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Discussion Starter · #1 · (Edited)
I was having a conversation about acceleration power the other night with another rider regarding manual shifting VS the use of a CVT such as in a scooter, a motorcycle like the Aprilia Mana 850, or even a car. Rather than explaining the back story outlined in threads such as this one, I just listened and drank my beer. Apparently, the common thought pattern to many people is that horsepower is the sole determining factor in acceleration, and that the only thing the gears do is keep the engine in a specific power range. Following that logic, it's assumed that a CVT will keep the engine at peak horsepower and therefore out-accelerate a manual shift.

Now, I told you that short story to tell you this: That theory is not true. Horsepower is only half of acceleration. The other half is gearing. Why make a thread about it? Because apparently, so few people understand it. I used to be one of those people who completely missed gearing as part of the equation, too. It works like this: The engine makes a specific amount of horsepower at the crank. That's nice and all, but how it's delivered to the rear wheel is dependent upon gearing, be it the transmission or final drive. By "how" it's delivered, I mean there's a trade off between top speed and acceleration power (specifically, the amount of leverage to the rear wheel). For example; 1st gear provides the lowest top speed, but the highest acceleration. Inversely; top gear provides the highest top speed, but the lowest acceleration. "Gear ratio" is referring, in it's most abstract form, to the rate that one gear is turning VS the rate another one is turning. There's ratios of rotaions at the crank VS the gears in the transmission; rotations at the crank VS rotations of the rear wheel, rotations of the front sprocket VS rotations of the rear sprocket, etc. It's really just a measurement.

So, how exactly do lower gears make more rear-wheel torque than higher ones? Glance at the animation below. It shows the general concept. Say the small front gear is the drive gear, and the large rear gear is the driven gear. A small gear is easier to turn from its center than a large one, and a large gear is easier to turn from it's outside than a smaller one. This means, horsepower being equal, a smaller drive gear and larger driven gear will create more leverage, hence why I swapped the sprockets on my motorcycle -1 tooth in the front and +2 teeth in the rear (same principle, except the sprockets are connected by chain). Without making any modifications to the engine tuning or exhaust, I now have substantially more acceleration power, all thanks to the laws of physics. The added bonus is that it also adds a few RPM's at any given speed to boost power even more. This is secondary to the increased torque from the gears, but worth noting since many people assume that's all that re-gearing does.



What about the transmission? How do you accelerate the hardest? Let's look at this chart of power per gear below (for reference, this is from a car). The 1st column of data is engine power alone. The stars denote peak torque and peak horsepower as reference points. The other columns are measuring power output from the transmission. Notice the gear ratios are torque multipliers for the engine. To make things pefectly even, you'd need a gear ratio of 1:1 (hence why any gear with a ratio of 1:1 or higher is considered "overdrive"). The reason 1st gear has a slower top speed but a heck of a lot more power is because it has so much lower of a gear ratio... from the looks of the chart; 3.54 turns of the crank for every 1 turn of the output shaft. It takes a lot more force to get weight moving than to keep it moving at a constant speed. Low gears have the power to get a vehicle moving from a stop; high gears can afford to sacrifice power to hold a higher cruising speed. This works out well because it accommodates how vehicles travel on roadways to begin with. The most critical performance observation to note though is that any RPM, including redline, produces more power at the rear wheel in any particular gear than upshifting to a higher gear.

Code:
       Engine     Transmission output torque (ft-lb):
       Torque      1st     2nd     3rd     4th     5th
 RPM  (ft-lb)     3.54    2.13    1.36    1.03    0.72  <- gear ratio
----  -------     ----    ----    ----    ----    ----
1000       50      177     107      68      52      36
1500       65      230     138      88      67      47
2000       80      283     170     109      82      58
2500       92      326     196     125      95      66
3000      104      368     222     141     107      75
3500      114      404     243     155     117      82
4000      120      425     256     163     124      86
4500      125      443     266     170     129      90
5000      130      460     277     177     134      94
5500*     133      471     283     181     137      96
6000      130      460     277     177     134      94
6500*     122      432     260     166     126      88
7000      110      389     234     150     113      79

Before you spend $750 on an exhaust and fuel controller, consider spending $75 on new sprockets 1st and see how you like it. If you want to target sprockets for a certain peak power/top speed for the riding you do, go look up your bike on gearingcommander.com. You're also gonna want to re-calibrate your speedometer. I recommend the 12 o'clock Labs SpeedoDRD. My speedometer is now dead accurate.

Anyway, I hope the animation and chart with explanations have been informative. If anyone has any corrections to make, just let me know and I'll do so. I want this to be as accurate of a resource as possible.
 

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Nice write up Rusty! Well written. I think this is the type of topic that, if written too technically, will just make people's eyes glaze over. I like the way you explained it and kept it simple. I, too, think this is one of those areas that needs more exposure about this bike and its hidden potential.
 

