Gearing...not an oil thread!

[Onder]

Best gear ratio, not as beaten a topic as best oil at least.
Im getting 3650 at 70mph (indicated). This is fine out on
the main line and 3100 at 60mph for A and B roads not
bad either. But on the green lanes and around town it is
less than ideal. First is going to be less than 2000 a lot of
the time. One thoughts wander to the cam.
I guess Ill have to face facts and pull the primary again
and go down one tooth on the gearbox.
Perhaps will stay with what I have and see if I get used to
it.
Comments?

 

[XTINCT]

What do you have now?

 

[Onder]

Just as stated.
But let me add what I failed to mention: TTI 5 speed box, BNR belt and I weigh 9 stone fully road ready!
74 850. It has the torque to handle it perhaps just needs more getting used to it.

 

[Fast Eddie]

Do you have the high, or low, first gear option ?

 

[Onder]

Hmm I forget! Whatever was standard for a street box at the time . I get your point though. Perhaps time to
pump more pounds out to Kiwiland and get a low set. But maybe a lower second gear would be more the thing.
Well we shall see how you make out choosing the right sprocket when your TTI arrives.

 

[o0norton0o]

Just as stated.

You actually didn't state your gear box drive sprocket size... When I go to gearcommander.com and plug in 3650 RPM's, and chose the stock commando tire sizes I get 68 mph in 4th gear when I chose the 22 tooth sprocket, so I assume that is the sprocket size you have.

Right now I am running a 21 tooth on my 750, and it's awesome everywhere except the hiway where it's ok for short blasts for a few exits, but my speed at 3650 rpm's is only 64 mph (that includes the calculation for my 18 rear tire too) I am actually doing 3977 rpms to go 70 mph on my commando. That's OK, but around me there's a lot of superslab going east and west, so 70 mph is around the average speed people drive. Certainly my 21 sprocket would be ideal if a commando had a taller 5th gear.

You can go to this website and play with the theoretical speeds, rpm's, tire sizes, and gear sizes and project the results of your theoretical combinations. https://www.gearingcommander.com/

My commando originally had a 19 sprocket when I got it. I can't imagine riding that on the hiway. At 3650 rpm, with a 19 tooth front sprocket and my 18" wheel, I am doing 58 mph where you are doing 68. I am certainly glad I have the 21 now rather than the 20, given that I have an 18" rear wheel. I wonder how the 22 would feel... A single aditional tooth in the 20 tooth range is about 5% difference. 2 more teeth is about 10% .

 

[Onder]

For you then 4k is 70. Not so far from where I am now and Im wondering if that would not be better as an all arounder. It is pretty hilly round
these parts and even when I go out east. I ride solo so no problem there and there is plenty of torque out on the road.
It is the slow stuff that is a bother now. Green lanes are often dirty and gravelly and a low gear is handy esp when meeting SUVs from the home
counties.
TTI sprockets not the same as stock besides being for 520 chain. Im using a 21 now.

 

[bill]

my overall gearing is 4.19 with a 100/90 19 rear tire and i have found that is about the best comprise wit a stock 4 speed box.if i had a 5 speed with a lower 1st than a 4.1 or 4.0 would be OK IMHO

 

[o0norton0o]

When I decided to switch from the original 19 tooth sprocket, I agonized over whether 21 would be too tall and make the bike feel gutted in the lower rpm's. I took the chance and skipped right over the 20 tooth, because I already thought the original 19 was obviously too small. I thought going up only 1 tooth would still feel on the small side, so I bought the 21 tooth sprocket and hoped it would be "just right". The idea of jumping up 3 teeth to the 22 tooth sprocket mean't it would be ~ 15% change. I feared that would be too much, so I settled on 21.

What makes sense to me is that if you were just doing hiway cruising, a 22 sprocket might be the best choice because it lowers the cruising speed RPM's, but it's hard to know if I'd like it around town... Maybe someone will chime in who's used both sprockets... and give an opinion from experience

 

[Nater_Potater]

I've been running a 22T for a couple of years since most of my riding is on the 65mph roadways, but it hasn't seemed to bother the around-town riding any noticable amount. That's with both the stock twin Amals and a single 34mm Mikuni. All you have to do is slip the clutch for just a couple of feet, then you're off. 'Really no difference from the 20T stocker. I know it's not quite what you were asking, but, based on my 20-22T change, your 19-22T suggestion shouldn't be too awfully drastic.

