doncojones

You're comparing two things that are both essentially measuring how fast the wheels are spinning. Of course they'll be similar. What I mean is that even if the mechanism or circuit or whatever controls the speedometer is in perfect working order, they tend to be optimistic about the actual vehicle's speed while in motion.

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.

jimlab

How do you think law enforcement vehicle speedometers are calibrated?

Go to any shop offering speedometer calibration and certification and you'll find a set of rollers involved. Not radar. Not GPS.

rzograbian

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?

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.

jimlab

Yes, they are, and law enforcement vehicles used for traffic patrol are required to be certified on a yearly basis. The certification card may be presented as evidence of the vehicle's speedometer accuracy in court, then returned to the records department afterward. That is why "pacing" is a legal method of ascertaining another vehicle's speed.

Well, in my experience it's fairly accurate, even in the upper extremes.

The comparator circuit which receives AC pulses from the VSS (vehicle speed sensor) located in the vehicle's transmission is responsible for converting those pulses to an analog output so we can watch a needle move on the gauge face.

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.

jimlab

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.

r0gu3

I think stock aerodynamics with front/rear diffuser and full flat underbody tray with 1 or 2 strakes would be highly beneficial.

DamonB

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.

DamonB

That doesn't mean a hill of beans if you're on rollers. All you need to know is the exact circumference of the roller and at what rpm that roller is spinning. From that you can get a velocity (ground speed) which you can compare to what the speedometer in the car indicates. In actuality this is the most accurate measure of speed possible; not GPS or radar guns.

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.

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.

rynberg

Correct. I'm not talking about MS or 99-spec stuff, which actually is an ENGINEERED product.

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.

Kento

Egads...simply "modeling" (read: styling) a piece of bodywork "after a race car", and actually replicating an engineered product are two different things entirely...

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.

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.

Fred Sickert

[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.

F=ma

How is negative lift not the same thing as downforce...

jimlab

You know, after reading Fred's posts in this thread (no offense, but damn...) and the **** 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...

https://www.rx7club.com/showthread.p...hreadid=256477

https://www.rx7club.com/showthread.p...hreadid=251537

Fred Sickert

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.

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.

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?

Fred Sickert

I don't know about the F1, but I'd like to find out. :-)

saxyman990

Wow! Yeah, Jim! By this logic, some of the retarded Civics around my house should be stable at speeds approaching 250! Guess I better watch out for them from now on...

Kento

[QUOTE]Originally posted by Fred Sickert

Quote:

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.

Crikey...the way some people on this board try to "read into" someone's reply with blinders on...

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...

salamander

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).