[Sourav@1977]

Hello guys
I own a Yamaha Gladiator RS 2007 125 cc model.
The bike is in very good running condition and has done 36000 km without any technical snags.
I often have to commute between 2 cities and cover 300-400km / day as the situation demands.
But I don't cross 60km/hr and so I could hardly cover 45kmph on average which strains my energy and time.
Of late I have made certain modifications to the sprocket system. The original setup was 14T / 45T in front and rear respectively.
I have swapped the the same with 16T/39T and found remarkable improvement in kmpl figures and top end speed. There has been more than 30% fuel savings and I could cover 60kmhr on average now.
My question is..
Will it take a toll on my engine longevity???
What are cons in such a setup except loss of acceleration which is manageable?

[ashwinprakas]

Originally Posted by Sourav@1977

Will it take a toll on my engine longevity???
What are cons in such a setup except loss of acceleration which is manageable?

The difference in final drive ratio is massive as you've dropped from 3.2 to 2.4, the SS125 being a square stroke the hit won't be as noticeable as on short stroke.

Provided you adapt to changes i.e slip a bit more clutch when required and ensure that the motor isn't lugging by maintaining adequate rev's then worst case scenario you'd just burn some more clutch, not a matter of concern as its a consumable any how.

Now if you do not adapt and rather strain the engine then you'd be reducing the life of your crank bearings and output shaft.

This is because there is really no torque being made or lost, you're just playing around with leverage, and leverage depending on your riding conditions may or may not be of advantage to you, which BTW from the looks of it seems it is working to your advantage.

So simply put the best course of action is to simply unlearn and relearn to make sure you do not put undue strain on the motorcycle.

To simplify even further just imagine your crankshaft, it is rotating as the piston goes up and down, the more load there is for the engine to overcome the slower the revolution gets, now this means that instead of evenly transferring load from the power-stroke it gets acutely targeted at a couple of balls in your bearing, which accelerates wear and tear, now the sensible means would be to take the load off by downshifting or if already in a lower gear slipping the clutch, but if instead you crack open the throttle then the power-stroke comes down harder and causes more damage. This over time results in the crank bearings going for a toss sooner which would require the engine to be completely dismantled to be replaced.

So do take care and make it a point to respect the power band.

Cheers,
A.P.

[vrprabhu]

Sourav,

If I can make a friendly suggestion - choose a bigger and better bike, if you travel 300+ km in a day.

This is in no way demeans the Gladiator (it is my bike too for more than 10 years now (see my avatar!) - I have also done 400 kms on a single day, with a pillion one way).

And, based on my experience, the following two characteristics of the bike needs regular attention :

1. The front disc brake is sharp. Problem is that, it takes a toll on the steering race - those who have driven and braked hard would have experienced the 'kat-kat-kat' sound from the steering joint.

2. Chain wears out fast - post 20,000 kms, the chain snatch is very much perceptible. By increasing decreasing the final drive ratio, I think your chain will wear down much faster.

(Note to Ashwin : have a small doubt - does changing the final drive ratio require change in idling rpm? The manual recommends ~1400 rpm, which I think is on the higher side (I prefer it to be in the 1000 - 1200 range))

[srini1785]

Originally Posted by Sourav@1977

Will it take a toll on my engine longevity???
What are cons in such a setup except loss of acceleration which is manageable?

Originally Posted by ashwinprakas

The difference in final drive ratio is massive as you've dropped from 3.2 to 2.4

Keep the ratio above 2.0. Anything lower and you get a wheelie at lower gears when you least expect it. Such modifications are common in RX100 wheelie groups.

[Mr.Boss]

Originally Posted by srini1785

Keep the ratio above 2.0. Anything lower and you get a wheelie at lower gears when you least expect it. Such modifications are common in RX100 wheelie groups.

He moved from 3.2 to 2.4, that's gonna make the bike sluggish initially. The opposite will make a wheelie

[srini1785]

Yes, I was wrong. I was thinking about RX100 which is a 2S. His is a 4S.

Btw, Lower the ratio, higher the chance of wheelie. Ideally 1:1 ratio on a 2S with a open throttle would definitely pop a wheelie. It may not be possible on a Gladiator due to weight , lower end torque etc.

[ashwinprakas]

Originally Posted by vrprabhu

(Note to Ashwin : have a small doubt - does changing the final drive ratio require change in idling rpm? The manual recommends ~1400 rpm, which I think is on the higher side (I prefer it to be in the 1000 - 1200 range))[/i]

Idle RPM is dependent on two things, the weight of your crank and decompressor.

Weight of crank because it needs to carry momentum from one power stroke to the next, hence why below a certain RPM the engine simply dies.

As for the Decompressor, it is not present in all motorcycles though all modern machines seem to have one. With a decompressor in place the RPM needs to be above the recommended mark else the decompressor being centrifugally operated would remain active and prevent the exhaust valve from completely closing i.e the compression loss from that would result in the engine dying.

