Topic: Road racing final drive gearing??

[slikwilli420]

With my new engine going in for this year and the additional top end RPM I will gain over a stock engine, what are thoughts for gearing? Last year was my rookie year and with a stock engine I ran 17/46. Im thinking that I would go up and down one size in the rear to have a range of 45-47 but then I started thinking about the additional RPM I will have available over the stock and figured I might need some more change. The reduced gyroscopic effect of a lightened crank vs a stock one will mean I need to stay higher in the revs as well. Vortex sells quick change sprockets in 520 pitch for a good price on ebay but I still don't intend to buy 6-8 gears, but would like to know what range would be most useful.

[turboguzzi]

The amount of variables that impact gearing is really large. Youre mostly mentioning tech/bike related stuff but what about track layout, rider skill/ lap times, riding style?

So I think that for baseline you need to talk with someone who rides a CB750 in the tracks you are going to ride. Cant believe you'll vary more than +1/-1 on the rear compared to that baseline. Add +1/-1 front sprockets and you are really well covered, no need to for 6-8 different rears (and such a wide range would require different chain lengths too). better to mix and match front and rear sprockets.

[slikwilli420]

I know there are a lot of factors at play, but the gearing likely doesnt change a ton from track to track, except for Road America, which has some of the longest straights of any track in the US. I have 17/46 gearing now, and was interested in understanding where to go from here as a general rule, considering that my bike will have a higher RPM ceiling and due to more skill on my part and lighter weight on the bike's part, a higher top speed vs last year.

[simon#42]

i changed gearing at every circuit and i recommend you do the same . unfortunately you are stuck with your individual ratios but you can change the overall
ratio to good effect . its not just about being flat out in top on the back straight sometimes having the right gear for certain corners is better for lap times .
the joy of racing purpose built race bikes was the option of seven different ratios for every gear .

[slikwilli420]

That makes total sense Simon. With me going into my second year and hopefully having shaken off all the bugs that needed working out last year, I can concentrate on fundamentals as well as refining my riding. I will be on a much faster bike this year and want to make the most out of it and with that will come more fine tuning for each circuit.

[Rocketman]

Unfortunately you will probably find each track will require its own unique gearing.  For example, here is what I run on the tracks I race on with my CB550 (stock is 17/37):

15/37 Barber
16/36 Mid-Ohio
16/37 Mosport
17/37 Road America

With my new close ration 5th gear this will all change this season.  I have 15 to 18 front and 35 to 39 rear sprockets.  Best case is to get a recommendation from someone running a CB750.  Otherwise I'd get at least a 16 front and a 45 & 47 rear to start with.  If you plan to race Barber you may also want a 15 front.

[Haybus]

Matt, I found it useful to have video of my tach to make sure I was hitting peak hp at the end of the straight, or thereabouts. That's a good starting point, then make sure you can use that gearing for getting out of important corners. I have a bad bog below 6500 so I used the video to understand what gear I needed coming out of a couple corners at our 2 tracks. Tougher when you only race a track once a year. I'll need to watch those videos again here pretty soon!! Ha ha.

Here's to hoping your bike revs to the moon and has gobs of torque out of the corners!!

Alan

[bwaller]

Simon is right, sometimes gearing properly for a long straight will make it suffer elsewhere, including the launch. You may find that 17/46 will be a good place to start, but you're going to get faster and that will change how you gear it.

Then as TG mentions managing gearing changes AND using the same chain saves hassles. Instead of 45/46/47 rears maybe think about 44/46/48 and two different fronts. Just for a greater difference in ratio. I have no idea what a 750 will need, but as an example calculate by the ratios to give yourself a few more options.

48/16= 3:1           48/17= 2.824
46/16= 2.875:1     46/17= 2.706
44/16= 2.75:1       44/17= 2.588

If you choose 17/45=2.647:1
                    17/46= 2.706:1
                    17/47= 2.765:1  this only gives you a 4.3% difference. Fine to fine tune if you ride a 2stroke with narrow powerband.

I manage with 2 front & 4 rears on the same chain. 2.625 to 2.117, a 19.4% difference for ten different tracks. I could probably tune finer, but 4strokes are forgiving. 

[scottly]

Matt, you must be running a small diameter rear tire to pull a 2.7 final ratio? On my 750 with a 120-18 and a 2.82 (17/48) ratio, the engine is only turning about 6250 RPM @ 92 MPH in forth gear, and over 100 MPH at 8500. Next time you have the bike on a dyno, do a pull in 5th gear, and find out how fast you can go and stay in the meat of the power band. You want the lowest gearing (highest numerically) that will allow the greatest speed you will be going on a particular track.

[slikwilli420]

Matt, you must be running a small diameter rear tire to pull a 2.7 final ratio? On my 750 with a 120-18 and a 2.82 (17/48) ratio, the engine is only turning about 6250 RPM @ 92 MPH in forth gear, and over 100 MPH at 8500. Next time you have the bike on a dyno, do a pull in 5th gear, and find out how fast you can go and stay in the meat of the power band. You want the lowest gearing (highest numerically) that will allow the greatest speed you will be going on a particular track.

