RPM range

[ClayBo2009]

After reviewing a few months of RPM data from the Alfano on my sons kart, there appears to be a fair amount of variability in how low his RPM get in the slowest/tightest turns of the track. I know some of it is due to atmospheric conditions but it seems that it could also be correlated to different gearing, or maybe how hard/soon he brakes going into a hairpin. Ultimately, I am wondering if there are ways to get the RPMs to fall off quicker. During races the engine is above 4000 RPM everywhere except the tightest turns, but peak torque on the 206 with yellow slide is in the low to mid 3000s so I am wanting the engine to shed RPM as fast as possible going into those turns so he can get the max torque coming out. We typically run two black and two white springs in the Hilliard clutch so I don’t see us getting to a place where the shoes will totally disengage during a race. Any tuning tips or driver style tricks that can help drop the RPM closer to the sweet spot? Thanks!

 

[Flash765]

After reviewing a few months of RPM data from the Alfano on my sons kart, there appears to be a fair amount of variability in how low his RPM get in the slowest/tightest turns of the track. I know some of it is due to atmospheric conditions but it seems that it could also be correlated to different gearing, or maybe how hard/soon he brakes going into a hairpin. Ultimately, I am wondering if there are ways to get the RPMs to fall off quicker. During races the engine is above 4000 RPM everywhere except the tightest turns, but peak torque on the 206 with yellow slide is in the low to mid 3000s so I am wanting the engine to shed RPM as fast as possible going into those turns so he can get the max torque coming out. We typically run two black and two white springs in the Hilliard clutch so I don’t see us getting to a place where the shoes will totally disengage during a race. Any tuning tips or driver style tricks that can help drop the RPM closer to the sweet spot? Thanks!

Roll speed in a corner is going to be way more important than trying to get him to hit a certain low RPM based on the published torque curves. The varying RPM in the same corner is going to be due to inconsistency in his driving most likely.

 

[95 shaw]

After reviewing a few months of RPM data from the Alfano on my sons kart, there appears to be a fair amount of variability in how low his RPM get in the slowest/tightest turns of the track. I know some of it is due to atmospheric conditions but it seems that it could also be correlated to different gearing, or maybe how hard/soon he brakes going into a hairpin. Ultimately, I am wondering if there are ways to get the RPMs to fall off quicker. During races the engine is above 4000 RPM everywhere except the tightest turns, but peak torque on the 206 with yellow slide is in the low to mid 3000s so I am wanting the engine to shed RPM as fast as possible going into those turns so he can get the max torque coming out. We typically run two black and two white springs in the Hilliard clutch so I don’t see us getting to a place where the shoes will totally disengage during a race. Any tuning tips or driver style tricks that can help drop the RPM closer to the sweet spot? Thanks!

You may have a common misconception of the role of peak torque and horsepower.

Torque is your engines ability to move a load over distance, ie, lb/ft.

The work your engine does is measured by the load over a distance over a specified time. Lb/ft per second.

Horsepower is the work your engine does, compared to a known quantity. Lb/ft/sec divided by 550.
One horsepower is defined as moving 550 lb one foot in one second.

A look at the engines torque curve shows the max load the engine will move, albeit at a slow rate. This is peak torque.
If we know how far it moves in a length of time, we know how much work is being done.

As rpm (a measure of distance over time) climbs, the distance the load moves climbs for a specified length of time.

At peak horsepower, the engine is doing the maximum amount of work it is capable of.

Since the weight of the kart is constant, the load stays the same. If the kart is moved farther in a second, more work is being done.
In reality, we would like to stay at or near peak horsepower because we are moving the kart farther in a length of time.

So, dropping rpm quicker is a poor idea, unless you need to slow down.


This is why everyone talks about maintaining momentum. You are moving the load farther, faster.

Hope that helps.

 

[CarlsonMotorsports]

I've found that with the yellow slide, you need to tune the clutch to a lower rpm engage than the adult slide.
Adult slide is typically 2 white and 2 black. Yellow, you can either go softer on springs, or simply add weights (if you're running a clutch like the Flame where you have room to adjust by adding weights.) By adding 1 heavy weight to each shoe, you will bring the engagement down closer to peak torque on the yellow slide (per our tune anyhow.)
Maintaining higher rpm at corner exit in tight corners seems to be the ticket to quicker lap times. Work on that, and you may find yourself pulling some rear teeth off and getting faster yet.


-----
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
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34 years of service to the karting industry ~ 1Cor 9:24
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com

 

[jjchat]

95 Shaw provides an excellent explanation that also support why you want to set you clutch stall speed at peak torque = your utilizing maximum ability to move weight a short distance (ie get the kart moving as quickly as possible) but want to spend as much time at or near peak HP.

 

[IronHorse]

When you are significantly over weight (400# in a 375# class) where should I be on clutch tuning? I have been running my Bully with 4 black springs with no weights.

 

[mike97760]

You might consider trail braking through that slowest corner in an effort to keep the rpm's up.

 

[gary10]

Riding the brakes is not trail braking. It’s probably the most improperly used term in karting. Trail braking is simply braking while you are still turning. It has nothing to do with the gas pedal.

 

[GT5050]

New to posting here - thank you Gary for correctly pointing out what trail braking is and isn’t. I would add to that that I have my doubts as to whether riding the brakes is helpful at all at keeping RPMs up in most corners unless truly slow enough to where the clutch disengages, which is rare and even then carrying good speed and getting back to throttle early would be my goal, not riding two pedals at once. From personal experience and looking at data from my driver and others, even with lifting I’ve never seen RPM drop below what correlates to x speed in a corner. The engine simply doesn’t have time to run down to the point where it needs to be spooled back up, except maybe a reeeaallly long lift in a really slow corner from a novice driver. From my perspective riding the brakes changes the way the chassis behaves (for good or bad), but best way to keep RPM up is just roll good speed through corner

This goes back to the OPs post, where you want to bring RPM down quickly. I would never dream of wanting to get his rpm down lower to get into some torque curve (gearing notwithstanding). More RPM on a given gear = more speed (exception is clutch slip), more speed = less time through the corner and faster out of it. In a Briggs there’s not much that’ll replace rolling speed. That said as was mentioned earlier, as corner speeds come up to ideal perhaps gear can come down.