Taller Gearing to Offset BS Speedo? - Page 5 - Honda CBR250R Forum : Honda CBR 250 Forums
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post #41 of 47 Old 05-30-2018, 12:49 AM
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Originally Posted by tegs View Post
With 15T/37T, 120 is at about 7200 RPM. As always, your mileage may vary.

One thing I have noticed is that the loss of power JKV mentioned leads to the bike really not wanting to accelerate in 6th gear, and it's struggling with headwinds and inclines. I'm choosing to view that as an opportunity to become a better rider by learning how to use the power band more effectively, but it might be a problem for you at higher speeds. In my area, people go up to 10 kph over the limit before being ticketed, and if that's also true of SA than the bike will really struggle at 130 kph in sixth. It defeats the purpose if you end up having to run in fifth to maintain the speed you want.

https://www.gearingcommander.com/ is a fantastic resource, but don't trust the default values. Another thread here says 74 inches or 1879.6 mm circumference is the correct tire circumference to get the speed at RPM range where it should be.
Thank you so much for the reply.
You are correct we can do 10 over before getting a ticket. Where I live the wind is heavy. It can average like 60 to 100 km per hour some days when I go home. We've gone into winter now so the bike is parked and on charge till I get a non raining day. (No Snow for SA) so we can ride in winter.

Looks like I'll then have to keep my ratio.

Thanks again for the info much appreciated.

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post #42 of 47 Old 05-30-2018, 10:54 AM
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Originally Posted by tegs View Post
I finally got around to swapping the sprockets, so I wanted to revisit this and report the outcome.

I went with a 15t front and 37t rear, which puts it at around a 9.12% difference. The speedo reads at 101-102 kph when I'm actually going 100, which I'm liking as margin to prevent tickets. No longer running the numbers in my head frees up some mindspace, and I found that especially important last night when riding in an unfamiliar area in heavy traffic.

There is a definite reduction in acceleration, particularly in 2cd gear. The tradeoff is that 100 kph highways no longer make my hands numb, and 110 kph highways no longer feel like I'm sitting on a paint shaker. The increase in comfort over long distances is worth the loss of power IMO.

All in all, I'm happy with the change. I'm still out accelerating everything I care to, and since most of my riding is at 80+ kph reducing vibration makes my trips a lot more comfortable.

I keep a spreadsheet to track all my expenses, maintenance, etc, and I've modified it to account for the new sprockets so I know what the odo should read despite the sprocket change.

Couple thoughts here... first is that by changing sprockets you do not "lose power" per se, rather what actually happens is that the relationship between the engine RPM (and it's horsepower & torque at a given RPM) and a given road speed has been shifted.

The other general comment I wanted to bring up regarding sprocket changes, is that on a bike with a fair amount of miles on the current chain & sprocket set, you really don't want to reinstall that worn chain onto new sprockets, due to the fact that the worn chain will accelerate wear on the new sprocket teeth. Same goes for installing a new chain on old sprockets, which will cause the new chain to wear prematurely.

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post #43 of 47 Old 05-30-2018, 01:54 PM Thread Starter
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Originally Posted by MotoMike View Post
Couple thoughts here... first is that by changing sprockets you do not "lose power" per se, rather what actually happens is that the relationship between the engine RPM (and it's horsepower & torque at a given RPM) and a given road speed has been shifted.

The other general comment I wanted to bring up regarding sprocket changes, is that on a bike with a fair amount of miles on the current chain & sprocket set, you really don't want to reinstall that worn chain onto new sprockets, due to the fact that the worn chain will accelerate wear on the new sprocket teeth. Same goes for installing a new chain on old sprockets, which will cause the new chain to wear prematurely.
Taller gearing reduces mechanical advantage, which translates to less power to the road. It's akin to shortening the near end of a lever; you don't need to move it as far to get the same result, but it takes more force to make it move. You can really feel it on a pedal bike; the rider's output power is consistent, but they still need lower gearing to get mechanical advantage to accelerate and go up inclines.

If we put the physics aside, I'm feeling the difference and it's too pronounced to be observation bias. The bike is accelerating significantly slower than it used to when in the same RPM range.

I swapped out the chain as well, and kept the old chain and sprockets in case I decide to go back. I'm not a fan of making permanent changes. :-)

One thing that bears mention is that there's a steel guard around the front sprocket to keep a broken chain from hitting the engine, and it had to be ground down a little to fit the larger 15T front sprocket.

Every bike is a good bike. The question is whether or not it's the best bike for what you want to do.

The Little Sport Tourer That Could - 2011 CBR250R with touring windscreen, Saddlemen seat, tank cover, frame sliders, mirror extenders, taller gearing via sprockets to fix speedo and reduce vibration at highway speed.
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post #44 of 47 Old 05-31-2018, 12:31 PM
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Perhaps a more accurate way to describe the different results from various final drive ratios, would be as a performance change, rather than a gain or loss of power.

Power output from a given engine throughout it's operating RPM range doesn't change unless that engine is modified in one way or another, or it's power output is affected by significant changes in elevation and/or ambient air temperature. While many owners have changed to a 15T front sprocket on their CBR250R's, I went in the opposite direction changing to a 13T front sprocket due to the fact that I live & ride at a relatively high altitude (9000 ft above sea level). Every 1000 ft increase elevation causes a loss of about 2.5% of an engine's rated sea level horsepower. And that net decrease in available HP is after the EFI system has adjusted the fuel/air ratio for elevation, or in the case of carbureted fuel systems, after jetting has been adjusted for the elevation change.

For me, changing to a lower final drive ratio with the 13T front sprocket helped to regain some of the bikes performance (acceleration) which was lost to higher elevation.
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post #45 of 47 Old 09-22-2018, 09:32 AM
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I tried the 15T front earlier this year when it was time for a new chain and it was terrible. Yeah the bike revved a little lower at X speed compared to the stock gearing, but it was woefully undergeared on pretty much anything but flat ground. Any slight wind or incline and it had to be wound out, thus it was actually much worse on fuel, by almost 20% in some cases. Quite possibly the worst modification I've done on any machine I've owned.
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post #46 of 47 Old 09-22-2018, 03:42 PM
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My fuel economy made a big improvement when I went to +1 front -2 rear. 15/36. 14% longer. Along with the other mods I have done with the Pilot Street tires being another nice improvement, I have been averaging over 105 mpgUS all summer. Last summer was hotter and dryer and I was over 110.


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post #47 of 47 Old 09-23-2018, 03:35 PM
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Gearing "up" 1T on the countershaft with a small engine is tricky, and many people that try it end up switching back to stock. Without a significant amount of torque you are really going to feel it when accelerating. But if you favor smooth cruising and only need moderate acceleration most of the time it can be an improvement.

I geared my SV650 up 1T on the countershaft and never regretted it, but I've got almost 50 ft.lbs of torque to work with so the acceleration penalty was minimal.
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