View Full Version : gearing for desert

07-11-2006, 04:47 AM
hey guys after some input.
building a desert racer using a zx12r motor, and am unsure of the gearing. the zx12r makes top torque at 7,500rpm and top hp at 10,500rpm.

what i would like opinions on is where i should set my target speed? should i run the speed at max torque or max hp?? in otherwords where do i gear the engine to hit the speed.

i have the ratios, tyre size etc. just not sure where the best place to have my gearing set for. alot of the racing is flatout desert straights but still needs quick accel;eration for the tighter tracks.

thanks in advance. luke

07-11-2006, 07:41 AM
what i would like opinions on is where i should set my target speed? should i run the speed at max torque or max hp?? in otherwords where do i gear the engine to hit the speed.

i have the ratios, tyre size etc. just not sure where the best place to have my gearing set for. a lot of the racing is flatout desert straights but still needs quick acceleration for the tighter tracks.

thanks in advance. luke

Okay, analyze where you're going to be driving - shorter course, long course?

Who's tracks are you going to be running on? Whiplash? MDR? FUD? SCORE? BITD?

The desert races I've run have had an average speed of 50-60 mph. There are a few sections where you'll top out, but in general, they're not that long, nor are they that often. Don't worry about trying to make your car run 100+ for 10% of a course. Most of desert racing is NOT flat out.

Can you set up so that you can change ratios for given courses? - that would be the ideal way. Just like some of the Class 10 guys have a long stroke torque engine for short courses and a short stroke higher revving engine for long courses.

Please, don't get offended by this next line - it's not meant in any sort of bad way:
Have you actually run your buggy faster than 80 or 90? If you have, have you done this for any extended period of time in the desert? - and where? - what were the conditions? A 1' tall whoop at 80 is like a 3' tall whoop at 50. Shitaki happens very quickly and it doesn't take much to get you totally out of shape - not the thing to have happen when flying across the dirt.

Trust me on this one - anything above about 80 is FREAKING FAST over desert terrain in a small(er) wheelbased car. I'll seen just over 90 in mine on a couple straights down in Mexico and in the mid 80's in Snowflake (Northern AZ) - it's fast enough. I've also been in similar sized cars right at 100 mph (at Glamis) and it's damn scary.

It's a natural tendency for us (we're testosterone poisoned) to want to make our vehicle go 343.2 mph, just to say "Mine's faster" or "mine will go XXX". Sorry, but WHO CARES - it's not important. What is important (and you are on the right track with your question) is getting the car to work for the majority of the course.

About the only thing out in the dirt that really haul arse are Trophy Trucks - they'll hit 140. Class 1 cars run 125-130 and Class 10 may top out around 110 - 120. Most of the time, these speeds are seen on dry lake beds.

Set your car up so that you stay somewhere in the middle of your two given RPM's - It will allow you to use the torque flexibly, yet still give you decent running legs. You don't want the engine screaming it's guts out except on the few short straights that you'll find.

I think you'll be surprised when you finally get out in the desert and start playing - speeds are a lot slower than you think for the majority of a course. Set the car up for the tighter, more technical sections or you may end up having to fight with an over-geared setup if set up for speed.

Remember - the KEY to a desert car is SURVIVAL of the car. You need to develope the mentality of "reliability over all out performance". - especially engine wise. Sure, you can make lots of HP, but is it at the risk of poof-kaboom? - no. Try sitting out in the middle of the Mexican desert for a few hours sometime... "Oh, look - another cactus."

The desert will break things that you never, ever thougth would break. NOTHING in the car is safe from the desert, so over-build and build with the mentality of "I wanna make it home" - RELIABLE!!!

Make the car run well w/o stressing the engine. Make the suspension work - that's probably the most key aspect... w/o a good handling, well suspended car, you can't use the HP very well anyhow.

With the engine you're going to use, start out gearing your ride so you top out in the 100 mph range. The powerplant should be more than flexible for everything inbetween.

The easier you can make it to drive, the better it will be on you. - so the better you can get your gearing, the easier it will be to drive (less shifting, less clutch work).

A couple other thoughts about making the car easier to drive - especially over a long distance/time:

1) Set the steering wheel up so that it's just in front of your chest - within a foot or so.

Most people have the wheel out in front of them too far - almost at an arm's length. WRONG. You have no power in your arms when they're extended. You end up using more of your shoulder and back muscles. Get the wheel up close - so that your elbow's either just rubbing the wheel or with in a couple inches when you steer. It allows you to use your chest and arm muscles to drive instead.

Think about it this way - go pick up something heavy - like a battery. Do it with your arms extended first. Now do it with your arms bent and your hands within a foot or so of your chest. See how much easier it is? Now think about doing this over 200 miles of rough terrain.