Scottly, the rear tire is the Avon AM road racing tire in 18" so its standard size. 17/46 was recommended given the type of riding I was going to do and the faster speeds I needed to achieve.

Simon is right, sometimes gearing properly for a long straight will make it suffer elsewhere, including the launch. You may find that 17/46 will be a good place to start, but you're going to get faster and that will change how you gear it.

Then as TG mentions managing gearing changes AND using the same chain saves hassles. Instead of 45/46/47 rears maybe think about 44/46/48 and two different fronts. Just for a greater difference in ratio. I have no idea what a 750 will need, but as an example calculate by the ratios to give yourself a few more options.

48/16= 3:1           48/17= 2.824
46/16= 2.875:1     46/17= 2.706
44/16= 2.75:1       44/17= 2.588

If you choose 17/45=2.647:1
                    17/46= 2.706:1
                    17/47= 2.765:1  this only gives you a 4.3% difference. Fine to fine tune if you ride a 2stroke with narrow powerband.

I manage with 2 front & 4 rears on the same chain. 2.625 to 2.117, a 19.4% difference for ten different tracks. I could probably tune finer, but 4strokes are forgiving. 

That makes more sense Brent. I would probably use the 17 and 18 up front as 16 is pretty small for a 750, but still go one tooth off of 46 and do 44 and 48, giving me a 15.5% range of final drive ratios.

Matt, I found it useful to have video of my tach to make sure I was hitting peak hp at the end of the straight, or thereabouts. That's a good starting point, then make sure you can use that gearing for getting out of important corners. I have a bad bog below 6500 so I used the video to understand what gear I needed coming out of a couple corners at our 2 tracks. Tougher when you only race a track once a year. I'll need to watch those videos again here pretty soon!! Ha ha.

Here's to hoping your bike revs to the moon and has gobs of torque out of the corners!!

Alan

Ive been thinking about a GoPro for a while now Alan. I wanted to get some track video but having film of my tach would be useful as well. With my new engine it will be especially important to not bog down and stay in the right RPM range for each corner.

[bwaller]

Matt, again the 750 guys will help with better direction on selection but yeah having a better spread in ratio is wise, especially if we don't want to carry tons of gearing options. Simon can attest during the 2stroke heyday guys had so many sprocket options in their spares kit it was a whole different science!

I never caught the bug, but Tyler is big on watching race vid. Asap after a race he goes over it to see where he can do better. I still say walking the track is important, so then combining that with GoPro coverage likely is best. Hell I used to walk the track backwards sometimes to find turn in points. 

[simon#42]

have a look at data loggers , you can get them much cheaper than you think . much better than a go pro , you can check your revs as well as
oil temp , head temp , oil pressure or you can go further with suspension travel lap timer etc etc .  have a look at some of the kart websites to
see some basic ones that you will find very useful .

[scottly]

Dennis went 137 MPH in 5th gear with 17/45 sprockets at Bonneville, and that was too high of gearing to achieve the maximum speed the bike was capable of. 
http://forums.sohc4.net/index.php?topic=135473.350

 

[slikwilli420]

Dennis went 137 MPH in 5th gear with 17/45 sprockets at Bonneville, and that was too high of gearing to achieve the maximum speed the bike was capable of. 
http://forums.sohc4.net/index.php?topic=135473.350

There is under a 2% difference between my 17/46 and stock 18/48 gearing. I would think for a top speed run, I would be having an 18 or 19 front and low 40's rear to get that ratio well under the stock numbers. My bike pulled well for a stocker in that gearing, likely because it was so close to stock specs.

[slikwilli420]

have a look at data loggers , you can get them much cheaper than you think . much better than a go pro , you can check your revs as well as
oil temp , head temp , oil pressure or you can go further with suspension travel lap timer etc etc .  have a look at some of the kart websites to
see some basic ones that you will find very useful .

Simon, I would pair video with data as well as GPS for track location. Software that Tyler uses allows a lot of that capability I believe. At this point, I want to see how I am entering/exiting corners as well as RPM to see where I can reduce lap times.

[Haybus]

+1 for walking the track. Best with other racers that want to talk about corner entry and exit. It's funny how you perceive where you are in most corners while walking the track, then realize you aren't there the next day during practice. That's come up a couple times with the friends I typically walk the track with. BTW I'm currently running 16/48 and like it a lot.

Alan

[scottly]

Dennis went 137 MPH in 5th gear with 17/45 sprockets at Bonneville, and that was too high of gearing to achieve the maximum speed the bike was capable of. 
http://forums.sohc4.net/index.php?topic=135473.350

There is under a 2% difference between my 17/46 and stock 18/48 gearing. I would think for a top speed run, I would be having an 18 or 19 front and low 40's rear to get that ratio well under the stock numbers.

