Rear Sprocket Question N/S/R
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eeee-bip
Benelli TNT 125 "Bean Alley" Kymco AK550 The War to end all Wars
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Sat Aug 10, 2019 12:35 pm quote
If I had a diminutive motorcycle which I was going to take on a fairly long ride and the aim was to take load off the engine by reducing the revs of the engine at higher speeds would I fit a larger sprocket to the rear wheel or a smaller one ?
Initial acceleration is not a concern.

Bill x

Last edited by Bill Dog on Sat Aug 10, 2019 1:08 pm; edited 1 time in total
Molto Verboso
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Sat Aug 10, 2019 12:49 pm quote
Smaller sprocket.
eeee-bip
Benelli TNT 125 "Bean Alley" Kymco AK550 The War to end all Wars
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Sat Aug 10, 2019 1:10 pm quote
Ask
Would that "load" the engine at low revs if had to produce more torque to turn the rear wheel ?

Bill x
Molto Verboso
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Sat Aug 10, 2019 1:36 pm quote
Changing the rear sprocket smaller will make it harder for the engine to spin the rear wheel....within modest changes, you'll probably just notice a bit lazier acceleration and the engine may feel a bit more gutless at speed too.

The smaller the engine, the more you'll notice the difference.

"Fine tuning" the gears is quite common, making very small changes like 1 tooth at front or a few teeth at the rear sprocket. You'd notice that, but not dramatically.
Molto Verboso
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Sat Aug 10, 2019 1:43 pm quote
.. and I'll have to add, it all depends.

Take my VanVan200. The gear box was designed so, that the 1st gear was a bit too tall and the largest gear a bit too short.

So, if I tried to lower the revs for traveling by changing a smaller rear sprocket, I'd have also made the first gear even taller... in the end, I kept it standard and suffered the revs, as I did quite a lot of slow speed crawling too.

Just an example...
eeee-bip
Benelli TNT 125 "Bean Alley" Kymco AK550 The War to end all Wars
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Sat Aug 10, 2019 1:53 pm quote
Load
So reducing the size of the drive sprocket and the rear would have the maximum effect, after lots of sucking and seeing of course.

Bill x
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Sat Aug 10, 2019 2:05 pm quote
Re: Rear Sprocket Question N/S/R
Bill Dog wrote:
If I had a diminutive motorcycle which I was going to take on a fairly long ride and the aim was to take load off the engine by reducing the revs of the engine at higher speeds would I fit a larger sprocket to the rear wheel or a smaller one ?
Smaller sprocket increases the revs but a higher speed increases the load on the engine.
The faster you go the (much) greater the drag.
Drag increases as the square of the speed, which is a big part of why a 24hp GTS300 HPE doesn't go twice as fast as a 12hp GTS125 i-get

I reckon the best way to give the engine an easy life is to reduce the weight it carries and take it easy on the throttle.
eeee-bip
Benelli TNT 125 "Bean Alley" Kymco AK550 The War to end all Wars
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Sat Aug 10, 2019 2:34 pm quote
Gatt
Taking it easy on the throttle should be achieved by changing the gearing by reducing the rpms in relation to the rear wheel.

The only object on the bike that would save weight is the exhaust but replacing the pipe for a lighter one may increase the mpg but it won't change the rpms.

Bill x

Last edited by Bill Dog on Sat Aug 10, 2019 2:42 pm; edited 1 time in total
Ossessionato
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Sat Aug 10, 2019 2:38 pm quote
If I were doing it, I'd add 1 tooth to the front sprocket. Easier than removing the rear wheel. If that feels like it's not enough, then replace the original sprocket and do the math and change the rear sprocket.
MV Santa
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Sat Aug 10, 2019 2:39 pm quote
Think of it like a bicycle with a rear derailleur. The larger sprokets in the rear will make the pedals spin faster with more torque to the wheel. The smaller sproket will cover more ground with each rotation of the pedals but with less power. Don't go too small with the rear sproket or the motor will not have enough power to maintain speed going up hill or into the wind. Then, again you can always downshift if that happens.
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Sat Aug 10, 2019 2:51 pm quote
Re: Gatt
Bill Dog wrote:
Taking it easy on the throttle should be achieved by changing the gearing by reducing the rpms.
You seem confused about gearing?
On a conventional manual transmission as found on nearly all motorcycles, varying the throttle does not alter the gearing because there are a range of fixed ratios.

