Sprocket Question....

[JustinBoots]

Quick question....You can obtain almost the exact same gear ratio on a bike using more than one sprocket tooth combination so is there an advantage to having a sprocket with more or less teeth in front? Maybe a 17T sprocket has an advantage over a 20T sprocket when matched with the rear sprocket giving the same end result ratio?

[Logan #34's Dad]

You know my answer on this one. 4 holeshots at Illinois National and that was the only change.
My thoughs: it takes less effort to turn a small sprocket than a large one.

[don bassani]

Just checked my notes Rock,21 tooth on my 70 at Ill and both holeshots,it's in the clutching ! You can run a clutch set up with tall gears or short,just adjust properly..That day you geared to the clutch.It is a shame your out of cvt's cause you were just figuring it out.Now you and I can clean pre filters and take it easy and enjoy those mods !

[Logan #34's Dad]

Originally posted by don bassani
Just checked my notes Rock,21 tooth on my 70 at Ill and both holeshots,it's in the clutching ! You can run a clutch set up with tall gears or short,just adjust properly..That day you geared to the clutch.It is a shame your out of cvt's cause you were just figuring it out.Now you and I can clean pre filters and take it easy and enjoy those mods !


And SO much less Advil! LOL. I wanna know what your "butt dyno" is gonna do all day now? No more runs through the pits. Bring on the wine!....

[don bassani]

50 stock limited for Brandon ,Hetrick built and ready for human dyno,we still gonna be playing with cvt's for a few more years !!!!!

[jake55]

hey don, feel free to visit me in the pits if you want to keep the built in dyno in tune with the bigger bikes, hate to see ya lose your feel before Brandon steps up in class.

[nutech]

Rock or Don would you share any tips on clutching (shuuu like the r/t 70 motors)

[JustinBoots]

Reason I asked is my top competitor was running a 20 Front and 34 Rear and I run was running an 18 Front and 32 Rear. Ultimately I felt it didn't matter one way or the other since ultimately it comes down to the ratio, but thought I'd get a couple opinions from you guys. My son was almost neck and neck with 1st on the holeshot out of the gate to the first turn, but slowly lost ground on the track. I need to bring my RPMs up and do some clutching adjustment for that though.

Thanks for the info.

[bloodswet&gears]

The bigger sprocket in the front has less of a chance for the chain to make contact with the swing arm at the frame, but a bigger sprocket in the rear gives you less ground clearance.

[JustinBoots]

Originally posted by bloodswet&gears
The bigger sprocket in the front has less of a chance for the chain to make contact with the frame, but a bigger sprocket in the rear gives you less ground clearance.

Good points...didn't think about that.

[#404's Dad]

Originally posted by bloodswet&gears
The bigger sprocket in the front has less of a chance for the chain to make contact with the frame,

You sure about that?

[JustinBoots]

Yeah no worries....I've got a frame saver already so automatically was thinking about the swinger.

[bloodswet&gears]

Guess it all depends on what parts you are using and what class you are racing. Not sure if it's against any rule to c-notch the frame above the chain.

[EthansDad]

I've got this figured out. As I've said before, you need to look at the total ratio from crank to rear wheels in the CVT world. that is ratio from CVT, ratio from tranny, ratio at final gearing - and wheel height. to level set all this data -its a good idea to equate it to theoretical top speed (based on motor rpms and final ratio - what is the top speed it could reach - not taking into account wind resistance or other junk).


the problem in our world is our rpms are "steady" and our ratio change (that is what a CVT does). most tools for figuring out best ratio assume that the ratio is fixed, and the rpms sweep from X-Y rpms - those won't work for us - CVTs are the opposite.

so - to chart this - a number of factors are known (like sprocket size, tranny size, rpms, etc) - BUT knowing what the CVT ratio is at any point in time is tuff. What I do know from measurements - that the INITIAL CVT ratio (of a malossi overrange) is about 4:1 and the final ratio is about .75:1. shimming, CVT setup and other stuff like that will change the CVT ratio -but only change it in little bits -like maybe move it to 4.2:1 or 3.8:1 it will still be close to 4:1 for initial. for final, say you don't have heavy enough rollers-well your not going to reach .75:1, you might hit 1:1 - which really, really changes the top speed (like 25% slower).

So given all that, and a "magic" formula I found - you can build out a chart like this. not all possible gearing sizes- but enough to get the point. Also dispels the "1 tooth in the front is = to 3 in the rear" rule of thumb - that is not what I see.

Using this data:
the top speed is based on the ratio (shown), wheel height (assumed 16in), and motor top rpms (13k used). you can see in the chart what changes in ratio do. if you go down in wheel height you go down in top speed. also, up in rpms, up in speed. the other variable is your final CVT ratio - good luck finding out what that actually is...but the more your shift out, the faster you go. i.e. .75:1 is shifted out more than 1:1.

Tranny:1 is the gear selection the 11 for example is 11:1

front/rear sprocket - what it is!

drive ratio -just the ratio of the front/rear sprockets.

total ratio - what it says! assumes .75:1 CVT final - no wheel height in this ratio

top speed - max theoretical based on all factors.

Enjoy.

-EA

[JustinBoots]

That's very similar to the chart I made except I didn't have top speed calculated...which now that I see it is very critical..thanks. Now my only challenge back to you, and I trust you are right, but I need to ask in order to convince myself is.....

Wouldn't your top speed increase, instead of decrease with each increase in wheel/tire size? I'm running 18" tires compared to stock height 16" because my bike is producing more than enough torque so I expected the sacrifice in bottom-end take off to be minimal compared to the increase in top-speed. I just keep thinking in my mind that at a set RPM, a larger tire will cover more ground per revolution than a small tire would resulting in a higher top speed.