Perfect example of this is to watch in-car camera shots of some of the NASCAR guys - notice how the wheel's amost in their faces? It's because it's the most ergonomic and efficent way to drive.

The other thing to think about is pedal placement. We just spent the better part of a day working on positioning the brake and throttle pedals in my new desert racer. Get your seat installed, get the steering wheel where it needs to be and then work on the pedals.

Start out by sitting in the seat and just let your legs flop down on the floor board where ever they manage to land. Take a close look at the position of your feet - do they point up? Do they point in? Out? - the toes of my right foot tends to point off to the right more than my left foot does to the left.

Work on placing the pedals to your feet - not your feet to the pedals. - comfort, ease of use - make it fit you, no you fit it. The easier it is to drive, the less energy you'll use, the better you'll be. You're gonna be beating yourself for countless miles, make yourself as comfy as possible. (yes, I keep beating this horse, but it really is key)

Draw a line on the floorboard where the heel of you shoes hit the floorboard. Now pull your feet back about 2" or so. This is where you want your heal rest to be. It will put your foot in a position so that, to be fully off throttle, you'll have to flex your foot up just ever so slightly and when you go to full throttle, you're not pushing down to the point that your hitting full extention of your foot.

Same goes for the brakes - probably even more so. You want the majority of your braking power (from your foot) to be just after what would be a relaxed state. You don't want to have to point your toes to fully push the brake pedal. What happens when one of your brake circuits goes out (run a separate master cylinder for the front and rear circuits)? If you're already fully extending your foot, then leg to get good braking pressure with a working system, what happens when you loose a circuit (like the rear brakes) and the pedal now suddenly has 3 more inches of travel? - I'll tell you - you scare the crap out of yourself! (gee, how do I know this???)

If you find that you're lifting either heel off the floorboard to get full travel on any pedal, then the pedal's too far away from you.

Make sure you get a dead pedal in position for your left foot too. You'll be amazed at how much you use this to help hold you in your seat. It should be about 1/2' to 1" closer to you than the pedal closest to it. This will keep you from riding the pedal or trying to brace yourself for a big hit and managing to hit the pedal.

I don't have a clutch - so my brake pedal's about 6" wide. I bend the ends of the pedal towards the front of the car. It allows me to slide my foot off my dead pedal and onto the brake w/o fear of catching the pedal or letting my foot accidently ride the pedal.

It's the little, attention to detail things that really make a difference.

Remember - you're gonna be driving for an extended period of time. The easier you make it to drive, the better it will be.

Hope this info helps, if not, sorry for the long ramble.

07-11-2006, 08:35 AM
That is all good info, maybe it should be a sticky for general guidelines on a build

07-11-2006, 09:44 AM
Definately something to remember. As usual, taking notes.... :shock:

07-11-2006, 09:52 AM
Boyd, Here's a starting point. My Hypersprint has a ZZR1100 in it--not too terribly different from your engine. The gearing was 3.57:1, IIRC, when I last drove it. First gear was pretty high speed...a bit high--I think I could hit 30 easily.

Yoshi said he's running 6:1, and that's common with the busa crowd...and then they just don't use first gear at all. This supposedly it saves 2nd gear, which people "miss" when shifting from 1st.

Here is an alternative calculation for you. Rather than gearing for a target top speed (hitting it or restricting it), why not gear for fun factor? I can't do it, but 2nd gear starts seem like a good idea; save 1st gear for the campsite. What doesn't sound fun to me is constantly shifting to keep it in the power band--I enjoy shifing, but hate the old peaky 2-stroke powerbands where you have to shift non-stop...it gets tiring. So, assuming your powerband is from 6,000 RPM up to 11,000, you can figure out how the gearing will work with whatever top gear you want at, say, 80mph. So, if you want to minimize your shifting, set it up so each gear change up moves the RPM from 10-11,000 RPM down to 6-7,000. On the other hand, if you don't want to scream your engine, then set it up to shift at a lower RPM.

Here is a calculator for you. You'll have to plug in the gear ratios for your tranny (primary reduction plus all of the gears, up to 5--these should be available in your manual, but it shouldn't be hard to find them online), and then you can change the sprocket sizes to see what happens, and pick a ratio you like.
http://www.sprocketspecialists.com/Spro ... izer2.aspx (http://www.sprocketspecialists.com/SprocketOptimizer2.aspx)

07-11-2006, 11:12 AM
That is all good info, maybe it should be a sticky for general guidelines on a buildEgad - not a sticky. That'll make my head big and then you guys won't be able to stand me! 8)

I was just passing on five years of learning (and frustration?).... Glad it helps.

07-13-2006, 02:16 AM
thanks for the time taken to answer. i had to read it twice lots of really good stuff there that i would have had to learn the hard way.

once again thanks.