Thinking about bicycles, gearing can be described by the term "development", this is the distance travelled per revolution of the crank.

(F/R)xC=D

F = front sprocket teeth
R = rear sprocket teeth
C = circumference of wheel
D = development

The longer the development the harder it is to pedal.
Same principle applies to motorcycle engines.

Best not to overthink it, suitable gearing has already been chosen by the manufacturer, just go and have fun riding it!
eeee-bip
Benelli TNT 125 "Bean Alley" Kymco AK550 The War to end all Wars
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Sat Aug 10, 2019 3:03 pm quote
.com
Not confused.

We're not talking about the transmission, just sprockets to up the revolutions of the rear wheel in relation to the revs.

If my rear wheel is being turned around more often by using either a smaller primary sprocket or a smaller rear sprocket than standard ones surely I'll be using less rpms ?

Maybe I'm not phrasing it very well. I'm looking for more revolutions of the rear wheel in relation to rpms so that the engine is reving lower at the same speed.

It's OK I think I got it 2 or 3 posts up.

Bill x
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Sat Aug 10, 2019 3:16 pm quote
What is this "diminutive motorcycle" you speak of? A Honda Grom?
eeee-bip
Benelli TNT 125 "Bean Alley" Kymco AK550 The War to end all Wars
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Sat Aug 10, 2019 3:18 pm quote
Clackers
A Benelli TNT 125.

1 gear more then a Grom with 3 HP more and 1000 less expensive.

Bill x
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Sat Aug 10, 2019 3:19 pm quote
Re: .com
Bill Dog wrote:
I'm looking for more revolutions of the rear wheel in relation to rpms so that the engine is reving lower at the same speed.
But if you do that the engine will be more stressed than if you left the transmission alone.
eeee-bip
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Sat Aug 10, 2019 3:20 pm quote
Biffa
Yes that was a concern and modifying it is out of the question so the adjustment is going to have to be attempted externally.

Bill x
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Sat Aug 10, 2019 3:26 pm quote
Re: Clackers
Bill Dog wrote:
A Benelli TNT 125.

1 gear more then a Grom with 3 HP more and 1000 less expensive.

Bill x
Very popular bike up here, see quite a few yoofs having a blast on them.
Veni, Vidi, Posti
LX190
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Sat Aug 10, 2019 3:38 pm quote
Can you start off OK in 2nd gear? First gear needs to be low enough so a XXL sized motorcyclist, with a pillion, can do a hill start. So it's normally lower than you need for normal riding.
Just make sure if you change the gear ratios you're not dropping the revs so low you can't hit maximum speed. Is there a dyno chart available for the Benelli?
Moderaptor
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Sat Aug 10, 2019 3:59 pm quote
Re: .com
Bill Dog wrote:
Not confused.

We're not talking about the transmission, just sprockets to up the revolutions of the rear wheel in relation to the revs.

If my rear wheel is being turned around more often by using either a smaller LARGER primary sprocket or a smaller rear sprocket than standard ones surely I'll be using less rpms ?

Maybe I'm not phrasing it very well. I'm looking for more revolutions of the rear wheel in relation to rpms so that the engine is reving lower at the same speed.

It's OK I think I got it 2 or 3 posts up.

Bill x
Fixed that for you...
Hooked
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Sat Aug 10, 2019 5:22 pm quote
Divide current rear teeth by front teeth. For example 43/16 = 2.6875

Divide proposed new rear teeth by front teeth e.g., 40/16= 2.5

Divide new ratio by old ratio: 2.5/2.6875=.93

The new combination will result in approximately 93% RPM compared to the original, or a 7% reduction in RPM. So, 4000 RPM will now be 3720 RPM.

You also have to factor in chain length (if you have enough adjustability to take up chain slack with a smaller sprocket).

On some motorcycles increasing the front sprocket size too much can cause the chain to rub the engine case.

And, you'll have to feather the clutch to get rolling if you change the rear more than a couple of teeth.
Hooked
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Sat Aug 10, 2019 6:02 pm quote
Sometimes, when you try to lower the operating RPM's on a small engine, you will actually loose MPG's because you are out of the power band it was designed to operate in during WOT. Bigger engines dont care because they have the power regardless, and most people only do it to increase top speed. So get some good timed runs with the 'ol butt dyno unless you have access to a real dyno.
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Sat Aug 10, 2019 7:30 pm quote
Just to get a little revenge, why do you want to modify a bike to do what it wasn't designed for when you could just buy a bigger bike?
eeee-bip
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Sat Aug 10, 2019 10:11 pm quote
Monster
Maybe you didn't read the post properly.

