How fast was I going???

Not quite flat-out.

1983 XJ750 Max -- 5th Gear running at 8,500 rpm.

My speedo was pegged. How fst was I going? Who knows the math?

RickCoMatic

 

Twice as fast as 5th gear at 4250RPM?

 

hey! or half as fast @ 17000 rpm.......

i'll guess if 5200 is 100k's then maybe 180k's?

 

Not sure of the maxim rim size but by by my calc then 114 mph if a 16 inch rim.

18 inch rim size would be 123mph.

 

Based on this

 

OK, so is the target speed or the max speed the correct one? Also, the Maxim's have a 130/90/16 tire rather than a 120/90/16, so that'll affect the results as well.

 

130/120 is width.....

 

Do i get a prize? since i was the only gesser , i just looked at veraverter and 180k is 111mph....a front tyre be nice

 

HooNz: Yes the 120 or 130 is width, however 90 is the profile - or rather the height of the tire expressed as a percent of the width.

120 * 90% < 130 * 90%

or the 130 tire is taller than the 120 tire.

As the circumference of the tire is based on the overall diameter of the tire, a 130 tire will have a bigger circumference. If you turn a tire with a bigger circumference at the same RPM, it will roll down the road faster...

 

ok woot , ya got me .

i thought 90 was 90 high wether 120 or 130 wide or even 500 ...really referenced from tractor/truck tyres so i'll claim i never read the specs properly...

so the calc is 120x90%=108mm high
130x90%=117mm high that right?

Cos i just measured my new 130/90 and its 90mm from rim to outer?... but thats on a 120 rim.. i confuzed..

 

I know what you mean about tractor tires... could be a different system.

I'm not sure where you measure from... I do know, however, if you add up the parts ( 16" rim plus 2 times the height of the tire) and calculate the circumference that it is DIFFERENT than the real circumference - meaning as you noted that the real life measurement is either not right, or you and I both measured from the wrong place.

interesting eh?

 

ok woot , this should be another post/inquiry , hope someone can clarify whats appening..

so what would the height difference between a 120 and a 130 static then technicaly? as one would think if it says 90mm from the rim it is! , also 2 inches gains 9mph by Hired_Goon , if its a cupla mm one would think ain't gunna do alot for the average blast up the road user..

also tyres do expand at rpm so there's another one to add in.....

 

Well.

The math of it is that circumference = diameter * pi

16" = 0.4064m = 406.4 mm.

c(120) = (406.4 + (108 * 2)) * pi
c(120) = 622.4 * pi
c(120) = 1955 mm per revolution

c(130) = 640.4 * pi
c(130) = 2011 mm per revolution

Every revolution you go 56mm further... that may seem like only a time bit but we're talking quite a few revolutions per minutes and ALOT over an hour. About 3% difference in speed readings.

 

soo.... how fast was he going?

 

Hired Goon's answer is a good one.

Remember that Hired Goon's is more correct than the speedometer, and might be out ~3% if the tire sizes weren't right.

Short answer - Hired Goons target mph was 114mph.

 

Book im dano.....

 

Yep, Give or take 5 mph 114 will be pretty close given Ricks target RPM of 8500.

Lots of variables involved that are too hard to calculate. Eg tire pressures, true circumference based on the flat spot on the bottom of the tire, incline/decline, wind resistance etc All have an affect on top speed.

Want a true reading Rick??? Strap a small handheld GPS to your handlebars. It will give average/current and top speed while your riding.


Or take one for the boys and scream past the highway patrol radar at 8500 RPM. I'm sure they will tell you how fast it really is.

 

He he he, are we all going to chip in to pay the fine he would undoubtedly receive? Not sure I would take one on the chin just for this honor, however it sounds like a way to chisel your name into the annals of our history book, securing lasting fame for crazy courageous acts. I will chip in $2, just to say I support my brother’s quest for the truth. That’s a worthy cause, no? I couldn't throw money to someone who was bent on breaking the law, that would be morally wrong. But I would support scientific research, it is an effort to find knowledge for the greater good (at least it sounds more palatable).

 

I vow to get a better spedo, and find out for sure :-D

 

"Not Guilty. I swear, Your Honor, my passing the State Trooper with the Radar Gun was not a flagrant situation. I was going that fast in the name of Science.

Your Honor ... I see that the Trooper has submitted a log of my speed from following me for 3-minutes. Any chance I can get a copy of that, so I can post it for my friends to see?"

 

Good one ricko , i bet there is a few of "them" lookin here and thinking [admitted guilt , proof , broke the law! , fine him make im paye] , we do not have a 5th admendment here! ....

Also on the other subject , as there was still no vertical number of size difference in the "numbers" between a 120/130 here's a what is goin on by h/g's 2 inch size from 16in to 18in and +9mph...

2inch = 50.8mm = 9mph / 14kph
1inch = 25.4mm = 4.4mph / 7.2kph
. 5inch = 12.7mm = 2.2mph / 3.5kph
.25inch = 6.35mm = 1.1mph / 1.8kph

As one can see "if" the vertical difference is +10mm that would equate to a gain of less than +2mph so in my no knowledge hindsight i'll stick with 120/130 IS width , wink wink nudge nudge.

 

The 120/130 is the width of the tire in millimeters (mm), measured from sidewall to sidewall. Since this measure is affected by the width of the rim, the measurement is for the tire when it is on its intended rim size.
from
http://auto.howstuffworks.com/tire2.htm