ahlmans formula - Honda Motorcycles - FireBlades.org
Off-Topic Discussion of anything that doesn't fit anywhere else. If it's related to motorcycles in any way, DO NOT post it here. Post it in General Discussion or a more specific forum.

User Tag List

Reply
 
LinkBack Thread Tools
post #1 of 16 Old 02-18-2004, 12:58 AM Thread Starter
 
Join Date: 08-29-2002
Posts: 1,147
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
   
ahlmans formula

ok, this is really driving me crazy. I got the ahlman's speedo recalibrator and I want to figure out what my mph should be. I am using the formula on this page http://www.jahlmandesign.com/instructions.htm but no matter what numbers I use for gear ratio it doesn't come out to a number that makes sense. Someone else give it a try for me please.
These are the numbers I have from the 954 service manual.
Primary reduction 1.52 (73/48)
Gear Ration (6th) 1.19 (25/21)
Final Drive Ratio 2.69 (43/16)
Rear Tire Circ. 24" (rough guess)

with these numbers, and rpm at 3,000 the formula says speed would be 168mph?

Please some smart math guy check this out. Am I being retarded or is this formula really screwed up??

Thanks!
MedTechRR is offline  
Sponsored Links
Advertisement
 
post #2 of 16 Old 02-18-2004, 2:06 AM
 
Join Date: 05-01-2001
Posts: 12,126
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Reputation Power: 0
             
Re: ahlmans formula

Just borrow a GPS, find the speed difference, and calculate the percentage. Much easier that way.
ConqSoft is offline  
post #3 of 16 Old 02-18-2004, 9:14 AM Thread Starter
 
Join Date: 08-29-2002
Posts: 1,147
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
   
Re: ahlmans formula

I know it'd be easier to do that, or to find one of those MPH radar signs but it's driving me crazy why this stupid formula isn't working for crap. I'm going to have to give up and blame it on the formula!
MedTechRR is offline  
Sponsored Links
Advertisement
 
post #4 of 16 Old 02-18-2004, 9:23 AM
Supporting Member
 
cbr99xx's Avatar
 
Join Date: 02-12-2003
Location: North Georgia
Age: 57
Posts: 2,692
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 27 Post(s)
Reputation Power: 27
             
Garage
Re: ahlmans formula

Quote:
Originally Posted by 2k2954
ok, this is really driving me crazy. I got the ahlman's speedo recalibrator and I want to figure out what my mph should be. I am using the formula on this page http://www.jahlmandesign.com/instructions.htm but no matter what numbers I use for gear ratio it doesn't come out to a number that makes sense. Someone else give it a try for me please.
These are the numbers I have from the 954 service manual.
Primary reduction 1.52 (73/48)
Gear Ration (6th) 1.19 (25/21)
Final Drive Ratio 2.69 (43/16)
Rear Tire Circ. 24" (rough guess)

with these numbers, and rpm at 3,000 the formula says speed would be 168mph?

Please some smart math guy check this out. Am I being retarded or is this formula really screwed up??

Thanks!

You might want to check your rear tire circumference... if the formula requires circumfrence rather than diameter your number is way off.

Life always looks better through bugs on a face shield.
cbr99xx is offline  
post #5 of 16 Old 02-18-2004, 11:56 AM
dB
 
Join Date: 04-17-2002
Posts: 1,666
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
   
Re: ahlmans formula

Quote:
Originally Posted by cbr99xx
You might want to check your rear tire circumference... if the formula requires circumfrence rather than diameter your number is way off.
Bingo. The formula uses circumference, not diameter.
dB is offline  
post #6 of 16 Old 02-18-2004, 2:08 PM
 
Join Date: 07-28-2001
Posts: 469
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
 
Re: ahlmans formula

Quote:
Originally Posted by dB
Bingo. The formula uses circumference, not diameter.
which would cut that by a little over a 3rd and put you around 55mph.

Oops! Guess I should look at the formula first. I assumed we were using the rear tire circumferance in division not multiplication. Nevermind.

Last edited by sweetonion; 02-18-2004 at 6:02 PM.
sweetonion is offline  
post #7 of 16 Old 02-18-2004, 2:24 PM Thread Starter
 
Join Date: 08-29-2002
Posts: 1,147
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
   
Re: ahlmans formula

ok, I checked the actual circumference, which is 78". The only problem is the way the formula is stated, this makes the mph 546?? The formula is obviously wrong, or my head is? It still doesn't make any sense, in fact it's getting worse?
MedTechRR is offline  
post #8 of 16 Old 02-18-2004, 2:32 PM
 
Join Date: 03-18-2003
Posts: 12
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
 
Re: ahlmans formula

Quote:
Originally Posted by 2k2954
ok, I checked the actual circumference, which is 78". The only problem is the way the formula is stated, this makes the mph 546?? The formula is obviously wrong, or my head is? It still doesn't make any sense, in fact it's getting worse?
78” is the perimeter of the wheel and not the circumference. If that was true you would have a wheel that was 78” x2= 156”… I would hate to see the tire for that. The Circumference is half of the diameter of the wheel… If you have a 17” wheel then you have an 8.5 Circumference.

