Originally posted by jimlab Maybe with an older cable driven speedometer, but I took my car up to 180 mph indicated on the dyno and it was off by 2 mph (178 mph measured). Electronic speedometers are fairly accurate when calibrated properly. Inaccuracy can creep in with the comparator circuit's conversion to output usable by an analog needle indicator, but it's still much more accurate than the cable driven speedometers. The problem is that the only way to figure out how far off any particular car's speedometer is from actual is to use testing equipment that measures something other than the rotation of the wheels and most of us don't have radar guns or GPS test boxes. At least I know I don't. |
Originally posted by doncojones You're comparing two things that are both essentially measuring how fast the wheels are spinning. Go to any shop offering speedometer calibration and certification and you'll find a set of rollers involved. Not radar. Not GPS. |
this might be a stupid question but what about hoodpins? when i did 165 in my car, the hood unlocked and started to shake. i have fixed the latch and now have installed hood pins for extra protection. will those make a difference that you can notice?
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I could be wrong here but I do not believe that law enforcement vehicle speedometer readings are admissable as evidence in a court of law--only speed gun readings are.
Honestly I have no way of knowing whether or not the FD's speedo is indeed accurate at high speeds or not. It has been my observation however that when in-car speedometer readings are compared with those from external testing equipment, the speedometers *generally* are on the optimistic side even if properly calibrated for lower speeds. |
Originally posted by doncojones I could be wrong here but I do not believe that law enforcement vehicle speedometer readings are admissable as evidence in a court of law--only speed gun readings are. Honestly I have no way of knowing whether or not the FD's speedo is indeed accurate at high speeds or not. It has been my observation however that when in-car speedometer readings are compared with those from external testing equipment, the speedometers *generally* are on the optimistic side even if properly calibrated for lower speeds. Any time you convert from digital to analog, there's a chance for error. For example, the analog speedometer in my Z06 was sometimes in disagreement (only by 1 mph or so) with the HUD's digital speed readout on the windshield. The quality or calibration of the comparator circuit on some cars may be the problem, especially since there's no regulation of how accurate the speedometer needs to be beyond 60 mph or so, and there's an acceptable margin of error even at those speeds. |
Getting back to the topic...
My car has an R1 front spoiler, no rear spoiler, and was lowered about 1.5". At 150-160 mph, it felt rock solid, which is not to say that a bad dip in the road wouldn't toss you into orbit. These aren't F1 cars we're talking about. They can't travel inverted at speeds over 120 mph because of the amount of downforce generated by their spoilers. :) Mazda knew the top speed of the cars was 158-160 mph in stock form, and I suspect they were designed to be very stable up to and including those speeds. Beyond 160 mph, however, all bets are off in my opinion. You're out of the operating range set by the manufacturer, and on your own. Although the FD was wind tunnel tested extensively during the design phase, I've never heard or read any evidence supporting the belief that Mazda wind tunnel tested the cars to anything above the stock attainable speeds. The C5 Corvette was wind tunnel tested and designed to be stable at 200+ mph, which is where the "billboard" rear end came from (it was a functional choice, not a design choice), but I've never found any information of this kind for the 3rd gen. RX-7. If anyone knows different, I'd like to hear it. That said, although I've been to 150-160 mph many, many times, I have been to an indicated 180 mph once and only once, and I know there are others who have done it also. I was on a ~5 mile straight stretch of road, the car was fully modified with stock twins in non-sequential configuration at the time, and it still took awhile (relatively speaking) to climb that last 10 mph or so. The car still felt "OK", but not nearly as stable as at 150 mph. I won't do that again, nor would I recommend anyone else try it. Personally, I'd recommend people with stock aero combinations (stock front and/or rear spoilers, '99-spec front bumper, etc.) stay under 165 mph if you value your skin. If you have a body kit, I would not assume that it had any sort of testing whatsoever, and it'd be a pretty shitty way to find out by having something going wrong on a top speed run because the body kit you bought was designed for the car show circuit, not for 100+ mph speeds, if you know what I mean. :) |
I think stock aerodynamics with front/rear diffuser and full flat underbody tray with 1 or 2 strakes would be highly beneficial.
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Base model front lip and no rear wing: I've seen 170 mph and there was maybe one or two mph left. Quite a bit of lift on the front end at that speed; I wouldn't have tried go around any curves. Any downforce at the rear at all would be bad in this case (assuming the R1 wing actually makes downforce) as it would pitch the car even more nose high and make the front float even more. In my car the front end really lightens up at 150+.
Haven't tried it with my front splitter in place though. Fastest I have run the splitter at the track is about 115 but since the corners aren't near that speed I can't say if it helps reduce front lift at all. I wouldn't expect it to though. |
Originally posted by doncojones You're comparing two things that are both essentially measuring how fast the wheels are spinning. |
Originally posted by rynberg Some of you guys are hilarious. If you think that an aftermarket body kit designed solely for looks improves the aerodynamics of a car that Mazda spent hundreds of hours in a wind tunnel on, you need help..... BTW, I've heard from more than one source that the Veilside kit damn near shakes itself off the car at speeds above 120 mph due to the terrible aerodynamics of the kit.... |
I'm pretty sure that the Mazdaspeed bodykits are windtunnel tested. As well as (obvisouly) the R1 spoilers. And I would imagine that the '99 spec body parts are tested as well.