Nathan

 

[WZ507]

As we extrapolate from our own gearing situations to Onder’s, we must be aware that he has a belt drive, that often results in taller gearing. Using his single data point of 70 mph@ 3650 rpm as the datum, his present gearing is slightly taller than a stock Commando with a 23 tooth countershaft sprocket. So even though he’s presently employing a 21 tooth countershaft sprocket, all things considered he’s pulling some serious gear.

 

[o0norton0o]

Onder's belt drive primary with a 21 sprocket is : 70mph @ 3650 rpms
A Stock commando with a 23 sprocket : 70 mph @ 3595 rpms
A Stock commando with a 22 sprocket : 70 mph @ 3759 rpms

So, Onder's ratio is between a 22 and 23 sprocket's ratio, appoximately a 22.333_ tooth ratio (If you trust gearing commander's calculation table)

 

[WZ507]

Onder's belt drive primary with a 21 sprocket is : 70mph @ 3650 rpms
A Stock commando with a 23 sprocket : 70 mph @ 3595 rpms
A Stock commando with a 22 sprocket : 70 mph @ 3759 rpms

So, Onder's ratio is between a 22 and 23 sprocket's ratio, appoximately a 22.333_ tooth ratio (If you trust gearing commander's calculation table)

I respectfully disagree with the conclusion regarding Onders gearing.

I’m sure the Gearing Commander data table is reliable, that said, the variation you observed has to be related to tire size differences (and you used a larger one).

The gear ratios given below are unambiguous, and in light of this the only way the engine speed can vary at a fixed bike speed is by applying different tire sizes or growth factors to these ratios. I used no growth factor in my calculations and measured the Dunlop K81 on my bike to determine a tire radius, so that is the basis of the data set for my plot.

We can also use the values from the table in the factory manual to determine the engine rpm at 70 mph for a 22 and 23 T countershaft. These results suggest the factory used a smaller tire than I measured, as their engine speeds are even higher.

In light of the foregoing, both my calcs and the factory calcs suggest that to equate Onders present gearing to factory Commando gearing puts him somewhere larger than a 23 T CS sprocket - which is damn tall gearing.

 

[Torontonian]

Yes a belt drive kit will change the gearing ratios.
After installing my belt kit with the 21 tooth sprocket ( that I had been previously happy with ) , the bike turned into an old dog.
Off came the primary again to fit a 20 tooth and all the around town life came back and good to go for the highway.
Note: I don't speed anymore.

 

[o0norton0o]

Tire size for Onder's calculation is included in his ratio overall, because his final ratio is determined by observation.... so whatever his tire size is, it's accounted for by his stated observation. Changing tire size of course would change his observed ratio too... as I'm sure you know.

The ratios I quoted from gearing commander are theoretical, not observed. Those ratios do count on chosen tire size to give correct ratios. Interesting that gearing commander's stock calculations are different than the factory calculated ones.... I'll have to look into that.

Like you WZ, I realized that the best thing to do is to measure my rear wheel circumference to do my calculations, so I have done that. When I do calculations on that site, I adjust the tire size input to give me the measured circumference of my tire. I've gone out on the road with my GPS on the handlebars and I have checked the accuracy of both gauges. They're pretty close to being on the money, except the speedo starts to read higher than actual speed above 90 mph. I can live with that.

My bike turns 3977 rpms to go 70 mph by gearing commander's calculation table, using my measured tire circumferance, my GPS to check the speed. Both my clocks reasonably verifying the speed and the RPM's.. Here's my set up below.

If you want to calculate my 70mph rpms with the factory gear ratios, My tire circumferance is 81.49" As I said, I get 3977 rpms @ 70 mph with a 21 sprocket. I know the speed is correct because of the GPS. Certainly these smiths tachs are not accurate, but mine was right on the theoretical calculation from gearing commander, so I think it's close to being right... although I've never checked gearing commander's ratios against the factory ones. I just assumed they used the factory ratios....

Thanks for posting those charts and all the other people for chiming in with their data and opinions on where they felt their gearing ratio was approaching it's limit of good performance. From what other people have said here, I think I could have gone to a 22 sprocket with my 18" wheel without sacrificing much low end grunt... Good to know. ( I have a custom rear wheel, so I could remove teeth from that sprocket for "half steps" too)

 

[Onder]

Just a note to say I have stock 19" TT100 tyres. The bike is an 850 so probably has
a bit more torque than a 750 and most importantly Im 9 stone kit included. That
is a lot less than most riders. The bike is not seriously hampered at any speed but
obviously a bit less sparkling. Ill keep what I have for the rest of the season and
see if I simply get used to it.

 

[Fast Eddie]

9 stone eh?

Nothing special about that...

I was 9 stone too...

When I was 12...!