So simply put, the idle RPM remains as advised by the manufacturer.

If you're worried about the load from going for taller FD ratio then don't bother, the change is minuscule as far as the first gear is concerned, worst case under extreme load you would have to slip some clutch and that would be at RPM's closer to the power-band, so again idle RPM is something that need not be bothered about since its function as the name says is to idle and while idling load is out of the equation.

Originally Posted by srini1785

Btw, Lower the ratio, higher the chance of wheelie. Ideally 1:1 ratio on a 2S with a open throttle would definitely pop a wheelie. It may not be possible on a Gladiator due to weight , lower end torque etc.

The ideal ratio is 3.00.

It's the opposite, higher the ratio, more the chance of wheelie. Say if I take ratio from 3.00 to 3.50 then it would be easier to wheelie the motorcycle.

For ease of understanding lets ignore the FD ratio and lets have a look at gear ratios, which gear do you think its easier to wheelie? 1st or 3rd?

Ideally the 1st gear of a motorcycle would have ratio closer to 3:1 whereas the 3rd gear would have a ratio closer to 1:1.

Hope that clarifies things.

[srini1785]

Originally Posted by ashwinprakas

The ideal ratio is 3.00.

It's the opposite, higher the ratio, more the chance of wheelie. Say if I take ratio from 3.00 to 3.50 then it would be easier to wheelie the motorcycle.

For ease of understanding lets ignore the FD ratio and lets have a look at gear ratios, which gear do you think its easier to wheelie? 1st or 3rd?

Ideally the 1st gear of a motorcycle would have ratio closer to 3:1 whereas the 3rd gear would have a ratio closer to 1:1.

Hope that clarifies things.

You sure?. I have seen RX100 stunt freaks actually lowering their rear wheel sprockets to get that much sought after "Lift". Lowering the rear sprocket teeth actually gets more traction on the wheel. They also tend to add more weight to the rear by using a bigger size wheels so that it shifts the weight to rear.

I have never wheeled so i could be wrong.

[vrprabhu]

Originally Posted by ashwinprakas

As for the Decompressor, it is not present in all motorcycles though all modern machines seem to have one. With a decompressor in place the RPM needs to be above the recommended mark else the decompressor being centrifugally operated would remain active and prevent the exhaust valve from completely closing i.e the compression loss from that would result in the engine dying.

Thanks for the explanation to my off topic query - clarifies one issue which I had been facing intermittently (albeit only during early cold starts), I think.

Another (stupid & off topic) question - in a carb bike, is the choke performing the task of a decompressor? (it has been referred so, in a couple of places in the manual). The decompression which I relate to - the priming done before starting which needed to be done in older days Bullet / Jawa, and sometimes the kickback was much more powerful thank your kick!

[ashwinprakas]

Originally Posted by vrprabhu

Another (stupid & off topic) question - in a carb bike, is the choke performing the task of a decompressor? (it has been referred so, in a couple of places in the manual). The decompression which I relate to - the priming done before starting which needed to be done in older days Bullet / Jawa, and sometimes the kickback was much more powerful thank your kick!

Off-Topic but Interesting Topic.

Function of Choke is to make the AFR temporarily Rich(More Fuel), in older motorcycles like my CT100B and Splendor when you activate the Choke a butterfly valve literally chokes the intake(Air), on modern motorcycles the Choke is actually an enricher i.e through the choke circuit it dumps more fuel.

As for the Decompressor, its function is to decompress the cylinder, cause during compression stroke both valves remain closed and the piston faces resistance going up as the cylinder builds compression, this is what causes kick-back, so in older Bullets we had to manually decompress the cylinder in order to start the motor, same is the case with all 4 Strokes just that we didn't get any of the large displacement ones in India hence our experience is limited to Bullets.

Here's an example of a Honda XR500R kicking back, you can see the fear in his eyes. :

Motorcycle Kicks like a Mule! - XR500R

Most modern bikes come with an auto-Decompressor, its centrifugally operated as explained in my previous post. It is essential to decompress the cylinder on motorcycle that have a self starter as without decompression the starter motor would go for a toss.

There are still some motorcycles that come without a decompressor, small displacement bikes like the CT100B and Splendor, compression isn't much to the point that it'd sprain your leg but at times when you slow kick you'd get a kick-back.

Other motorcycles that lack a Decompressor but still come with a self-starter are some older Honda engines i.e the one on the ZMA and the Pulsar series from Bajaj where with the exception of the Pulsar 150 that came with a kick-start the rest of the self only models lacked a decompressor. How these engines take care of kick-back is mostly by form of Jugaad, when the starter turns the engine and if it is against compression then the starter clutch bears the kick-back preventing it from being transferred to the starter-motor.

This is why on such motorcycles sometimes when you thumb the starter you hear a loud metal clunk/thud.

