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It was Dezzie who set that up - nice one Des.
 

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bummer, am i reading the chart correctly? i think without modifying the grom the top speed is 56.9 mph?

why do honda advertise 65mph? i know they will have an asterisks "under optimum conditions"
 

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All this site does is plot theoretical speeds at certain revs for a given gearing combo. It does not mean that you bike will be able to do it. For example, my bike will now rev to 9,200rpm. If I put a +2 front cog on, my bike *could* do 85mph. It does not mean that it actually will - not without a strong tail wind and a bloody steep hill. Also, my experience from actually trying a +2 cog shows that top speed on the level is no better than normal, but that hill/wind performance is very much worse, requiring you to drop down to 3rd for most of the time.
 

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All this site does is plot theoretical speeds at certain revs for a given gearing combo. It does not mean that you bike will be able to do it. For example, my bike will now rev to 9,200rpm. If I put a +2 front cog on, my bike *could* do 85mph. It does not mean that it actually will - not without a strong tail wind and a bloody steep hill. Also, my experience from actually trying a +2 cog shows that top speed on the level is no better than normal, but that hill/wind performance is very much worse, requiring you to drop down to 3rd for most of the time.
O ok ,is that with a -2 rear as well or would that not make a difference ?
 

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That calculator is cool. It should have a disclaimer that says:

Please keep in mind the the calculations are maximum speeds based on the mechanical gear ratios of your specific combination. If your engine does not have enough horsepower/torque to maximize the gearing used, over gearing or undergearing your motorcycle may actually reduce performance

I copied that from the gear calculator that is set up specifically for Honda minis that I use here.
 

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Hi guys, it's all about fractions.
 

Think of the front cog. Its rpm is controlled by the engine rpm and the gear you are using. Most motorcycles run at high rpm but don't make much torque (for example, they wouldn't run well at 60rpm which is 1rev per second). So the aim of the final drive (chain and sprockets) is to reduce rpm and hence increase torque. So the front cog is smaller than the rear cog. 15t against 34T for the MSX.

Treat this as a fraction (front divided by rear) and you get the effect it has on gearbox output rpm. So for the msx it would be 15/34 = 0.4412. So if your front cog was spinning at 1,000rpm, your rear would be doing 1000x(15/34)=441rpm. Final drives also work the other way.
 

On push bikes people peddle slowly but can exert quite a bit of torque (about 60rpm). To use this to get the bike going faster the front cog is normally larger than the rear. My single speeder is 28t front and 15t rear. This gives a final drive of 28/15=1.867 (front over rear remember). So peddles spinning at 60rpm will make the rear wheel turn at 60*(28/15)=112rpm.
 

Now, the reason I went through this is so that people know how it all works before changing the gearing. When changing the number of teeth, the circumference of the cog will change, meaning the distance the chain has to travel will change. So the length of chain needed changes roughly inline with the total number of teeth. Roughly. This means that to maintain the correct chain tension, you either have to move the wheel using the adjusters, or you have to fit a longer/shorter chain.
 

When considering your options, to avoid having to get a new chain, you should try to minimise the change in total circumference of both cogs. So imagine we have a bike with simple gearing, 15t front, 30t rear. This gives a ratio of 0.5 . Since circumference is directly proportional to the number of teeth, then these cogs have a circumference of 45t in total.
 

Now imagine we wanted to get a final drive of 0.6 to make it more of a cruiser. Changing only the front would mean going to 18t to give 18/30=0.6 . Total teeth = 48T, so a change of +3. If we changed just the rear, then we'd need to go to 25T rear to give 15/25=0.6 . Total teeth = 40T, so a change of -5. You can see that changes to the smaller cog have the least effect on chain length. However, note how they have opposite effects.... for the same effect on final drive one make needs a longer chain, one needs a shorter chain. If we change both together they can be make to cancel out.
 

If we try 17t front and 28t rear we get 17/28=0.607 , so almost right. But notice that the total circumference is the same as 17+28=45T. So this gearing would not change the length of chain much if at all. For most people though, it all comes down to what's easiest and cheapest, so that means changing the front cog. Going one tooth up or down won't have much effect on chain length, but as it's the smallest cog, it will have a strong effect on gearing. It's also easier as you don't have to take the rear wheel off. Also, small steel cogs are cheaper than big aluminium rear cogs.

 
I hope all that helps people. I may have got the odd bit wrong, so please feel free to comment
 

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I've also made something like that... to calculate ratios for cars - www.shiftchart.eu. You can also enter your torque curve, if you check the 'optimal shift' checkbox you get the optimal shift points for fastest possible acceleration.

Anyone got a torque curve of a standard grom/MSX? (saving power curves will be the next feature I'll add :) )
 

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Bottom line if you FAT/BIG GUY/GIRL you are going to go slower than a Thin person. Weight of payload will determine max speed for a given motorized vehicle on a flat roadway.
 

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Old topic, but still fun topic.
A 125cc can go round about 60MPH (roughly) top speed.

if you want to get better gas mileage, and let the engine run lower RPM; while still keeping round about the same top speed here's a few suggestions:

Stock sprockets: 15/34T
Stock top speed: ~57mph @ 7k RPM

Custom sprocket setup: 17/31T
Custom Top speed 3rd gear: ~57MPH @ 7k RPM

3rd gear on a 17/31T setup is very close to stock 4th gear (0.1MPH faster actually).
Speed reading will be off by ~24%
4th gear RPMs will be down by ~24% compared to stock.
4th gear top speed guestimate will be ~55MPH @~5k RPM; that is ~5MPH slower in 4th gear from stock ~60MPH @ 4.6k RPM.

Lots of numbers...

Benefits of a 17/31T over a 15/34T are:
1- More usable 1st gear (not too fast shifting)
2- Lower RPMs
3- Lower engine heat below ~50MPH.
4- Possibility to maintain top speed thanks to nearly identical 3rd gear ratio vs 4th gear stock.
5- No chain cutting necessary
6- Theoretical MPG up by ~24% (110MPG -> 136MPG)
7- Better for chain (less wear, because counter sprocket is larger, and chain pull is lower).

Cons:
1- Slower acceleration from 1st gear
2- $36 on sprockets (more if you have to get them installed instead of install them yourself)
3- When pushing the engine in 4th gear to the max, the engine will get hotter.

I know it sounds silly, but you could do an 18/30t setup.
Numbers won't change too much.
- Top speed ~1000RPM lower than with the 17/31T setup @ ~50MPH (guestimate).
- Bad starts (clutch acrobatics needed to depart from a stop), especially with a passenger
- Theoretical MPG's of just under 150MPG (which I think will be impossible)
- Final gear acceleration virtually zero.

It wouldn't make sense, but I'd love to hear if anyone actually would try out a 18/30T setup.
17/31T makes a lot of sense; it allows the bike to have an overdrive; to save engine life, and even more gas.
 
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