calculating speed of rev counter

[crocket]

 As we all know all boxer spedos are way out so does anyone know the speeds in top gear for a 1981 R65 at say 30/40/50/60/70  by using rev counter ? (Standard diff)

[tvrla]

Since there were three different final drive ratios for the R65 over the years, I'll give you figures for all three. These will be the RPMs at 55 and 70mph:

3.56 (32/9) ---- 3734 - 4752

3.44 (31/9) ---- 3608 - 4592

3.36  37/11 ---- 3525 - 4486

Using the 3.36 as an example, dividing 4486 by 70 will give you the RPM difference for every mile an hour, which works out to just slightly more than 64. Then multiply that by the speed you want. If you want to know the RPMs at 50 - multiply 64X50 and you get 3200.

Or, looking at it another way, 1,000 RPM in fifth equals 15.6 mph.

At 15.6mph in fifth, the tach should read 1,000. 31.2mph = 2,000. 46.8=3,000.



  
 

[Bob_Roller]

Can't tell where you reside, are you in the US with an 85 mph limit speedometer by chance ?

[Barry]

What's your final drive ratio ?

Here's a table for the 3.44 final drive assuming stock 4.00x18" tire. If you have a different tire or want to be absolutely precise then substitute a different rolling circumference which by the way is the not the same as just measuring the static diameter or circumference because of tire deflection as the wheel rolls under load. Then if you are really picky and want to complicate things more there is a centrifugal effect at speed working in the other direction to increase the rolling circumference slightly. I would make  a stab at compensating for that by measuring the rolling circumference without being sat on the bike. Easiest way to measure the rolling circumference is to make a vertical chalk mark on the tire, position the rear wheel so that mark is exactly perpendicular and make a corresponding mark on the ground then roll the bike forward one complete revolution of the rear wheel and mark the ground again. Measuring between the two marks will give you the rolling circumference.  

If you want to be really obsessive a 3.36 is only the nominal final drive ratio. It's really a 37/11 so the precise ratio is 3.363636 recurring and a 3.44 is really a 31/9 which is 3.444444 recurring
 
Must be easier to use a satnav   img][/img]

[Barry]

If all the above seems too complicated and you just want to calculate the MPH @ 1000RPM this is the fomula for a a 37/11 final drive :-


1000 RPM  ÷ gearbox ratio in 5th ÷ Final drive ratio x Rolling Circ'  ÷ inches in a mile x 60 (to convert minutes to Hours)

= 1000 ÷ 1.5 ÷ 3.3636 x 81.69 ÷ (5280 x 12) x 60

= 15.33 MPH per 1000 RPM


Something to do on a wet Easter Sunday afternoon.

[tvrla]

It seems like in the USA the most common R65 final drive is the 3.56. Over there it's the 3.36?

My LS came with the 3.56, converted to the 3.36 and am considering trying out a 3.20 or maybe even taller.

[crocket]

Thanks for info all,sorted now just wanted to see how accurate my spedo was. By the way I am from Nr stoke staffs England and have been a biker for 45yrs owning in the past British/Jap and various BMW (K75/R100rs/R1100rt/R80gs) but find the R65 more manageable as I am getting older and I am not a Big chap! Bike has done about 160kand just had its first clutch.Hoping to do a bit more Europeon touring (one up as wife tied up.....no I meant busy!) Am finding site very interesting/helpful. Reguards Chris

[Barry]

Welcome Chris.
I appreciate the low seat height and relatively low weight too.

[tvrla]

So give us your findings! Don't leave us with a mystery - how close was the speedo?

I can still manage the larger bikes just fine and have been riding as long as you. But there's something about these bikes that just feels so right! Light weight, nimble, and flickable. They feel like baby beemers with almost the same amount of power as the big brothers. How can you go wrong with that???

And when I hop on the RS it feels like such a huge monster for the first few miles!

[crocket]

 My last bike was a R1100rt which I managed OK until it came to pushing it around or if it fell over,did 3 Eur  tours on on it two up,before that I ran a K75 Ultima for 45k,turbine like motor plenty of torque 60mpg but very top heavy,best non boxer.My R65 suits me as  I am 5" 8"/10stone and dont find it a maul also easy to work on /get spares,if I had to be "picky" it would be the engine/exhaust noise when riding (even with earplugs in) Lovely handling bike though! Well the  spedo was 5mph too fast at 30mph but nearly correct at 60mph.