mark kibort

Yes, nascar does run some fat tires, but then the drawback or advantage is in the tire being a much larger part of the suspension set up. diameters are still near the same as we all see at the tracks.

watch the comment about "improved" gearing. smaller tires wont improve gearing nessesarily. it can hurt it, just as easily as it can help it. It depends on the speed range you will be using and the gear ratios you have available. In my racing, with my hp and speed ranges and gearing, a 26" tire is optimal for the tracks I visit. going to a 23" tire would be a disadantage, forcing me to spend more time in less hp range of the engine over a lap. And thats what its all about at the track.

sig_a

In my case, the first thing in evaluating potential benefit moving from 17 to 15 was using a web based torque and horsepower calculator. It's pretty interesting predicting on paper that which works in practice.

mark kibort

usually the simulators are pretty good. But , it is a matter of the input values.

usually, you need to know the changes in weight for the tires and wheels. also, the rate of acceleation will change the effects of the weight on the wheels/tires, but that will be incorporated in you set up values of overall weight and hp, with gearing.


what did you see as far as acceleration changes from a 17 to a 15" wheel and tire. what values did you use, for weights and overall tire diameters?

sig_a

There are a number of variables, but they are knowable. 17x9, 17x8 ('87 911) with nt01 (DE only) 215/45, 255/40 vs 15x8, 15x7 Fuchs with nto1 225x45 on all four wheels is about 18 pounds total weight savings. 1 inch lower C/G (less roll compensates to some degree for smaller total contact patch area (900mm vs 940mm). This sounds trite, but true: car is quicker not faster, particularly in tight corners because of +/- 9 percent "better" gearing. 3.2 engine operates centered within torque power curve operating about 400 to 500 rpm higher average per lap. 5th gear is used on long straights then direct shift to 3rd -- not overly difficult in G50. See Buckley Racing as back up.

UncleMaz

+1 Noticeable gains in low end grunt and loss in highway gas mileage when you go to lower total wheel size than stock. I have had every size wheel from 15 to 18 on my 944 with AR between 70 and 35.

mark kibort

If you save that much weight, its a good thing and can be significant if it is per wheel
as far as the gearing, there is arguments both ways. it depends on the track. you show me that "better" gearing around a tight corner, and Ill show you a not as tight corner where it would be worse. remember, gearing doesnt make hp, it only optimizes the hp you have for a given corner, straight or track overall. (i.e. it "depends") . If what you say or referece to be true, there would not be gear change for the pro teams for each track, and its not always about the final drive speeds. its about optimizing the HP for most all the turns and straights. remember, lowering your overall gearing, just shifts the sweet spots down. it can mean that that shift can make you spend more time at a lesser hp depending on the speed ranges you are operating at.

Your "10%" better gearing can result in 20% less power depending on the speed range you are operating at and through. . optimally, you want to maximize the HP-seconds you can operate at over an entire lap. sometimes a lower overall gearing works, sometime higher. closer, usually always helps.

mark kibort

more factors than we have time for. But, keep in mind, I dont think the 928 with a 2.75 vs the 2.2 made any differnce in highway gas mileage. It more depends on the burn efficiency, and that has a sweet spot in the rpm and load curves. as far as low end "grunt". again, it depends. yes, it might be perceptable in each gear because they are all lower, but overall, vs actual speed, there are equal trade offs. Point is, you find me a slow speed that feels better, and Ill find a slightly faster speed, that might be in the next gear that will actually be slower. It depends on your definition of a slow turn.

mark kibort

The easiest way to describe performance due to gearing, tire size, etc, is to ask yourself, or evaluate one simple factor. at the speed you want to compare or the speed range, which gear or tire size offers the most HP-seconds. If the power at any vehicle speed is the same, the acceleration will be the same. use your imagination on how that works within the relm of gear changes and tire sizes. its all a series of trade offs. If we had infinitely variable gearboxes, it wouldnt matter at all. you would run at max hp all the time when you wanted max acceleration.

