Sprocket and Clutch

Racing K Models

Sprocket and Clutch

Postby DaveAus » Wed Oct 26, 2022 1:27 am

Hi,

I’m still working away on getting my KR on the track and I’ve moved onto the clutch area.

My aim is to use the bike as a road racer and at the moment it has a 25t engine sprocket. I was wondering what the recommendation is for ideal size.

Also the clutch basket cover has holes drilled around it so oil can get into into the clutch basket. I was wondering if I’m better off buying a new cover and running a dry clutch or is the current set up ok.

Thanks
David
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Re: Sprocket and Clutch

Postby panic » Fri Oct 28, 2022 3:32 pm

what the recommendation is for ideal size

What top speed will you reach at the fastest track?
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Re: Sprocket and Clutch

Postby Ferrous_Head » Fri Oct 28, 2022 6:50 pm

Road Racing is generally all about top speed. At any track.
You gear for the fastest part of the track. Front or back straight. If the straights are long enough for you to reach the bikes maximum capable speed, then you gear for that. eg if the bike will pull 115 MPH, that's where you aim your gearing.

The 25T engine sprocket is too small, meant for the dirt. You really need the 34.

Tyre manufacturers will give you the circumference of the tire your using. It then just becomes a matter of math working out the final drive ratio to determine the speed reached at maximum revs.

To win, you need to be fast in the fast places.
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Re: Sprocket and Clutch

Postby DaveAus » Sun Oct 30, 2022 6:31 am

Thanks for the advice.

I’m looking to run the drum on the left side so I can change rear sprockets too.

I’ll go out and get a 34t sprocket and for that.

What’s your view on the wet vs dry clutch?
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Re: Sprocket and Clutch

Postby Ferrous_Head » Sun Oct 30, 2022 8:44 am

OK, Wait, let's be clear on our terms here.

The engine sprocket is on the primary drive. HD actually refer to it as a "motor sprocket". HD made several size sprockets for the engine. The smaller sizes were intended for short dirt track racing. The engine sprocket drives the clutch shell with it's integrated sprocket. That's the primary drive.

The rear wheel sprocket is driven by the transmission sprocket. Often refereed to as a "counter-shaft sprocket". But it is on the end of the main shaft.

Tp get any serious top speed you will need to be running a 34T engine (motor) sprocket.

The transmission sprockets are commonly available and cheap. They range from 15 to 24 (25 ?) and you may want several. The size you pick will depend on how much HP you have and what size rear wheel sprocket you run.

51T is commonly used on Sportsters and a good Sportster will handle a 24T transmission sprocket but your more likely to be looking at something a little smaller to start with. Get in the ball park with changes to the transmission sprocket first. Changes on the rear wheel sprocket are less "dramatic" than changes on the transmission sprocket.

Gearing changes can also require chain changes. The larger the sprockets the longer your chain needs to be.

There is no "set" gearing for the bikes. It will depend on the track. What works well at Nelson Ledges is not going to be suitable at Barbers.
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Re: Sprocket and Clutch

Postby panic » Sun Oct 30, 2022 9:15 am

A track with a very short straightaway may produce better results with gearing that does not allow maximum speed, in favor of higher number ratios for acceleration off corners.

The overall gearing is only part of the plan. The layout of a track affects which transmission gear set to use as well.
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Re: Sprocket and Clutch

Postby Ferrous_Head » Sun Oct 30, 2022 4:17 pm

There is a basic principle to follow.

To be fast you must be fast in the fast places.

10 mph faster down the straight than everyone else is much more important than being 10 MPH faster in the corners.

At the end of the day it's a timed event. It's how long it takes you to travel the required distance.

For those with deep enough pockets there are something like 56 different internal (gearbox) ratios. They really are for "fine tuning" for a given track.

But to begin with, you must first establish the final drive ratio that will give you the highest speed in the fastest section of the race course. Yes, it's also important about how fast you get there. The shorter the straights the more important that becomes.

But the firsy syep is always going to be establishing the gearing that provides the highest speed you can achieve in the straights.

Road racing is not drag racing. Yes, being able to accelerate faster than your competition from the start and from each and every corner can help. But consider a track like Daytona. Consider a bike that can accelerate from 0 to 100 MPH in 5 seconds. But 100 MPH is the top speed of the bike. You might be first into Turn 1 but by Turn 3 your race will basically be over as the rest of the pack just considers you a moving chicane.

Work out the maximum RPM you are willing to let the engine run at, work out what you think is the top speed you need and work out the final drive ratio from that.

If you have a rear wheel that will accept sprockets smaller than 51, go there. If you can run a 45, great. Start there. Smaller sprockets = less weight (shorter chains). Plus you will be looking at smaller transmission sprockets.
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Re: Sprocket and Clutch

Postby panic » Sun Oct 30, 2022 5:11 pm

10 mph faster down the straight than everyone else
What a great idea, and all you have to do is use the right sprockets!
And have a much bigger engine...

more important than being 10 MPH faster in the corners
10 mph faster in the corner enters the straight 10 mph faster...
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Re: Sprocket and Clutch

Postby Ferrous_Head » Sun Oct 30, 2022 6:29 pm

Of course all of this is only my opinion. Others may disagree.

There is so much more to winning races than just working out gearing. That's very true.

And clearly, I am not the fastest racer in the world. After 40 years of racing I only have one single Australian Championship to show for all my efforts. Mind you I have never had a sponsor and raced on a shoestring budget.

This is how I arrange my transmission sprockets, Two of them are not on that tree as they are on race bikes at the moment. Along with the transmission sprockets I have chains for each gear. The chain boxes are marked for each gear set as in 19T/51T so changes can be made easily and quickly.


[img]
http://www.xlch.com.au/images/Gear%20Box%201.JPG[/img]


I have very little to go on other than my own racing experience. I just remember it was all "Secret Men's Business" when I first started racing and nobody offered gearing (or any other) advice to me.

How do I post images inline ?
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Re: Sprocket and Clutch

Postby Ferrous_Head » Sun Oct 30, 2022 7:12 pm

OK, I'm old. I forget things.


Gear Box 1.JPG
Transmission gears in the gearbox toolbox
Gear Box 1.JPG (55.78 KiB) Viewed 53494 times
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