So to sum up, Choke affects fueling to make starting easy whereas Decompressor relieves cylinder of compression to make starting easy.

They're not the same.

[swapnil.awate]

With taller ratio, now engine is spinning at lower rpm for the same bike speed (kph) which usually leads to better fuel economy. So, for the same distance, number of revolutions run by the
engine are lesser leading to more life of rotating components.
However, the downside is less torque at wheel, leading to poor acceleration (pick-up) and load carrying ability. Owing to this there will be more load on engine to bring about any change in speed. This shouldn’t be an issue while cruising on highway but you should not lug the engine. Avoid low speed-high gear situation. This can affect the engine life adversely. Do not hesitate to downshift.
As you are reaping benefit of this setup without any discernible shortcomings, keep it.

[landcruiser123]

Here are the things I can think of:
1. Low acceleration and power to climb inclines
2. Piston Slap due to high resistance - might happen eventually due to extra torque required
3. Higher carbon build up in the engine - might eventually happen due to richer A/F ratio because the throttle will be wide open

3 shouldn't happen if you've an FI motorcycle

[gauravanekar]

I have a Yamaha Saluto RX 110cc and it is used for office commute at least 2 times in a week by me.
The terrain is flat and 80% of the route is in Jaipur -Ajmer Highway, also I weight only 65kg including my backpack.
Hence I have been procrastinating on my overgearing plan since a long time now. This thread motivated me to get up and check the stock sprocket setup.
Front 14 (single hole mounting), Rear 42, chain pitch unknown.
Final drive ratio works out to be 3.0,
Experts here can answer some of my questions:
1. Is 2.8 ratio ideal mod ? (42/15)
2. Is 2.4 doable? (39/15- RX 100)

My take on overgearing:
Most motorcycle riders upshift intuitively based on torque increase curve, while revving if we feel that torque is increasing then we continue to rev in the same gear, the moment we feel torque dropping off we tend to upshift.
Hence I feel gear changes between 1-2 2-3 up to 5-6 won’t be any different at all, only the speeds at which the gear change happens is increased throughout.
Now the tricky part is to get bike moving from standstill N-1, This is where the rider has to consciously move the machine making sure that there is not too much resistance at the get go i.e., an incline or raised manhole cover infront of front wheel. To put it in short, engine should not lug while releasing clutch in 1st gear.

[Sourav@1977]

I thank all the experts and enthusiasts for explaining the concepts. I am adapting to the new engine behavior after the modifications.
In over a month time I have made 2 rides.
One is from asansol to Kolkata to and fro in a day approx 450km.
Another one durgapur to haldia approx 570km to and fro on single day.
Though the initial acceleration was sluggish the bike was noiseless and smooth till 70kmph
So i didn't feel much tired after the journey as the engine vibration was less.
Post 75kmph their was lot of fluid friction due to wind resistance.
I maintained a speed of 58-62kmph on the whole journey and it returned around 75+ kmpl on the whole journey. (Tank ful method used).
I am really surprised by the figures.

In the city I rarely use 5th gear as it may damage the clutch plate. Kmpl figures is around 60.

Please suggest if I can switch to fully synthetic oil from mineral to prolong engine life and drain period.
If so which brand and grade should I look for.
Presently use 15w40 with 3000km drain period.

[xsskbalsara]

Hi All,

Newly joined this portal.

My bike originally started as a Bajaj Avenger 200 and has now been modded to that of an Avenger 220. For the last 7 years I've been using a 150/90/15 Michelin rear tire which after fitting on my custom alloy wheel would be like a 140/100/15. Due to that and on the stock Avenger 220 chain sprocket kit of 14 (F)/36 (R)/110 (Links) 520 pitch set, my top end (with a Pulsar 220 BS4 carburetor tuned for power), I used to top out at about 125 Km/h. Unfortunately with the current restrictions on tires and the availability, I've been forced to used the HD Street 750 stock tire from Michelin which is a 140/75/15. Due to this my top end has taken a solid whack to the point that I can barely cross the 3 figure mark at 8000 rpm (I have the old Pulsar twin pod meter set installed). That has forced me to ground the "Devil" (nickname given by my kids) from highway running.

With a lot of research and calculations, I've been doing a lot of homework about what I need now to get the "Devil" highway worthy again.

Options available to me are to switch
1. The front sprocket with one of 15 teeth OR
2. A rear sprocket (6 bolts) with one of 32 teeth keeping the same chain pitch. Worst case if I can get a 4 bolt sprocket, I will have it modified for a 6 bolt sprocket.

I've been contemplating using the smaller pitch chains but the last thing I need is the power of my engine ripping up the chain and causing damage that I can do without.

I've been scouring the interwebs for the past couple of weeks trying to find someone who sells custom chain sprocket kits but no luck.

Does anyone have any ideas as to who would make this and if someone in Mumbai actually has the expertise to modify an existing kit?

Thanks in advance
XHB