And you would be moving in the wrong direction. Dennis' 17/45 was 2.647:1, while 18/48 is 2.666:1; very close. The stock gearing was best suited to cruising down the super-slab at 80 MPH at a mild RPM.  He was able to hit 140 by changing the sprocket ratio and only using 4th gear, which allowed the motor to rev further into the sweet spot. I calculated the equivalent ratio for 5th gear would be 17/50, or 2.941:1. My bike is going 105 MPH at 8500 RPM with 17/48 in 4th, and hits 110 below 9000. I ran a 16/48 combo on the street for years, but found out during the first relay that it wasn't a good ratio for cruising at 75 MPH, as I found myself looking for another gear. The 16/48 combo pulled much better, and unless you are planning on hitting over 130MPH on the fastest straights, I would consider a larger rear sprocket.
Alan, a 16 tooth front sprocket is harder on the chain than a 17 or 18; a 17/51 would be the same 3:1 ratio as 16/48.

[slikwilli420]

Dennis went 137 MPH in 5th gear with 17/45 sprockets at Bonneville, and that was too high of gearing to achieve the maximum speed the bike was capable of. 
http://forums.sohc4.net/index.php?topic=135473.350

There is under a 2% difference between my 17/46 and stock 18/48 gearing. I would think for a top speed run, I would be having an 18 or 19 front and low 40's rear to get that ratio well under the stock numbers.

And you would be moving in the wrong direction. Dennis' 17/45 was 2.647:1, while 18/48 is 2.666:1; very close. The stock gearing was best suited to cruising down the super-slab at 80 MPH at a mild RPM.  He was able to hit 140 by changing the sprocket ratio and only using 4th gear, which allowed the motor to rev further into the sweet spot. I calculated the equivalent ratio for 5th gear would be 17/50, or 2.941:1. My bike is going 105 MPH at 8500 RPM with 17/48 in 4th, and hits 110 below 9000. I ran a 16/48 combo on the street for years, but found out during the first relay that it wasn't a good ratio for cruising at 75 MPH, as I found myself looking for another gear. The 16/48 combo pulled much better, and unless you are planning on hitting over 130MPH on the fastest straights, I would consider a larger rear sprocket.
Alan, a 16 tooth front sprocket is harder on the chain than a 17 or 18; a 17/51 would be the same 3:1 ratio as 16/48.

What you're saying contradicts what I keep hearing, hence my confusion. I get there is a sweet spot for any motor, but generally a larger front and smaller rear is needed for more top speed. The gearing you quoted (17/45) is theoretically 19mph slower than a 19/44 combination. While I do need torque to drive out of the corners, I will also need to get my top speed at 130-140 on the longer stretches.

[bwaller]

Often a long straight is preceded by a hairpin so it's most important to get off that corner quickly. I can't imagine you'll ever want a 19T front plus what scottly says is true about small front sprockets working chains.

Whatever you used last year should be your baseline. Your engine will be stronger and the absolute biggest boost you'll feel is acceleration and that's what you want. You spent that money shedding weight for that very reason too. Acceleration and corner speed will win the day. Good top end is important, but who get's there first is moreso. Keep thinking towards gearing from your baseline first and if you run it into the limiter way before the end of a straight, it'll be easy to know what to do. If over geared I think sometimes we get complacent.

Your getting it done Matt.

[slikwilli420]

That works for me Brent. I went with wider spread of rear sprockets for more change and now will have 44/46/48 rears along with an 18 front if I can find it. Rebel Gears no longer does custom fronts. The gave me a number for a guy but nothing yet.

[bwaller]

Doesn't Kenny have 520 sizes?

[slikwilli420]

Doesn't Kenny have 520 sizes?

Only 17 and 19 tooth, not 18 tooth.

[Rocketman]

Sprocket Specialist may be able to help.

http://sprocketspecialists.com/

I always call and so far they have had (or made) all the 520 gearing I needed.  1-800-782-8200

[gschuld]

Sprocket Specialist may be able to help.

http://sprocketspecialists.com/

I always call and so far they have had (or made) all the 520 gearing I needed.  1-800-782-8200

Same here...

George

[bwaller]

I like SS and always use their rear alloy sprockets. I just don't like the full narrow 520 fronts they sell. Do they still use that "C" clip front sprocket attachment?

[Rocketman]

The fronts I sourced from Sprocket Specialist use the stock Honda retainer that fastens to the sprocket with two 6mm bolts.  Can't recall ever closely comparing them to a OEM Sprocket to see if there were any differences.

[bwaller]

That's good about using a stock retainer, early ones didn't. I prefer a thicker spline section, not always easy to find.

[slikwilli420]

PBI was the original supplier of my 17 tooth front through rebel gears. They had the thicker flange at the spline. I'll try them and report back.

[gschuld]

FWIW, at Mosport(2016), both of Harry Peters cb750 racers ran 17/46, while a stable mate in the 412 bike(with factory carbs) ran 17/47 gears.  Mosport is considered a fast track by most on account of the LONG slightly uphill section(+/- 130mph by the end).  The Harry Peters bikes(both factory frame and the Rickman) were fast...

George

[bwaller]

That's helpful George. Justin's #412 is a nearer stock engine, but Harry's is the same size bore Matt will use. I think Justin should use an larger rear for there.