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Gearing selection

Gearing selection

Gearing selection

Hi. I'm an amateur working on my first race car. It's an Ultra4 series car, which is off road, fast stretches similar to rally, but then with some serious off road bits, like traversing boulders.

I'm now working on gearing to help select a suitable gearbox and final drive ratio.

I've been reading Carroll Smith's Tune to Win but there's only two pages there on gearing. I'm stuck on this bit though:


Looking at the engine power curves with a view toward gearing, a couple of things become immediately evident. First is the fact that, on any given race track, we want our maximum rpm in top gear to coincide with the maximum BHP of the engine- as installed. If we don't reach that rpm
because we are geared too short, we will give horsepower away and will lose both top speed and laptime. If we exceed the rpm by much, we will sacrifice horsepower again with the same result.

I don't understand how you can make maximum RPM of an engine match the maximum BHP. The maximum BHP is at a certain RPM and the max RPM is elsewhere so how can these be matched?

If anyone can also suggest other books that cover gearing, that would be good :)

RE: Gearing selection

It's not "maximum rpm", it's "maximum rpm in top gear". In other words, the rpm at the fastest speed the vehicle reaches.

RE: Gearing selection



It's not "maximum rpm", it's "maximum rpm in top gear". In other words, the rpm at the fastest speed the vehicle reaches.

How do I work out what that is? Or do I choose a top speed? With my car it won't need to do over 70mph because it's an off road vehicle. I will road register it, but even then, no need for high speeds because of the type of vehicle it is, with it's huge 40" tyres.

RE: Gearing selection

Am I right that according to the rules the engine and transmission are "open' (unlimited!) .

What transmission will you be using?

I see a requirement is a transfer case, with a "low range.".
"All vehicles must be capable of transmitting power to all four wheels/tires,
and must be equipped with a functioning low range. Low range is defined as
a gear ratio that is lower (numerically higher) than 1:1. "

What is the top speed you anticipate while in low range, if that is where you expect to do all your racing to take place.

RE: Gearing selection

The top speed in question is the "top speed at a given track", which will vary.

Most people will talk to other people driving similar vehicles and see what they say they do for top speed at that track (being aware of the possibility that what people say they do may differ from what they actually do), take an educated guess, go out for a practice session, see how well that guess corresponds to reality, and change ratios in the field accordingly. (My thing is motorcycle roadracing, so we get to select front and rear sprocket sizes in one tooth increments! you may not have that option)

If you have more budget you can put data logging on the vehicle and use that data for what is actually happening in place of what the driver/rider says is happening.

If you have enormous budget you can run a simulation of the race course.

RE: Gearing selection

Ok so I will find out what the top speed is likely to be at events (I've got no race experience so I don't know). I know that MSA regulations state that courses for off-road events should be planned that average speed should not exceed 30mph. For road use (ie when in high ratio) I'm happy with 70mph cruising speed as it's an open vehicle with high tyres.

Ok so that settles how to find the top gear ratio. So I need to find out how to find the 1st gear ratio.

I've been looking at the 1UZ engine, most adaptors mate it to Toyota transmissions R150/R151 or WXX series or RXX series. For transferbox I am finding out if the Landcruiser box can attach to any of these transmissions, or I can use a divorced Landrover LT230 transferbox which allows changing of the high ratio gears, which is useful.

I've ordered another book which has a chapter on gearing as I really know nothing about designing gearing.

RE: Gearing selection

"For road use" ...

Is it a race car, or is it a road car?

If you gear it for peak HP rpm (likely 5,0000-ish) at 70 mph highway cruise, that's going to get old fast. Noisy, thirsty.

There may be other considerations here.

I know only of pavement roadracing, not off-road. But there are times you don't want the shortest possible gearing that achieves peak-power at the max observed speed on track in the tallest gear:

(1) The actual road speed at other places on the track may put the rider "between gears" - the higher of two choices is too tall and the lower of two choices is too short - and if that spot leads onto a straightaway, it may be more important to be in the right gear coming out of that corner than what happens later in that straightaway.

