Powerband, Gearing, and Sprocketing

After a hell a lot of headbreaking, i think we are finally reaching a simple conclusion as to what's holding us back from ETs in the low 13s - we are simply dropping out of the powerband - during the second shift or so - and never climbing back in. And on a pepped up 2stroke, if you are out of the powerband, you're going nowhere fast.

While pondering over it, hacked up a simple small C program which prints out a simulation of a run, provided a few basic paramters - at what rpm the powerband starts, what's the max rpm limit in 1st gear (for stock sprocketing), and if needed, the front and rear sprocket sizes. the following assumptions are made:

1. The bike revvs 1000 rpm less in a gear as compared to the next lower gear.
2. A higher sprocketing allows a higher rpm limit.
3. A linear relationship is assumed for the above.

This is the code. Maybe calling it a program and all is an exaggeration - all it does is a few multiplications and divisions.

/\*
 \* pb.c - Compile as:
 \* gcc -o pb pb.c
 \* 
 \* Example usage:
 \* pb 8000 12000
 \*/

#include <stdio.h>

const double RATIO_FIRST = 2.571; 
const double RATIO_SECOND = 1.777; 
const double RATIO_THIRD = 1.318;
const double RATIO_FOURTH = 1.040;
const double RATIO_FIFTH = 0.888;
const double RATIO_SIXTH = 0.785;

const double STOCK_FRONT_SPROCKET = 15;
const double STOCK_REAR_SPROCKET = 40;

int pbstart;
int rpmlimit;

void error(const char \*s, int r)
{
        printf("Aieeee! you dropped out of powerband in %s (rpm %d)...smell smoke
        and DIE!\\n", s, r);
}

void info(const char \*s, int r)
{
        printf("into %s, and still in powerband (rpm: %d)\\n", s, r);
}

void shift(const char \*gear, float ratio)
{
        int newrpm = rpmlimit \* ratio;
        if (newrpm < pbstart)
                error(gear, newrpm);
        else
                info(gear, newrpm);
}

int main(int argc, char \*argv[])
{
        if (argc != 3 && argc != 5) {
                fprintf(stderr,
                        "Usage: %s   [front sprocket size] [rear sprocket size]\\n",
                                argv[0]);
                exit(1); 
        }
        
        pbstart = atoi(argv[1]);
        rpmlimit = atoi(argv[2]);
        if (pbstart <= 0 || rpmlimit <= 0 || rpmlimit <= pbstart) {
                fprintf(stderr, "error: bad input values for
                                   powerband start / rpm limit\\n");
                exit(2);
        }       
        
        if (argc == 5) {
                double fs = 0, rs = 0, sprocketing = 0;
                fs = atoi(argv[3]);
                rs = atoi(argv[4]);
                if (fs == 0 || rs == 0) {
                        fprintf(stderr, "error: bad input values for front/rear
                                         sprocket sizes\\n");
                        exit(3);
                }
                sprocketing = (rs / fs)
                                        /
                        (STOCK_REAR_SPROCKET / STOCK_FRONT_SPROCKET);
                rpmlimit \*= sprocketing;
        }


        // Assume that the max rpm decreases by 1000 as we go up a gear
        // This is not probably a very logical assumption, but you
        // will agree with me that it's something similar that happens
        // in real life.
        shift("2nd gear", RATIO_SECOND / RATIO_FIRST);
                rpmlimit -= 1000;
        shift("3rd gear", RATIO_THIRD / RATIO_SECOND);
                rpmlimit -= 1000;
        shift("4th gear", RATIO_FOURTH / RATIO_THIRD);
                rpmlimit -= 1000; 
        shift("5th gear", RATIO_FIFTH / RATIO_FOURTH);
                rpmlimit -= 1000;
        shift("6th gear", RATIO_SIXTH / RATIO_FIFTH);
}        

If we run it with our current configuration - stock gearing - this is the output:

bash-2.00$ ./pb 8000 12000
into 2nd gear, and still in powerband (rpm: 8294)
into 3rd gear, and still in powerband (rpm: 8158)
Aieeee! you dropped out of powerband in 4th gear (rpm 7890)...smell smoke and DIE!
Aieeee! you dropped out of powerband in 5th gear (rpm 7684)...smell smoke and DIE!
Aieeee! you dropped out of powerband in 6th gear (rpm 7072)...smell smoke and DIE!
bash-2.00$ 

The powerband starts at 8000 rpm, that's when it really comes on the pipe and that explosive surge appears.

So the idea is to sprocket for a shorter gearing. Say we bump up the rear sprocket to 41. Then:

bash-2.00$ ./pb 8000 12000 15 41
1.025000
into 2nd gear, and still in powerband (rpm: 8501)
into 3rd gear, and still in powerband (rpm: 8381)
into 4th gear, and still in powerband (rpm: 8127)
Aieeee! you dropped out of powerband in 5th gear (rpm 7940)...smell smoke and DIE!
Aieeee! you dropped out of powerband in 6th gear (rpm 7337)...smell smoke and DIE!
bash-2.00$ 

As seen above, we drop out only on the shift to 5th. Suppose we bump the rear to 42 now:

bash-2.00$ ./pb 8000 12000 15 42
1.050000
into 2nd gear, and still in powerband (rpm: 8708)
into 3rd gear, and still in powerband (rpm: 8603)
into 4th gear, and still in powerband (rpm: 8364)
into 5th gear, and still in powerband (rpm: 8196)
Aieeee! you dropped out of powerband in 6th gear (rpm 7602)...smell smoke and DIE!
bash-2.00$ 

More improvement...That should be enough actually, because i don't think we are gonna be using sixth.

Comments:

thanksssssssss

Posted by oyun indir on mars 13, 2009 at 12:47 PD PDT #

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