Please school me as I am still trying to snatch the grasshopper from your hand. :D

[EthansDad]

you're right. changed the original post to correct. up in wheel height = up in top speed.

BTW - measure your wheel height, don't go on what's stamped on the side. my 18" wheels are 15.75 inches tall.

-EA

[JustinBoots]

Woohooo! Grasshopper will be mine one day! LOL

I'll measure per your suggestion, but admittedly right now I don't see the formula by looking at your chart. As long as the curve moves together based on tire size, I won't know my actual top-spee, but still can tell what gear ratio would work best depending on whether I'm running a short tight track or a long track.

[EthansDad]

I stated clearly the formula is "magic" - which means you can't see it :-)

That I won't share, but I can build this table out for any combo of gear ratio, cvt setup, motor/tire size, etc. when you start to get way off center (very big/very small sprockets) you can see that you'll cross over the tranny ratio boundaries - which is what a lot of these CVT holeshot pros are talking about. when to go big, when to use one setup VS another - that part I'm trying to figure out too. this chart just helps me visualize it better.


-EA

[#404's Dad]

Originally posted by bloodswet&gears
. Thank you for checking my post.


Surely not checking your post by any means, got much better things to do, just reading what was new today on here and read your post and even though I knew better I still had to go look at a DRR just to be sure I wasn't crazy lol.

Been doing this for a while and ya had me questioning myself, imagine a new to this guy reading that and maybe not coming back to this topic to follow up on it and see your correction.


EthansDad, nice work as usual, chart will surely be helpful to many.

[jakesdad6]

I wish you could dumb this down a little so I could understand it. What is the 11 9 and 7 mean under tranny is that the gearing. What tranny come stock in a Drr 70. Use there tranny other tranny gear you would prefur. Have you ever ran a 17 front didn't see that on the chart

[jerkyboy]

11.1 9.1 7.1 internal tranny ratios.

[jakesdad6]

And how do you acheive them ratios yous what tranny gears

[EthansDad]

hey Jake - to get different tranny ratios, you switch out your tranny gears - there are three different kinds.

As for "making it simpler" - that is why I put this in terms of max speed. I don't get ratio either. if someone was to tell me which is better - a bike with a 1.6 or a 1.94 ratio, I'd just look at them like they were trying to explain atomic power to me. I do get top speed changes and can figure in my mind, based on track/rider if I want to accelerate more (lower top speed) or run it out (higher top speed).

for your Q on sprocket like what does a 17T do, you can figure that from this chart.

for example - look at just the 9:1 tranny gear numbers..then look at a fixed rear sprocket, and what the speed does when you just change the front.

20:35 = 52.39mph
19:35 = 49.82mph
18:35=47.84mph
17:35 = ?

I'd go out on a limb here and say dropping to a 17 will reduce my top speed by about 2mph, which will give me more acceleration.

so there is a simple rule of thumb - drop 1 tooth in front, drop 2mph, drop 1 tooth in rear, gain 1 mph.

you can also use that drop 2mph in front / gain 1 mph in back rule of thumb to figure sprocket sizes. so say you start at a 19:35 and then drop to a 17T front, but want the same top speed as before -what rear do you need?

I dropped 2T in front, so 4mph slower, so I need at least 4T LESS in the rear which would be 31T. since they don't make that, I'd go with a 32T. I bet if you go to rebel gears and look at the ratios between 17:32 and 19:35 they are close, yes? someone check me. so we get to the same result on gear selection, just using MPH to help guide instead of ratio.

you can use this MPH figuring on tranny gears too...

11:1 with 19:35 sprockets = 40.76mph
9:1 with 19:35 sprockets = 49.82mph
7:1 with 19:35 sprockets = 64.06mph

not as clear here, but still usable. Say I start in in the middle -9:1. If I drop to 11:1, I loose 9mph, and if I jump to 7:1 I pick up 15mph.

If you make a tranny switch, and want to get your sprockets close to before, then figure out the combo of sprockets to pick up 9mph, or loose 15mph. make since?

[jakesdad6]

Do you have a prefrence on dropping the front or rear to get better bottom end. Like for holeshots

could you tell me the 3 tranny gears you where talking about ??

If I gear my sprokets for bottom end to help on the holeshots but run high speed tranny gears does that help maintain both good bottom and top end?

[EthansDad]

Do you have a prefrence on dropping the front or rear to get better bottom end. Like for holeshots
nope - its Trial and Error for me. just put this together for those trying to answer that Q.


could you tell me the 3 tranny gears you where talking about ??
if you call any DRR shop and say "I want the 9:1 tranny gears, or 11:1" they will know. same gears, just plus/minus a few teeth to get the different ratios.



If I gear my sprokets for bottom end to help on the holeshots but run high speed tranny gears does that help maintain both good bottom and top end?
let us know when you find out! I'm no expert here, but I'm going to come back to ratio is ratio is ratio no matter how you get there. is there an advantage in making a higher part of your ratio in the tranny VS final gearing, or is it best to have the CVT do all the heavy lifting on ratio? I really don't know, but I do think that is the Q to answer.

[jolokken13]

I personally would not try to gear off tire size use that as a last resort. Everyone knows the lighter something is the easier it is to spin (less hp used). Run a light tire (small tire) unless you need it for ground clearance. It's much easier to turn sprockets then a big tire. That's why you see all the flat track guys running 12 inch tires and just gear the hell out of it.