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Originally posted by EpitrochoidMan I'm pretty sure that the Mazdaspeed bodykits are windtunnel tested. As well as (obvisouly) the R1 spoilers. And I would imagine that the '99 spec body parts are tested as well. Originally posted by Fred Sickert Actually, if you sacrifice ground clearance, I'm sure it's easy to improve on the amount of lift on a stock front end. Also, aren't most of those kits modeled after race cars ? I would think that if they are solid, and solidly mounted, they should be fine. |
Originally posted by Fred Sickert Actually, if you sacrifice ground clearance, I'm sure it's easy to improve on the amount of lift on a stock front end. Also, aren't most of those kits modeled after race cars ? I would think that if they are solid, and solidly mounted, they should be fine. |
That was meant as a joke right? If not, you obviously have no understanding of aerodynamics and are giving C-west, Veilside, Ings, etc, far too much credit. |
Originally posted by Fred Sickert I don't think you understand aerodynamics. You don't need wind tunnels, although I'm sure they help. Pretty much anything that blocks air from flowing under the car will be good for aerodynamics (reduced drag and less lift). This is why "air dams" work. Its actually better to block the air than to let it flow under the car. There are countless instances where race teams have been chasing an instability problem, and found that the slightest modification to the bodywork paid major dividends in stability and aerodynamic efficiency, air dams included. |
[QUOTE]Originally posted by Kento
When you're talking about something as simplistic as an air dam, sure, but even then, at the elevated speeds that this thread is referring to, simply throwing any old "lip" to block the air can create just as many problems as it might solve. There are countless instances where race teams have been chasing an instability problem, and found that the slightest modification to the bodywork paid major dividends in stability and aerodynamic efficiency, air dams included. [/QUOTE} Actually I don't think you will create problems by limiting lift, which is what is dangerous at speed. Maybe if it broke and you suddenly had more lift. More to the point, I don't think driving your fd at 165 mph is safe just because the front end is "engineered" and wind tunnel tested. Do you ? If your body kit is unstable you'll probably feel it. If it broke off at speed, that might be a real problem. |
Originally posted by doncojones You can't "increase" aerodynamics. There are basically three aerodynamic forces that act on a car in motion: lift, downforce, and drag. Lift sucks the car up off the pavement, downforce pushes it down, and drag pulls it backwards. [color=red]You can also have negative lift where the car actually sucks itself down to the pavement, this is not the same thing as downforce.[color] Generally it is impossible to change the way a car responds to or creates one of these forces without also changing its response/creation of the others. For example if you set up a wing to create more downforce, that also creates more drag. |
You know, after reading Fred's posts in this thread (no offense, but damn...) and the shit that Greg Schroeder is trying to pass off in the kills forum, I'm starting to think that they don't teach Physics in Arizona... :p:
https://www.rx7club.com/showthread.p...hreadid=256477 https://www.rx7club.com/showthread.p...hreadid=251537 |
You don't need physics. Just limit the airflow under the car. Look at bonneville cars. They just lower them to the ground. Physics explains it, but it is a simple concept.
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Originally posted by jimlab You know, after reading Fred's posts in this thread (no offense, but damn...) and the shit that Greg Schroeder is trying to pass off in the kills forum, I'm starting to think that they don't teach Physics in Arizona... :p: https://www.rx7club.com/showthread.p...hreadid=256477 https://www.rx7club.com/showthread.p...hreadid=251537 |
Originally posted by Fred Sickert BTW Jim, feel free to post some physics equations that explain the aerodynamics of stock and aftermarket front ends, so we can discuss. |
I don't know about the F1, but I'd like to find out. :-)
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Originally posted by jimlab What's the point... you just lower the car a lot, which is why a McLaren F1 is stable at 240 mph, and an FD flips at somewhere around 215... right? :rofl: |
[QUOTE]Originally posted by Fred Sickert
Originally posted by Kento Actually I don't think you will create problems by limiting lift, which is what is dangerous at speed. Maybe if it broke and you suddenly had more lift. More to the point, I don't think driving your fd at 165 mph is safe just because the front end is "engineered" and wind tunnel tested. Do you ? If your body kit is unstable you'll probably feel it. If it broke off at speed, that might be a real problem. Did I ever say that driving the FD at 165 mph is safe? Did I say that you can "create problems by limiting lift"? All I was stating was that your own post implies that all these aftermarket bodykits "modeled after a race car" are safe to use at those speeds, and that you "don't need wind tunnel testing" just because whatever lip or air dam looks good enough to block airflow under the car. You speak of blocking airflow under the car as the end-all to high speed aerodynamics, but that is a totally misleading statement. Suffice it to say that simply "blocking the air" from going under the car with any old bodywork that looks like a race car will be "OK" at high speed is backyard engineering at best, which is why production and race car manufacturers use a wind tunnel in the first place. Would I "feel safer" driving at high speeds in a car that has actually been wind-tunnel tested and engineered, versus a car with bodywork that uses "race car styling" as its design basis? Yes. Also, I wonder if the FD driver that Jim speaks of sensed that his car was unstable just before it flipped... |
I had heard that Mugen actually wind tunnel test their products but that was about the only aftermarket firm that does (I don't count Mazdaspeed since its part of Mazda now).
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