Equation for RPM=MPH

Jeff Williams · 02-20-2003, 11:26 PM

Hi all,
Can someone please give me the Equation to calculate Mph. I know the variables are: tire height, final drive, trans ratio, rpm=mph. I used to have a calculation wheel but it has grown legs and I cannot find it any longer. I need to calc the different rpms vs final ratios on the gear set I put in my bike. I'm also interested in rpm vs mph in different gears. I have narrowed the final drives between 2.91 and 3.00(?).

thanks,

Jeff

"Boom,Boom,Boom, Boom, gonna shoot you right down."- Eric Burton and the Animals

Joe · Joe's Garage

Jeff

Ill do one better ............go here and plug in your ratios it should tell your speed in each gear.

good to 8k only

http://www.smokemup.com/auto_math/mph_range.php

You can use this one backwards...........whats the top speed that you want?

http://www.smokemup.com/auto_math/rear_end.php

shovelstrokeed · 02-21-2003, 05:03 AM

Jeff
((RPM*60)*(TireCircum/12))/(RearRatio*TransRatio*5280)

Ed

Jeff Williams · 02-21-2003, 08:32 AM

thanks guys for the needed help.

Joe,
I feel top speed doesn't need to be more than 130mph. But it's not that critical for me if it's 140 or 125, I don't ride that fast, it doesn't interest. I'm more interested in the twistys and where the rpm is going to be for the type of riding I'm doing. I'm about to change my tires at 2300 miles. The rear is bald and the front has some very interesting cups on the side grooves that peak my interest when the tire is cold,(felt like a hop, scared the hell out of me).Tire PSI front 35, back 38.5. Anyway I'm looking to put the rpm about 1000 to 1500 higher thru the same corners while in the same gear. Where the top speed end up so be it. I'm betting that I won't feel that I'm caught between gears as I am now. Also when I put on a pipe,(as well as some other tidbits), the power band will rise in the rpm.

Ed,
Thanks for the Equation. I see that you have a turbo bike. How did that work out. Did you get the lag sorted out. Would you do it to another bike or supercharge it.

Thanks again guys,
Jeff

You know your a Redneck when you drive by a sign that says"Don't do Crack" and you pull up the back of your pants.- Jeff Foxworthy

Dave Hopkins · 02-21-2003, 08:48 AM

Jeff
The chart below is my way of looking at it.
The left columb is ratio, 2nd columb is pinion teeth, 3rd is ring gear teeth, 4th is one model that has that ratio, 5th is the % of change in RPM from standard "S" ratio of 2.75. What you will get is a lower gear ratio in low & high which as you feel the same way about top speed as I it will not matter. In the twisties you will just be in a different gear, where you had been in 3rd you may be in 4th however the big gain will be your ratios will all be closer! With each upshift the gear ratios change is less than the one before, or 3-4 shift has more RPM drop than 4-5, etc. So by lowering the ring & pinion ratio the change you are going to realize is a closer ratio gearbox which unquestionably will make the bike livelyer. I have my eye on that 2.91 myself, hope you keep us posted.

2.54 13 33 K1200LT 92.31%
2.55 11 28 R1200C 92.56%
2.75 12 33 R1100S 100.00%
2.82 11 31 R1150RS 102.48%
2.91 11 32 R1100RT 105.79%
3.00 11 33 R1100R 109.09%
3.36 11 37 R850R 122.31%

Jeff Williams · 02-21-2003, 01:08 PM

Dave,
I also feel that the 2.91 may be the correct ratio. But, the 3.00 area may be of interest if it isn't too low for most riding. Having a bike that goes 130mph at 8000rpm isn't what I'm looking for. I'm going to take apart the light assembly for HID light. While their I will check out the spedometer and see if it meets the requirements of Palo Alto Spedo for adjustment. If it does without cutting into the back of the gauge then it "rear end time",(not a gay joke).

Jeff

"Bomb,Bomb,Bomb, Bomb, Bomb Iran"- Take off of a Beach Boys song-1980

shovelstrokeed · 09:05 PM

Jeff,
The turbo bike, when it is right, is the stone moritfied nuts to ride. It requires different technique from a normally aspirated bike in that you must get into the throttle way early in a turn to overcome lag. The hit from a good turbo is somewhat disconcerting on a bike like the K100 as it would break the rear tire loose in either first or second gear while straight up and down and wheelie in 3rd or 4th. Needless to say with that long wheelbase and small tire, twisties were hard to ride.

Supercharger is the way to go. I had a blown Z3 1.8 that would eat a 2.8 for lunch and had wonderful driveablilty and ease of power use. I could hang out the back end with the throttle with no problem and no danger of spinning. Turbo's are great on funny bikes with NOS to help them spin up. A lot of fun but not real practicle on a Busa. Henry Louie from LA has a turbo GSXR that does over 200 mph at dry lakes, runs in the low 9's in the 1/4 mile and is a perfectly docile street bike. He is even fast in the twisties. Lotsa balls!!!.

I think a blower on my R1100S would be a wonderful thing. 8 lbs of boost would give around 125 rear wheel HP with 110 lb/ft of torque. Wheeee! Wish I had the time and money to engineer one.

I'm ranting!
Ed

Jeff Williams · 02-22-2003, 08:03 AM

Ed,
You were not ranting, you were inspired!!! I have alway wanted to put a blower on a bike,(and no not the girl next door). I used to ride every so often a Honda 650 turbo that was made in the 80s by Honda. That little bike could almost knock you off the bike when the burst hit. I would like to see an S with a blower. Now that I think about it, it would probly be more cost effective than what people are doing now. Now I'm ranting, must be addictive.

Jeff

"Hey buddy, how about a couryesy flush". - Tom Arnold (Austin Powers)

shovelstrokeed · 02-22-2003, 04:06 PM

Lessee,
Remove alternator and mount Eaton style blower so snout replaces alternator pully. Nope!
Custom machine alternator rotor with extended thru shaft to direct drive blower snout. Rework belt drive for alternator for stronger/wider belt. Assorted brackets and tubage, a reworked or auxillary fuel pressure regulator. Still run closed loop and with the bypass valve on the Eaton no parasitic drag at no boost. Maybe $7000 in parts and machining for the first prototype, a least a month, more likely 2 to kinda fit everything and do a little prototyping. Rework the major flaws and oops's over the next 2 months. Yep, $25,000 minimum for the first production kit. About $4500 retail thereafter.
Need to move about 50 units to recover R&D costs and then maybe a 12% net profit. Oh well! Its fun to dream.

Ed

Jeff Williams · 02-22-2003, 10:03 PM

Ed,
Darn it Ed, you just take the fun out of it when you talk about that dirty word ($)

. I was thinking that their must be a company that makes superchargers for our bikes. It would be fun reading.

Jeff

"Just looking for a girl that can suck the chrome off my exhaust pipe." - Sunny Barger (Oakland Hells Angels) '71