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Message started by badwolf on 11/27/15 at 18:40:14

Title: Magic gear numbers
Post by badwolf on 11/27/15 at 18:40:14

Alright, we have no stock tach on our bikes. Folks have posted charts to tell us our speed vs. rpm in all gears. But if you, like I have changed final drive ratios, and or tire sizes, you just have to make a guess at your rpm at speed. NO MORE! I have done the math to get the ''magic numbers'' for each of the 5 gears.

1st - 44535
2nd - 30200
3rd - 21790
4th - 18349
5th - 16820

They don't make any sense on their own, but here is how to use them.
First, measure the circumference of your back tire. Blow it up to normal pressure, and have a friend layout a tape measure 7 feet. While sitting on your bike, start with the valve stem straight down at 0 on the tape. Roll forward till the valve stem is all the way around to bottom again, have your friend read the number, this is the circumference of your back tire, write it down.
Second, write down the final drive ratio on your bike. If you are running stock pulleys, they are 23 and 68 for a ratio of 2.96 to 1.
Third, divide the tire's circumference in inches to the second decimal point by the gear ratio. Write this number down. It is the key number for your bike as setup.
Example= My bike's tire is 73.50'' and my final drive ratio is 2.60 (Kaw. pulley conversion) giving me a key number of 28.27.
Now divide each gear's magic number by your bike's key number. This will give you the engine rpm at 10 mph in each gear.
Example for my bike is -
5th gear- 16820 / 28.27 = 595 rpm at 10 mph x 6 = 3570 rpm at 60 mph
Do each gear,
Example for me-
1st = 1575
2nd = 1068
3rd = 770
4th = 650
5th = 595
You can now multiply these numbers to see your rpm at different speeds for each gear, or divide your selected rpm by these to see your speed at that rpm.
Example= Say I want to shift at 4,000 rpm,
1st - 4,000 / 1575 = 2.53968253968, or basically 25 mph
2nd - 4,000 / 1068 = 3.745, or 37 mph
3rd - 4,000 / 770 = 5.194 or 52 mph
4th - 4,000 / 650 = 6.153 or 62 mph

Now you can ''try'' different final drive ratios or size tires by making a new key number.
My bike's stock tire was just 73'' and with the stock pulleys at 2.96 to 1 the key number was 73 / 2.96 = 24.66.
So at 60 mph my old rpm was 16820 / 24.66 = 682 x 6 = 4092
My current key number is 28.27 - 16820 / 28.27 = 595 x 6 = 3570
So the Kaw pulley conversion and tire change has dropped me 522 rpm at 60 mph cruise. (Hey, I'm old and live in south Florida = no hills or twisties. I am on the Superslab a LOT and always have to throw away rear tires with the sides looking like new.)
Now you can make-up charts and graphs CORRECT FOR YOUR OWN BIKE till your heart is content.
This is in relation to REAL SPEED, not the stock speedo. Get a GPS app for your phone and you can see how much your stock speedo is off, but that is for another post.

Disclaimer...I have worked the math several times for this, and it will give you a good idea of the numbers for your bike. If someone wants to go to 29 decimal points and re-figure it they may find it could be off by a fraction. BIG DEAL!! We aren't plotting the path of the Space Shuttle here. I am giving you a way to figure how changes in your final drive and tire size will affect your rpm on YOUR BIKE.

Best wishes, Badwolf

Title: Re: Magic gear numbers
Post by badwolf on 11/27/15 at 18:56:05

About the speedo being off...
WalMart is selling cheap Android LG smartphones for $10.00! Yea they are tied to Tracfone, but you aren't going to activate it as a phone, it has full Kit Kat and the Play Store. Just go on Wi Fi and install a GPS speedo app and POOF! you have a new GPS speedo. I use DigiHUD and am very happy with it. Rig-up a mount and you may never look at the stock speedo again. Mine is off by 15% 100 miles is 115 miles on the GPS.

Title: Re: Magic gear numbers
Post by pg on 11/27/15 at 19:22:15

Take a look at this:

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1341162964

Best regards,

Title: Re: Magic gear numbers
Post by chzeckmate on 11/27/15 at 23:36:03

7E7C6F716F6D757C746F1D0 wrote:

I'm not so sure about using a cheapo tracfone for that. I'm an Android mobile developer (among other things) and I have tons of phones for testing purposes. Some phones are better than others for this and some carriers are better than others. Here's an example. I have a cheapo HTC One SV that's on a prepaid service like tracfone. If it's not active the GPS features aren't accessible. When it is active its GPS isn't all that reliable. It generally does get the 3d fix but uses only 8 satellites so the accuracy isn't great. Another example, I have a Sprint Samsung Galaxy Note II that isn't active but you can access the GPS features and it generally uses most if not all of the visible satellites. Compare the accuracy of these two old devices at the same time and you'll see the Note II using 19 satellites with pinpoint accuracy and the HTC using 8 satellites is way off. Compare that to the Samsung Galaxy S6 or Note 5 and the difference is even more pronounced. Bottom line is you want a device that has a very good GPS processor or you might as well use the stock speedo.

