Setup gearing for an MX track

WeDaFaKaWe · 06-27-2008, 06:36 AM

How do mechanics decide on what sprockets to use for an MX track?

Please help me out with some issues.

With a 5 speed gear box I would guess that the fastest you can go on the bike would be flat out in fifth gear. So on a motocross track unless you’re hitting the rev limiter in fifth at some point, you’re never using the bike to its full potential.

Often I’ve overheard people and mechanics saying that you should bottom your suspension at least once somewhere on the track or your bike isn’t setup correctly.

Would that be the same for the gearing?

Surely if you can’t hit the rev limiter in fifth somewhere on the track your bike isn’t geared (sprocketed – I don’t think that’s a real word) correctly.

Is there more to it? If the standard sprockets mentioned in the service manual for a 2007 KX250f only go up to 50 tooth rear why do the aftermarket companies make much bigger sprockets (Renthal 52 tooth)? Or is that only for use when you change the front sprocket as well?

tigmaster · 06-27-2008, 08:25 AM

So you are using 5th on a track? Most people don't even ever see 4th on a track. However, the best solution if you would like to change gearing is to drop one tooth on the front sprocket. Still pushing stock hp, this is the best choice. You will lose rotational mass vs. gaining if you had went up in the rear. The suspension thing is true too.

Wopstar · 06-27-2008, 08:30 AM

Gearing has just as much to do with acceleration and bottom end as top speed. In fact on a MX track top end has almost nothing to do with how you gear it. You basically want to gear it so that you are in the power band as much as possible.

tigmaster · 06-27-2008, 08:34 AM

5th on a track gets me though. I can't believe it.

JThompson · 06-27-2008, 08:36 AM

One long straight away maybe....

Wopstar · 06-27-2008, 08:36 AM

Maybe an enduro course or an SX track but not on an MX track

tigmaster · 06-27-2008, 08:42 AM

Well anyways...

What Wopstar said is correct. It only matters that your bike is making peak hp and torque as much of the time as possible. Therefore we would need to know average speed on the track, top speed, and lowest speed (obviously 0 but you know). Then I could math hard on it and tell you things.

tjscott2012 · 06-27-2008, 08:47 AM

or maybe a drag strip lol! i dont know about the whole suspension thing though, the only time i bottom out at the track is if i screw up and overshoot the landing. then i hit the ground and bottom out, but if i had a perfect day, i dont think i would ever bottom out

tigmaster · 06-27-2008, 08:50 AM

Then you are not riding hard enough. The pros even say it is ok and that they do bottom out once or twice even with their ridic stiff suspension.

Wopstar · 06-27-2008, 08:53 AM

Quote:

Originally Posted by tigmaster

Then you are not riding hard enough. The pros even say it is ok and that they do bottom out once or twice even with their ridic stiff suspension.

I have heard that as well.

06-27-2008, 06:36 AM	#1 (permalink)
WeDaFaKaWe C Rider Join Date: Jan 2008 Posts: 1	Setup gearing for an MX track How do mechanics decide on what sprockets to use for an MX track? Please help me out with some issues. With a 5 speed gear box I would guess that the fastest you can go on the bike would be flat out in fifth gear. So on a motocross track unless you’re hitting the rev limiter in fifth at some point, you’re never using the bike to its full potential. Often I’ve overheard people and mechanics saying that you should bottom your suspension at least once somewhere on the track or your bike isn’t setup correctly. Would that be the same for the gearing? Surely if you can’t hit the rev limiter in fifth somewhere on the track your bike isn’t geared (sprocketed – I don’t think that’s a real word) correctly. Is there more to it? If the standard sprockets mentioned in the service manual for a 2007 KX250f only go up to 50 tooth rear why do the aftermarket companies make much bigger sprockets (Renthal 52 tooth)? Or is that only for use when you change the front sprocket as well?

