2.25 2.12
37 3.70 3.36 3.08 2.85 2.64 2.47 2.31 2.18
38 3.80 3.45 3.17 2.92 2.71 2.53 2.38 2.24
39 3.90 3.55 3.25 3.00 2.79 2.60 2.44 2.29
40 4.00 3.64 3.33 3.08 2.86 2.67 2.50 2.35
41 4.10 3.73 3.42 3.15 2.93 2.73 2.56 2.41
42 4.20 3.82 3.50 3.23 3.00 2.80 2.63 2.47
43 4.30 3.91 3.58 3.31 3.07 2.87 2.69 2.53
44 4.40 4.00 3.67 3.38 3.14 2.93 2.75 2.59
45 4.50 4.09 3.75 3.46 3.21 3.00 2.81 2.65
46 4.60 4.18 3.83 3.54 3.29 3.07 2.88 2.71
47 4.70 4.27 3.92 3.62 3.36 3.13 2.94 2.76
48 4.80 4.36 4.00 3.69 3.43 3.20 3.00 2.82

What Dynos Do

The humble dynamometer - well call them dynos for the rest of this story because dynamometer takes to long to type, its also hard to say.- has been in existence since the early 1800s in some forms but the dyno systems that we now know came into existence around the early 1930s in no other than the spiritual home of the ATV the good old US of A.

What do Dynos do.

The principal of what dynos are for is pretty basic, they measure the rotational speed, power output and tourque of any kind of engine, though in the modern age they have other uses too such as measuring Air Fuel ratios for tuning both carburetted and EFI systems.

There are two main types of dyno, the engine dyno, which is generally driven directly from the engine and the chassis dyno which does its work from the drive wheels, hence its name, and so takes into account the power output of entire driveline. Chassis dynos are the most common dynos to be found being used by ATV and motorcycle tuners here in Australia as they generally off the convenience of being able to ride straight on and do your testing without any special adapters other than maybe a change of tyres.

Out of this then there are many different sub-designs of dyno, once again though there are two main dyno designs in use locally, the intertia dyno and the eddy current dyno.

Inertia Dynos

An inertia chassis dyno basically consists of two big heavy drums on an axle that are then connected to a computer via a speed sensor. Horsepower is determined by measuring the maximum rotational speed of the drums and the time taken for the drums to get to that speed coupled with the RPM of the engine. The computer then extrapolates the tourque figure from these readings. To add consistency to the readings there are also sensors for air temperature and density as well as air/fuel ratios in the exhaust gas. The dynos software factors all of the data from these sensors to help give consistent horsepower readouts.

Inertia dyno runs are generally done in 3rd or 4th gear and starting from just above idle the throttle is then opened fully until the engine hits its rev limit. The up side of this is that you can do many runs quickly as the process of each run takes only seconds, the down side is that you are only really measuring at one throttle position, wide open.

Eddy Current Dynos

This type of dyno is generally accepted as the best tuning engine tuning dyno around, the eddy current type dyno can present a measurable force for the engine to push against. This is achieved via a system of rotating and static electromagnets which, when powered, produce the resistance load to the engine. What this means in real world terms is that you can measure torque,power and air/fuel ration at any throttle position which makes for a handy tool for EFI remapping and carburettor tuning both. Also because of the ability to vary the load on the engine it means that it is easier to replicate real world conditions in the workshop and therefore tune out flat spots in the power delivery

Results

Power figures will vary from dyno to dyno, even two different dynos made by the same manufacturer will give different readings for the same engine depening on the operator and the calibration of the machine. This means it is best to try to get all your dyno testing done on the same machine for accurate results.

The results that everyone wants to see however are how much power their engine makes after modification. The Best process for this is to get your ATV tested before and (obviously) after the modifications to best see the differences in power output.


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