Thought this might be of interest to some of you... Robert K. Kuhn CRX Owners Group President (http://www.crx.org/southcal)
1990 Honda CRXsi (http://drive.to/jiggy) ICQ # 3714283 (nickname: godzilla) ---[ Forwarded Message to follow ]-------------------------- >To: [EMAIL PROTECTED] >From: Citizen Kang <[EMAIL PROTECTED]> >Subject: Re: [hooligan racing] Calculate gear ratio >Date: Mon, 22 Jul 2002 20:05:26 -0700 >Reply-To: [EMAIL PROTECTED] > >On Mon, 22 Jul 2002 09:14:46 -0700, Beatrice L. Reynolds wrote: > > > How do you calculate gear ratios? > >It can be kinda simple, yet kinda complicated. > >Calculating the ratios of individual gear sets is easy: count the number of >teeth on the driven gear and divide by the number of teeth on the driving >gear. For example, if you have a drive gear with 10 teeth and a driven >gear with 30 teeth, you have a 3.0:1 gear ratio. > >If you multiply the new ratio with the "final gear ratio" of your car's >transmission (which is basically your ring & pinion gear ratio in the >differential), you can get the overall gear ratio in that particular gear. >If you have an older muscle car with a 4.11:1 rear end and a fourth gear >that is basically a straight thru design (ie: 1.0:1), the engine will be >turning over 4.11 times for every complete 360 degree revolution the rear >wheels make. An overdrive gear ratio (basically anything with a less than >a 1.0:1 ratio) means that you can reduce your engine's RPMs at the same >axle speed. > >My last tranny had a 4.125:1 R&P and an overdrive 0.89:1 4th gear, for an >effective 4th gear final ratio of 3.67:1. IIRC, Sharkey's 245/50ZR15 drive >wheels have a diameter of 26", providing me with a tire circumference of >about 82". Now everyone knows that 1 mile in 1 minute equals 60 mph. One >mile equals 5280 feet, which equals 63,360 inches. My drive wheels will >revolve 772.7 times over that distance. Since my engine revolves 3.67 >times for every tire revolution, my engine should be humming along at about >2837 RPMs at 60 mph. > > > And how do you determine if the ratio is best-optimized for the engnie > output horsepower (bhp at the wheel)? > >It all depends on what you plan to do with that horsepower. Drag racers >essentially pick a top gear/R&P gear final ratio to allow their engines to >hit a few RPMs above the engine's red line the moment the car hits the end >of the 1/4 mile. They graph everything out and then try and pick suitable >1st, 2nd, 3rd (and 4th if applicable) gear ratios that are evenly spaced. > >You'll need a piece of graph paper and your known gear ratios. You'll >first need to calculate the final ratio at the wheels in each of your gears >and plot that out with RPMs on the vertical axis and vehicle speed >(calculated using tire diameter) on the horizontal axis. These lines >should appear as multiple diagonals with 1st gear being the most vertical >and top gear being much closer to 45 degrees. Then draw a horizontal line >across the graph that corresponds to your engine's shift point (ie: RPM at >max horsepower). Erase everything above this line and connect the highest >point of the 1st gear line *straight down* until it intersects the 2nd gear >line. Now do this for each remaining gears to create a sawtooth pattern. >These are your shift points. Studying this graph will show you what you >might have to do to each individual gear ratio to ensure that your engine >is at the right RPM when you make the shift into a higher gear. > >It's all about experimentation. Sharkey now runs a 3.88:1 R&P which gives >me a lower engine RPM at highway speeds. The downside is a reduced rate of >acceleration as the drivetrain doesn't rev up as quickly. If I ever decide >to do something about it, I'm taking the 3.88 out and putting a 4.125 back >in. I made the swap to 3.88 because the physical number of teeth on the >ring gear is fewer, making for larger, stronger actual teeth. In the ACVW >world, this is commonly thought of as being stronger and therefore better >for severe driving. On the downside, the 3.88 pinion gear actually has >more teeth than my old 4.125 set, which technically makes it the weaker >pinion gear out of the two. No matter... Sharkey simply doesn't accelerate >as quickly as she used to. She does really *fly* at higher speeds, though, >which means that unless I want to kill myself at excessive speeds in fourth >gear, I better limit my high RPMs to third and leave fourth for the >freeway. > > >---------------------------------------------------------------- >Citizen Kang website: http://members.shaw.ca/sharkeysgarage > (if necessary, remove "_" from email address to reply) >---------------------------------------------------------------- >Southern DOS: Y'all reckon? (Yup/Nope) >---------------------------------------------------------------- ---[ End of Forwarded Message ]-----------------------------