That might help you with the math.
ranger31 is offline  
post #9 of 16 Old 02-18-2004, 2:35 PM Thread Starter
 
Join Date: 08-29-2002
Posts: 1,147
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
   
Re: ahlmans formula

umm, circumference is the length around the outside of the tire, or how far the tire will travel in one rotation.

I think the denominator of the equation they give should be 880, this makes the numbers all make sense. I'm guessing that was their error. Now I can sleep at night!
MedTechRR is offline  
post #10 of 16 Old 02-18-2004, 2:48 PM
dB
 
Join Date: 04-17-2002
Posts: 1,666
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
   
Re: ahlmans formula

Ok...I tried the math for myself and you're right, it doesn't work. I suspect the problem is the "88" in the denominator (not sure where that came from?).

The top of the equation seems to work. I'll use 5000rpm because I know that means about 75mph on my bike. Also, you have the ratios calculated backwards in your first post...

Primary reduction 48/73 = .657
6th gear 21/25 = .84
Final drive 16/43 = .372
Circumference = 78"

so: 5000rpm x .657 x .84 x .372 x 78in = 80,067inches/minute

There are 63,360inches in a mile, so: 80,067/63,360 = 1.26miles/minute

Multiply by 60 to convert to hours= 75.6miles/hour Which seems right.

I'm sure the 88 is a factor to do the inches-miles and minutes-hours conversion, but I think it is wrong.

edit: By my math, the correct denominator should be 1056
1 / ((60min/hr) / (63360in/mile))

Last edited by dB; 02-18-2004 at 2:59 PM.
dB is offline  
post #11 of 16 Old 02-18-2004, 3:07 PM
 
Join Date: 05-01-2001
Posts: 12,126
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Reputation Power: 0
             
Re: ahlmans formula

Quote:
Originally Posted by ranger31
78” is the perimeter of the wheel and not the circumference. If that was true you would have a wheel that was 78” x2= 156”… I would hate to see the tire for that. The Circumference is half of the diameter of the wheel… If you have a 17” wheel then you have an 8.5 Circumference.

That might help you with the math.


Diameter
The diameter of a circle is the distance across the circle, through it's center. It is the circle's width and is usually represented by the letter d. It is twice the circle's radius.

Radius
The radius of a circle is the distance from it's center to it's edge:
It is half of the circle's diameter

Circumference
The circumference of a circle is the distance around the circle. It is the circle's perimeter.
ConqSoft is offline  
post #12 of 16 Old 02-18-2004, 3:13 PM Thread Starter
 
Join Date: 08-29-2002
Posts: 1,147
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
   
Re: ahlmans formula

I know the ratio's I posted are backwards but this is the way the service manual listed them. When I entered them into the formula I swapped them around so they were correct. Well, now I can sleep easy because I know it's the formula and not me.
MedTechRR is offline  
post #13 of 16 Old 02-18-2004, 3:30 PM
 
Join Date: 11-21-2002
Posts: 705
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
 
Re: ahlmans formula

Just go find a radar 'sign', or use mile markers, worked for me.
slowpoke is offline  
post #14 of 16 Old 02-18-2004, 4:06 PM Thread Starter
 
Join Date: 08-29-2002
Posts: 1,147
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
   
Re: ahlmans formula

well, that would be the easy way out. I coudn't just give up. Plus, now I have a fancy excel spreadsheet that others can use to easily determine what their speedometer should read and also what affects changing gearing will do on speeds. Just enter your rpm, countershaft sprocket, rear sprocket, and tire circumference. Stock info is entered. Maybe this is good enough to stick in the KB or something
Anyway, hope it helps someone else!

Last edited by MedTechRR; 01-02-2008 at 7:54 PM.
MedTechRR is offline  
post #15 of 16 Old 02-18-2004, 4:12 PM
dB
 
Join Date: 04-17-2002
Posts: 1,666
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Reputation Power: 0
   
Re: ahlmans formula

Sweet!
dB is offline  
Reply

  Lower Navigation
Go Back   Honda Motorcycles - FireBlades.org > Other > Off-Topic

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Honda Motorcycles - FireBlades.org forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
A valid e-mail address is REQUIRED. You will not have access to any site features until you activate your account using the activation e-mail that is sent to this address.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Similar Threads
Thread Thread Starter Forum Replies Last Post
Rob Tuluie: Renault Formula One tigerblade Road Racing: Club / Amateur 1 01-08-2004 1:04 PM
USGP - Formula 1 BizJetGuy Off-Topic 12 09-26-2003 9:54 PM
Formula Racing School grafixman Off-Topic 4 06-23-2003 5:45 PM
Ahlmans Speedo corrector larddog954 Riding Gear / Luggage 4 06-12-2003 1:28 AM
Ahlmans Speedo corrector larddog954 Honda FireBlade 0 06-11-2003 7:18 PM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

 
For the best viewing experience please update your browser to Google Chrome