James Bailey

Mark " I dont think the 928 with a 2.75 vs the 2.2 made any differnce in highway gas mileage.... " so you think you get the same gas mileage on the highway in 3rd as you do in 5th ? because that is about what you are saying....3 rd is about 29% gear reduction going 2.75 vs 2.20 is about 25 % . The maximum RPM drops between gears is about 15 % down to 13 % for 4-th to 5th. Seems to me that if you shift it a say 6,200 RPM the maximum new low RPM is still about 5,300 RPM pretty much up on the curve and maximizing horsepower.....And YES zero to 50 mph is just about ALL about gearing again simply start out in second gear to feel the difference

dr bob

The real measure is total lbs/ft available in some defined period of time. That's real definition of horsepower. Since the horsepower calc includes torque and RPM values, the M in RPM is already the time component.

Generally, the perfect total ratio would have you at max horsepower point right as you reach max speed on your particular track. Then the individual gearsets are chosen as you suggest, generally to alow maximum exit acceleration from turns with the longest straight stretches immediately following. There's more to it than that because there will be more than a couple turns where exit acceleration will be different but speeds will be similar, and from that you'll be looking at total area under the wheel torque curve including the following straight track section.

Since we don't have a vast selection of individual gearsets available for the 928 boxes, the next-best option is to fine-tune final drive ratio as much as possible to have as many available gears as possible usable in that maximum horsepower range.

"Good" gearing doesn't create horsepower, it just gives you a chance to best use what you have.

dr bob

Jim, there are a couple very basic thermodynamic principles here that back you up. Anybody with an '89+ car with the digidash can play along. Drive the car at 60 in third and note the fuel consumption displayed. Do the same in fourth and see the difference. Except when I get dusted by a hotrod Civic between traffic signals (happens all the time...) I'd generally rather have the 2.20:1 final drive from the pre-'89 box so I could have some fuel economy and passenger comfort improvement.

MK--

You surely remember the Holbert car and the Bonneville history where they tried both the Euro and the US final drive sets during the speed record attempts. Both went the same speed, since the torque fell off at the higher RPM's at a rate greater than the net difference in gear ratios. Higher numerical final drive would get them to top speed sooner, but that wasn't that important in the top speed runs.

karl ruiter

I would think the biggest factor would be the way the tire size effects how your gearing matches up to the particular track you are on and your driving style. I think we can all agree that the motor has sweet spots and you want to be in a sweet spot at the critical times coming out of your critcal corners. I would argue that since Mark and Jim are doing vastly different kinds or racing, they might prefer different setups.

mark kibort

Jim, stop that. you are putting words in my mouth. we are talking about gas mileage as we all have come to know it. in 5th going 2200rpm with a 2.2 vs 2700rpm with a 2.75, it would be very debatable if there was any real reason why either one of these would have a efficiency advantage as far as gas mileage. sure, I would lean toward the 2.2, and it probably is better, but its not 25% better gas mileage now is it? Thats the point. There is a thing called BMEP that determines the answer to this kind of a question.

and no Jim, 0-50 is not "All" about gearing but it is important. Its about many other factors as well. 0-50 for most performance cars is in one gear, no shift. gear it down even further, any gains would be debatable. Its feble to think that just gearing the car down even further, gurantees better 0-50 performance. How many shifts do you want jim, 0-50mph, 1, 2 , 3 , 100? Even with paddle shifting, 1 gear is really about it.

You are missing the point of gearing. As Bob says, its to opitmize the HP you have available.

mark kibort

Yep, no argument here! +1

ahh, but one comment. . the Perfect ratio would not nessarily be one that puts you at max hp at the end of the straight, it might be a well after max hp to maximize hp over that speed and distance. you know this, its an integration.
In its simplest form, its HP-seconds. the more hp seconds you use, the faster you will be. (or use ft-lb seconds, which is the same thing for this discussion)

mk

UncleMaz

This is the formula to find your new final drive ratio by going with a smaller wheel/tire setup:
Original Tire Diameter / New Tire Diameter X original Final Drive Ratio = New Final Drive Ratio

In the off road world, gearing and tire size is much more significant. When I used to off road my Jeep and went from 30.5 to 35 inch tires and had to compensate by changing my R and P (I also did a rear disc brake conversion) Here was the formula I used for picking a new R and P: New Tire diameter / Old Tire Diameter X Original R&P Ratio = New R&P Ratio