This is a about lowering the rpms, not increasing speed or performance.

Thank you for all of your help.

Bill x
eeee-bip
Benelli TNT 125 "Bean Alley" Kymco AK550 The War to end all Wars
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Sat Aug 10, 2019 10:46 pm quote
Numbers
The Benelli is very easy to hoon around on because it's all about keeping it in the power which is around 7000 rpms so you're kept busy while feeding it the gears needs to keep it there.

In top gear that's around 50 mph which is enough for a small bike but if I was going to ride it long distance any faster I'd like to reduce the rpms a little.

Granted this could increase the load on the engine and as it's under warranty I may have to wait a while.

Either that or buy another one.

Bill x
Molto Verboso
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Sun Aug 11, 2019 12:09 am quote
The only way to guaranty that stress will be reduced is to lower the rpm by riding more slowly. This implies using less throttle which almost by definition lowers the engine load. If you try this on the road but find you can no longer cruise in top gear on the level then you should increase the rear ratio, i.e. fit a larger rear sprocket. In the more unlikely event that at your chosen reduced cruising speed the engine is still quite close to the red line you could do the opposite.

Most small engines are tuned so that maximum torque delivery is not far away from maximum power delivery and if your cruising speed is such that rpm drops below the engines peak torque (in your partially open throttle condition) it will be difficult to maintain cruising speed without changing down and defeating the object of reducing stress.

In practice this is all dependant on the rider weight, clothing, and possibly luggage. So its really a question only you can answer by testing in your particular conditions. If you are talking about a regular route or type of terrain you would also have to take into account the frequency of hills etc.

If I/we have misread your intentions and what you really want to do is crouch over the tank with your chin on the headstock on a wide open throttle then yes you could fit a smaller rear sprocket, but you won't be spending much time in top gear if you hit a headwind or a hill, and your point to point journey time might even be longer.
eeee-bip
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Sun Aug 11, 2019 12:32 am quote
Stuff
No. I didn't mention wide open throttle.

You're assuming I want to go faster and I'm not.

I'm looking into using less revs at the same speed by changing the sprockets.

Now if that loads the engine because it has to work harder to turn the rear wheel at lower rpms there's no point as it's probably designed to give it's peak power at higher rpms.

I've got that.

I think that what I was trying to avoid was it having it rev high up for prolonged periods but ironically it looks like adjusting it's gearing will probably cause more damage than leaving it as it was.

Sorted.

Bill x
Molto Verboso
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Sun Aug 11, 2019 1:30 am quote
Jimc corrected this, but just in case you'll decide to give it a try:
for a "taller" gearing that you've been thinking, it is either a smaller rear sprocket or/and LARGER primary (front) sprocket.

Makes sense when you'll think it for a while.

Also, if you'll find any discussion forums etc. about the same Benelli, it may be that others have thought about this too... often you'll have a limited choise of sprockets available and someone is bound to try them out.

The easiest gearing change I've ever done was to put a 1 tooth larger front sprocket to a late Honda of mine... it had enough room for the chains run without touching the cover with this slightly changed angle, the chain lenght was still within adjustment range.... and the effect was into the right direction of lowering the revs at higher speeds. I honestly do not believe this had any observable effect on the engine durability... but that's just me.
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Sun Aug 11, 2019 1:37 am quote
Don't forget. A small wind deflector will decrease the load
Hooked
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Sun Aug 11, 2019 3:52 am quote
Wrong question
The real point is, it takes the same amount of power to go the same speed, regardless of the gearing. You can generate that power with a moderate throttle opening with the engine in its powerband, or a greater throttle opening if not. So slowing down the engine might make it operate in a less efficient manner, actually increasing heat production and stress.

Modern cars with aero refinements require far less than maximum power at typical cruising speeds. And engines in many cases have variable valve timing so they are efficient across a wide rev band. So most are geared (in top gear) to drop the revs substantially, actually more to improve fuel efficiency that reduce engine stress.

The combination of smaller engines and much worse aero efficiency in bikes and scooters means the engine needs to be in the powerband to achieve and maintain highway speeds.