06-27-2008, 08:25 AM	#2 (permalink)
tigmaster Administrator Join Date: Aug 2006 Location: Missouri, United States Posts: 7,229	So you are using 5th on a track? Most people don't even ever see 4th on a track. However, the best solution if you would like to change gearing is to drop one tooth on the front sprocket. Still pushing stock hp, this is the best choice. You will lose rotational mass vs. gaining if you had went up in the rear. The suspension thing is true too. __________________ --'03 CRF450r-- --FMF Pipe, Pro-Tech Revavle-- --Vortex Basket With Tusk Disks And HP Springs-- --Pro Taper Triple Clamps And Bars-- --Devol Full SP With Moose HGs-- --Renthal 13/50 With R1 Chain-- --Now It Is The End, Probably...-- --'06 CBR F4I-- --Bone Stock-- Check Our Latest How-To Here

06-27-2008, 08:30 AM	#3 (permalink)
Wopstar A Rider Join Date: Sep 2006 Location: Lake Havasu Arizona Posts: 858	Gearing has just as much to do with acceleration and bottom end as top speed. In fact on a MX track top end has almost nothing to do with how you gear it. You basically want to gear it so that you are in the power band as much as possible. __________________ Stickers Add Horsepower!!

06-27-2008, 08:34 AM	#4 (permalink)
tigmaster Administrator Join Date: Aug 2006 Location: Missouri, United States Posts: 7,229	5th on a track gets me though. I can't believe it. __________________ --'03 CRF450r-- --FMF Pipe, Pro-Tech Revavle-- --Vortex Basket With Tusk Disks And HP Springs-- --Pro Taper Triple Clamps And Bars-- --Devol Full SP With Moose HGs-- --Renthal 13/50 With R1 Chain-- --Now It Is The End, Probably...-- --'06 CBR F4I-- --Bone Stock-- Check Our Latest How-To Here

06-27-2008, 08:36 AM	#5 (permalink)
JThompson Administrator Join Date: Aug 2006 Location: Bellingham WA Posts: 4,614	One long straight away maybe.... __________________ Tired of ads? Become a member for an almost ad free site! Also we give away monthly prizes just for posting! Register today! Looking for places to ride! Want to review a riding area? Check them out! PRE-ORDER MOTOTIPS Stickers now. Help keep mototips running! Get your order in today! Buy NOW! Special Discounts for members only! Check out our new discount forum for retailers offering to help you save money! Mototips Discounts

06-27-2008, 08:36 AM	#6 (permalink)
Wopstar A Rider Join Date: Sep 2006 Location: Lake Havasu Arizona Posts: 858	Maybe an enduro course or an SX track but not on an MX track __________________ Stickers Add Horsepower!!

06-27-2008, 08:42 AM	#7 (permalink)
tigmaster Administrator Join Date: Aug 2006 Location: Missouri, United States Posts: 7,229	Well anyways... What Wopstar said is correct. It only matters that your bike is making peak hp and torque as much of the time as possible. Therefore we would need to know average speed on the track, top speed, and lowest speed (obviously 0 but you know). Then I could math hard on it and tell you things. __________________ --'03 CRF450r-- --FMF Pipe, Pro-Tech Revavle-- --Vortex Basket With Tusk Disks And HP Springs-- --Pro Taper Triple Clamps And Bars-- --Devol Full SP With Moose HGs-- --Renthal 13/50 With R1 Chain-- --Now It Is The End, Probably...-- --'06 CBR F4I-- --Bone Stock-- Check Our Latest How-To Here

06-27-2008, 08:47 AM	#8 (permalink)
tjscott2012 The Spam man Join Date: Nov 2007 Location: Hurley, Mo Posts: 4,197	or maybe a drag strip lol! i dont know about the whole suspension thing though, the only time i bottom out at the track is if i screw up and overshoot the landing. then i hit the ground and bottom out, but if i had a perfect day, i dont think i would ever bottom out __________________ This is my signature. Its awesome

06-27-2008, 08:50 AM	#9 (permalink)
tigmaster Administrator Join Date: Aug 2006 Location: Missouri, United States Posts: 7,229	Then you are not riding hard enough. The pros even say it is ok and that they do bottom out once or twice even with their ridic stiff suspension. __________________ --'03 CRF450r-- --FMF Pipe, Pro-Tech Revavle-- --Vortex Basket With Tusk Disks And HP Springs-- --Pro Taper Triple Clamps And Bars-- --Devol Full SP With Moose HGs-- --Renthal 13/50 With R1 Chain-- --Now It Is The End, Probably...-- --'06 CBR F4I-- --Bone Stock-- Check Our Latest How-To Here

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