(2) Since the higher gears in the transmission are closer spaced than the lower gears, forcing the rider to use the higher gears in a place where the rider has to make a large speed adjustment in a short time might make the rider too "busy" - it may be better to downshift twice (from, let's say, 4th to 2nd) than thrice (from, let's say, 6th to 3rd).

(3) It may be pointless to make the lowest gear even lower. When first gear is low enough to send the front wheel skyward there's no point making it any lower, you'll just force the rider to short-shift into second while in a wheelie!

There is one track configuration in my area which is tight enough that I don't get past 4th gear (out of 6) and I don't want to gear it even shorter because 1st would become ridiculous and I'd have to change gear too many times (and too quickly off the start).

RE: Gearing selection

Brian, it's a race car but I'm also following IVA rules so that I can road register it. The car is about fun and about the build mainly and road registering will make it more usable. The transferbox will be two speed to switch between low and high range gearing like on any 4x4.

I know what you mean about making gears too short. Though still don't know how you figure out your first gear ratio.

RE: Gearing selection

If it has to do rock crawling, you probably want idle speed to be walking speed or less, and whatever torque the engine can make down that low to be able to give enough forward thrust to equal the entire weight of the vehicle or more.

RE: Gearing selection

Brian, there is crawling involved, though in Ultra4 the tendency is to go at obstacles at speed and use momentum to get the car over. Best example is to compare US rock crawlers vs rock bouncers. Same kind of obstacles but the crawlers crawl and the bouncers are fast 800bhp trucks that can quickly accelerate to speed to carry themselves over obstacles.

You might be right that 1st gear can be a crawl gear. I could always move away in second or third. But then as you said before, the worry is that the gaps between higher gears decrease with every gear.

I watched a video of Rakeways Ultra4 (Rakeway are a pro 4x4 builder). This is their test run: https://www.youtube.com/watch?v=g6dU5Kg6BfA
To me it looks like the car is undergeared for the speed they're doing in the field. They are constantly changing gear. Rakeway use their own custom transferbox which is single speed so there's no switching from low to high for the fast bits.

I'm starting to think my best bet might be to ask other drivers how they have geared.

RE: Gearing selection

Tmoose, that's an axle right? I'm going for fully independent. I've also seen reverse drive transferboxes that are so quick change you can do them by the side of the track. But expensive.

RE: Gearing selection

Hi rcx194,

The image on the previous link does happen to be a complete live rear axle.

Quick change "center sections" are commercially available for use with IRS.

Enterprising hot rodders have been doing that kind of thing nearly forever.

I think a best-effort choice of your gearing will be sufficient for a few years.

Looks like the R series transmissions you are considering are 5 speed with pretty deep first gears and moderate 4th-5th drop less than 20%.

RE: Gearing selection

Tmoose, that's a nice array of products!

I'll be running tyres in the region of 35" - 40", that's bus size, so those parts are probably not beefy enough. So far my best choice seems to be the Land Rover transfer box which has a few ratio options in high range. Most transfer boxes are straight through 1:1 in high-range so they won't allow changing changing gear ratio. The exception is the Land Rover box and the Suzuki Samurai (too weak for my application).

I'm considering keeping the original auto gearbox that comes with the Lexus V8 engine (1UZFE) that I'm fitting, to avoid the cost of mating a manual gearbox, but also because auto is fitted to almost all top class 4x4 racers like Trophy Trucks (no one has been able to explain why though). This means I only need to adjust the gears for the new tyre size and for new top speed. If the donor car is geared for 150mph, and I'm fitting the engine/gearbox that will never go over 90mpg, then do I gear down for the new top speed? Won't that make my first gear very short though?

I'm thinking of contacting a few other Ultra4 builders to get tips on how they geared their cars. They are all away on the main Ultra4 event of the year at the moment though.

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