Fortunately modern engines don't seem particularly rev-sensitive, presumably a consequence of better materials and machining, as well as better oils. Running the engine at higher revs (within bounds) won't cause excessive wear. Many motorcyclists change 'gearing' on their bikes to rev the engines higher (not lower), in order to get better acceleration. While fuel economy may take a hit, engine wear does not seem a problem.
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Sun Aug 11, 2019 4:17 am quote
Re: Wrong question
Engines tend to have a sweet spot where they feel smoother. On my guzzi that was unfortunately around 150kph in 4th gear. On the LX it's around 60kph which is just nice for city riding.
On a long ride that might be more important than anything else.
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Sun Aug 11, 2019 4:21 am quote
Re: Stuff
Bill Dog wrote:
No. I didn't mention wide open throttle.

You're assuming I want to go faster and I'm not.

I'm looking into using less revs at the same speed by changing the sprockets.

Now if that loads the engine because it has to work harder to turn the rear wheel at lower rpms there's no point as it's probably designed to give it's peak power at higher rpms.

I've got that.

I think that what I was trying to avoid was it having it rev high up for prolonged periods but ironically it looks like adjusting it's gearing will probably cause more damage than leaving it as it was.

Sorted.

Bill x
lots of trade off when changing the final drive gearing. What you might gain in 1 area you will probably lose big in the other. So while you might achieve lower rpms at a given speed you will suffer getting there and when you need to do a roll on for passing or something it will be gutless to do so.
You also have to consider excessive strain and wear on the clutch plates as well.

Bottom line is can it be done ,YES

don't drop more than 2 teeth on the rear sprocket.
You can increase the front pocket by 1 tooth and that would probably be equal to around 3-4 tooth decease in the rear sprocket.
eeee-bip
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Sun Aug 11, 2019 8:06 am quote
Clap
Thank you Doug.

Excellent work.

Bill x
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Sun Aug 11, 2019 1:28 pm quote
Like what was said above, go nack to your 10 speed bicycle days. I'm probably dating myself by calling it a 10 speed!

Larger gears in gr
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Sun Aug 11, 2019 1:32 pm quote
Like what was said above, go back to your 10 speed bicycle days. I'm probably dating myself by calling it a 10 speed!

Larger gears in front make for fewer pedal strokes at the same speed. Smaller gears in the rear do the same thing.

Talk to people in the geared motorcycle arena, they can point you towards the limitations of what you're trying to do. Usually moving the front sprocket up by a tooth makes a big difference, if there is room for such a sprocket. Going smaller in the rear is a good idea too.
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Mon Aug 12, 2019 3:53 am quote
Because I tour a lot, I decided to do what you are planning, but in a different way.

To reduce revs at touring speed, I chose to change the rear tire on the Vespa GTs from 130 to 140 width.

The extra width combined with the height to width ratio, added approximately 1.5% to the diameter of the tire.

So maybe a 100 RPM drop at 70 mph.

If there is room in the wheel well of the little Benelli, it may get you what you want with minimal fuss.
eeee-bip
Benelli TNT 125 "Bean Alley" Kymco AK550 The War to end all Wars
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Mon Aug 12, 2019 4:00 am quote
Fair
If I go for smaller gears there won't be a problem.

If I'm going for a bigger rear one it may well foul the chain guard.

More thought needed.

Bill x
eeee-bip
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Mon Aug 12, 2019 4:04 am quote
Lips
Actually you raise a very fair point and it's a shame that I've just had City Grips fitted as a tyre with a higher profile may well have created the same affect.

Difficult to compare and contrast heights though.

Bill x
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Mon Aug 12, 2019 6:21 am quote
Re: Lips
Bill Dog wrote:
Difficult to compare and contrast heights though.

Bill x
Most tyre manufacturers post the tyre diameter in the specifications.

Simple maths.
eeee-bip
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Mon Aug 12, 2019 6:26 am quote
Biffa
Yes and totally bombed out by the bias towards City Grips.

If I found a shit tyre that had a high profile would I still fit it ?

Hang on. I'll just get my calculator.

Bill x

Last edited by Bill Dog on Mon Aug 12, 2019 7:20 am; edited 1 time in total
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Mon Aug 12, 2019 7:18 am quote
I had to change from City Grips to Power Pure tyres to go to 140.

Also very good tyres.

I am running City Grips on the Xmax now.
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