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Discussion Starter · #3 ·
Thanks, I appreciate it. I agree with you; some of these topics can get pretty abstract, so I try to keep it in layman's terms as much as possible without losing the message. I really think gearing is one of those things that should be changed to suit the rider right out of the gate. For example, Sendler went taller for fuel economy, and I went lower for more control and variation of speed in the city. We all have different needs, and the bikes come out of the factory with a very "middle of the road" gearing. It's expensive to change on a Jeep or truck, but so cheap/easy to change on a bike that everyone should at least consider it as their 1st performance mod. A lot of people sat they put on a slip on pipe and didn't notice but so much of a difference and to be realistic about your expectations. When you go -1/+2 there's a glaring obvious difference in power. It'll also have you shifting for a phantom 7th gear on the interstate. The bike runs completely different.
 

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I'm with you on this modification. As far as my bike goes the engine and exhaust are still stock. I think this bike has plenty of power as is. For the time being I'm trying to dial the bike in and strengthen its weak points before doing anything with its power output. I'm one of those guys that would rather have the suspension, transmission and braking components beefed up 'before' adding more power. Plus, I'm on a budget and I can get more bang for the buck mods done with the added benefit of being able to work on my bike more.
 

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Discussion Starter · #5 ·
I know what you mean. The sprockets were a good deal for me at $75. I recall the SpeedoDRD being about that price too. I try to go for the best bang for the buck like you, particularly given that the CBR250R is such an inexpensive bike. I'll likely add the Zero Gravity Corsa windscreen to cut out some highway wind/drag and call it done. The grand total in parts for all my mods will be $250... $265 if you count the ProGrip carbon-black tank pad.
 

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Keep in mind that every CBR250R already has -1 front, +3 rear gearing. It's called 5th gear. By changing the final drive ratio shorter there is no magic increase in power on the road. It just moves the spacing of each gear closer together so you can stay closer to the power peak after shifting at the redline without dropping down below it as much. First gear is lower by a very small amount but the real change is the spacing and the low 6th gear that now feels like 5th used to.
 

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Discussion Starter · #7 ·
Keep in mind that every CBR250R already has -1 front, +3 rear gearing. It's called 5th gear. By changing the final drive ratio shorter there is no magic increase in power on the road...
You're one of the most detailed gear heads I've ever seen on a forum, so for this part, I just assumed you were referring to the similarities between 5th gear stock, and 6th gear -1/+3. This is true if only factoring in that particular VS that other particular gear.

It just moves the spacing of each gear closer together so you can stay closer to the power peak after shifting at the redline without dropping down below it as much...
Coming from you, this is the part I'm confused about. Changing the final drive ratio does a lot more than closing the space between the gears. It adds a substantial amount of torque at the rear wheel because the smaller the front sprocket; the easier the engine can turn it from the center. Inversely, the larger the rear sprocket; the easier the chain can turn it from the outer edge... much like socket wrench with a long handle VS a short one.

Am I missing something you're saying?
 

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Coming from you, this is the part I'm confused about. Changing the final drive ratio does a lot more than closing the space between the gears. It adds a substantial amount of torque at the rear wheel because the smaller the front sprocket; the easier the engine can turn it from the center. Inversely, the larger the rear sprocket; the easier the chain can turn it from the outer edge... much like socket wrench with a long handle VS a short one.

Am I missing something you're saying?
This is dangerously close to a statement claiming a free lunch. Nothing is "easier" than before, just as picking up a 10lb medicine ball straight off the floor is no "easier" than rolling it up a ramp.

To be clear: the engine puts out exactly the same power as before. Power is the key determinant to how fast the bike goes. Closer gearing will change the acceleration characteristics but will do so at the expense of drivability (frequent shifts) and a lower top speed, and can increase stress to the drivetrain.
 

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Discussion Starter · #9 ·
It's certainly easier (the bike has way more rear wheel torque now... night and day), though yes, at the expense of top speed and a little shorter shifting... but with a 10.5k redline and no need to even use my current top gear's 93mph max, I'm not complaining. You're also right about the drive train. It'll theoretically wear out the chain and sprockets a little quicker, but it's worth it to me. I just wish I would've been able to find someone who had steel rear sprockets not on backorder. Oh well, maybe next time.
 

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This is dangerously close to a statement claiming a free lunch. Nothing is "easier" than before, just as picking up a 10lb medicine ball straight off the floor is no "easier" than rolling it up a ramp.

To be clear: the engine puts out exactly the same power as before. Power is the key determinant to how fast the bike goes. Closer gearing will change the acceleration characteristics but will do so at the expense of drivability (frequent shifts) and a lower top speed, and can increase stress to the drivetrain.
This is true but that correlates with rusty's statement about horsepower and acceleration. There will be a tradeoff so managing both, in terms of what you're actually going to be using the bike for, are key.

What is confusing me the most is the gearing really. To clarify, because I'm a simpleton, to get better acceleration but with a sacrifice in top speed you decrease the size of the front gear/increase size of the rear gear? On the flip side, to increase top speed but sacrificing your acceleration you increase the size of the front gear/decrease the size of the rear gear?
 