Title: Re: Magic gear numbers
Post by Serowbot on 11/28/15 at 07:16:45

There's an app for that...
http://www.gearingcommander.com/

Title: Re: Magic gear numbers
Post by justin_o_guy2 on 11/28/15 at 07:25:27

Hey, Wolf, that's seriously cool. I'm sure an explanation would go over my head and waste your time, but th e stuff you did made me wonder How you derived , I wanna say, the constants?
Whatever, I don't wanna waste your time. I've been interested in math, just not real good at it.

Title: Re: Magic gear numbers
Post by badwolf on 11/28/15 at 09:00:02

Justin, You have to figure how many times your tire rotates to go 1 mile then use your gearing to see how many rotations of the engine that is. 60 miles a hour is 1 mile a minute so there is your rpm. Other people's charts work fine if your bike matches there set-up. Some brand tires vary in size to others, along with tire pressure, loading, and rim size. My tire is on a 3.5'' wide rim. If I put it on a stock rim the circumference and profile would change. This method allows you to see what effects changes in gearing and tire size does to your rpm on paper where it is a lot cheaper. My 4th gear almost matches what my stock 5th was so 5th now is like a extra gear compared to stock. Yes I have a higher 1st gear now, but I got used to it quick.(south Fla. = no hills) I was in north Ga. in Sept and It never bothered me. Now a little bigger tire and I should hit my target of peak torque (3400) at 60 in 5th.

Title: Re: Magic gear numbers
Post by badwolf on 11/28/15 at 09:07:07

Serowbot, Wow! Thanks! If I knew about that app it would have saved me A LOT of time. Oh well retired now so I have to do something on rainy days.

Title: Re: Magic gear numbers
Post by justin_o_guy2 on 11/28/15 at 09:36:52

I understand, I think, I think I get it, but, coupla beers and a few cocktail napkins, a few
What if we used this tire?
analyses, and a coupla more beers,, I'd have it.
I slapped the Dunlop on it, the Speedo and reality started to agree and the bike ran well, some days better than others, and I started understanding the racers back in the old days, ripping into their carbs, changing jets, based on temperature and humidity and, I guess, barometric pressure. I'm pretty sure it broke the 100MPH Mark once. Right conditions, long straight stretch of cement on I 20,
I just ran it, had fun, practiced in certain corners, working on exit speed and knowing whatever RPM it was turning at a given speed was just outside of my ability. I can see how knowing that could help with choosing the Right gear to be in at an apex, to get the best acceleration from apex to exit.

Title: Re: Magic gear numbers
Post by Kris01 on 11/28/15 at 11:57:01

The formula for rpm is:
rpm = mph X 336.1524 X (primary gear x 2ndary gear x trans gear) ÷ tire diameter

Tire diameter (for stock 140/80-15):
((bead width x (aspect ratio / 100) x 2) / 25.4) + wheel diameter

ex.:
((140 x (80 / 100) x 2) / 25.4) + 15
= 23.8189..." diameter

@65 mph in top gear:
rpm = 65 X 336.1524 X ( (67/37) x (68/23) x (23/26) ) ÷ 23.8189
rpm (@ 65 mph) = 4344.4671...

Title: Re: Magic gear numbers
Post by Kris01 on 11/28/15 at 12:02:53

4-speed:
1987-95
1.888 (68/36) Primary
2.956 (68/23) Secondary

5-speed:
1996-2015
1.8108 (67/37) Primary
2.956 (68/23) Secondary

4-speed:
2.2143 (31/14)
1.5000 (27/18)
1.0952 (23/21)
0.8750 (21/24)

5-speed:
2.3333 (35/15)
1.5789 (30/19)
1.1429 (24/21)
0.9565 (22/23)
0.8846 (23/26)

Title: Re: Magic gear numbers
Post by badwolf on 11/28/15 at 17:20:17

OK, I started this thread, and I thought I knew the numbers, but I guess there are a LOT of ways to figure them. Whatever works for you and seems easiest go for it! My head is spinning with all the numbers, maybe I should just go buy a damm tach.

Title: Re: Magic gear numbers
Post by Kris01 on 11/28/15 at 22:50:20

Your way works except you have to know the "magic number" for each gear.

I provided the above info for anyone who wants to do the math. :D

Really you only need to know your RPM at 10 mph in top gear. That way you can, hopefully, multiply your speed (divided by 10) times that RPM to know how fast you're spinning the engine.

Ex.:
10 mph in top gear with a 5-speed and stock 140/80-15 tire is approx. 668 RPM.

Multiply your speed divided by 10 (65 mph=6.5) by this number.
ex. 668 * 6.5 = 4342 RPM

You'll be real close.

You'll only need to remember 1 number: 668. ;)

45 mph = 4.5 * 668 = 3006 RPM
50 mph = 5.0 * 668 = 3340 RPM
55 mph = 5.5 * 668 = 3674 RPM
60 mph = 6.0 * 668 = 4008 RPM
70 mph = 7.0 * 668 = 4676 RPM

etc.