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It adds a substantial amount of torque at the rear wheel
Now that I think it through I guess you are right. And the rpm drop between shifts stays the same with either set up. It is the speed in gears that is closer with the lower gearing. What gets me me is when you compare 6th gear stock to a modified 6th gear with -1/+2 teeth. 12% shorter. The modified 6th has more torque at the rear wheel than the stock 6th gear for any given engine output. But the stock 5th gear has more torque than the modified 6th. And can run in almost the same speed range. Almost. And so on down the line. You just end up sometimes riding around in a different gear than before for any given speed. But in a drag race, the time that the stock bike is in the next lower gear is less than the time they are in the same number gear so the torque advantage is really with the geared bike on average. But it is not an advantage of the entire 12% as there will be times when the stock bike will still be in the next lower gear after you have been forced by the redline to shift. On a drag race to 75 the geared bike will use up 5 gears while the stock bike will only use up 4. So I would predict that the geared bike would average about a 9.6% torque advantage (80% 0f 12)? for the run.
 

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What is confusing me the most is the gearing really. To clarify, because I'm a simpleton, to get better acceleration but with a sacrifice in top speed you decrease the size of the front gear/increase size of the rear gear? On the flip side, to increase top speed but sacrificing your acceleration you increase the size of the front gear/decrease the size of the rear gear?
Correct, the way I found it simple to think about was to envision a ten speed bike and how hard it was to pedal or how fast you could go in different gears. (of course that realization came after a rather noobish pm from me to the OP)

With safe regards from the mostly frozen north.
 

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Discussion Starter · #13 · (Edited)
...On a drag race to 75 the geared bike will use up 5 gears while the stock bike will only use up 4...
That's the trade off right there from shorter gearing. What I gained in return though was that all 6 gears pull way harder than they did stock since dropping the ratio of the final drive obviously cascades the effect over what the transmission is outputting. Easily the best bang for the buck in performance gains, IMO. I just used Gearing Commander to pick sprockets that would eliminate the gap between max cruising RPM in top gear and peak power (which, by no coincidence, both now happen at 75mph at 8.5k rpm). The same as there's 2 ways to increase the bottom line: increase revenue or cut cost; there's 2 way to gain acceleration; raise the horsepower or lower the gearing.
 

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Here is an example of the point I have been trying to convey. You are on the highway and need to gun it from 65 mph to 75 to make a pass. The stock bike will beat the geared bike because it can use 4th gear whereas the geared bike will have to use 5th. Stock 4th has 3% more torque than the geared 5th.
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Nup, the bike with OVERALL lower gearing will build revs in 5th where as stock it will just sit there.
Think of it as better leverage, his 23hp can do more work than yours.

he'll be into 6th long before you , and get to 10,800/96.5 mph easily

The only time you might pass him is after 3 miles down a bloody long straight downhill on a pefect day.
(given same weight and rider skill)
But he's already been on 96.5 mph while you are waiting waiting waiting for speed we all know rarely happens.

eg Look at the top speed thread.. many new riders say they can only do in the 80-90mph range which is no faster than you can do in 5th.


I run -1 front, its good but still a bit high.
Rusty's is a bit low for road use,
Around 100mph at redline would be about perfect.
 

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Discussion Starter · #17 · (Edited)
if you go 1up in front and 1 down in the back do you need a odo correction or not
Assuming you meant speedo, technically yes. A 1 tooth change in the front is equal to almost a 3 tooth change in the rear.


Nup, the bike with OVERALL lower gearing will build revs in 5th where as stock it will just sit there.
In Sendler's defense, are you factoring in the fact that 4th gear is a lower ratio in the transmission? That should theoretically apply the same torque and rev, right? The overall leverage would come from the transmission and front/rear sprockets.


Think of it as better leverage, his 23hp can do more work than yours.
This is the best way I've heard it described.
 

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Sorry but you are mistaken. You didn't read my statement carefully enough or look at the graphs carefully enough. To accelerate from 65 mph to 75, the geared bike only has two choices. 5th and 6th. 4th has already redlined by that speed so it would have to pull in 5th. The stock bike can do the same pull in 4th. The stock 4th is 3% lower than the geared 5th. The stock bike wins in that scenario. The geared bike will beat the stock bike from 0-75 mph because any time the two bikes are in the same number gear, it will have a 12% torque advantage. But there will also be times at the top of each gear where the stock bike will still be using the next lower gear after the geared bike has been forced to shift by the redline. All in all the geared bike will average a torque advantage over the stock bike but it will be less than the 12% that it might seem at first glance.
 

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Is his bike is doing 8500 in 5th gear at 65mph when you both nail it?

or am I reading the graph wrong?
 

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So if you rarely or never travel 70 or 80 mph (city riding) rusty's setup is an improvement. If you do lots of highway speed travel (commuting) then stock is a better choice. Please correct if wrong.


Sent from my iPhone using MO Free
 
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