io_burn

Hey dudes,

For my birthday last Friday my girlfriend bought me this totally bad-assed GPS navigation unit. (Garmin eTrex Legend for any of you GPS-heads.) So yesterday we decided to take it out to the back roads near my house and see if we could get the maximum recorded speed up over 100 miles an hour. This is when I noticed how blazingly inaccurate my speedometer is at high speeds. Is this normal? I thought I was going about 115 when the GPS only read 101mph. Am I wrong to assmume the GPS is the more accurate of the two, or are they equally bad at high speeds? A 14mph difference between the two striked me as a little odd, as any normal driving speed the two match up almost exactly.

When I was stopping from going that fast, my car was practically shaking. (Which leads me to concern #2.) I don't think I have warped rotors or anything because, again, during normal driving my car is absolutely perfect in the brake department. Is it normal for the car to vibrate/shake when braking quickly from high speeds with the stock brake setup, or is something possibly wrong with my car?

Thanks guys

Nate B

Slowdown bungplug!!!!!!

io_burn

It was for the sake of SCIENCE!

Nate B

well in that case, top it out and see what the GPS reads

Rob :190:

warped rotors most likely, in my case, they only shook under hard braking.

typer.org

same for me

mrlegoman

I have no clue but this would be nice to know.
Can you give some more detail on the shaking? Was it a rough patch of road? How rapidly did you slow down? (did you threshhold brake or just ease on down?)

io_burn

Is it even anything to worry about then? I've never experienced any vibrations during daily driving, even when driving like a jackass on my way to work... Only when quickly decellerating from over 100mph

Nuts

if your tires wore out unevenly (left compare to right), it might cost the vibration at high speed.

CS

io_burn

No rough road, I live on this long, long, long country road that was just recently paved (used to be gravel) so the road is as perfect as ice, surface-wise. I'm not sure what the differences in braking techniques are that you're talking about, I'd say I put about medium constant pressure on the pedal, and my car was vibrating up and down. Not a 'Holy **** Jim! We're breaking up!' vibration, just enough to know that something isn't kosher with my brakes. It was kind of like those strips of grooves in the road on highways before toll plazas.

nsxtasy

Eli,

The shaking you describe is a very common condition that develops only when the brakes get VERY hot - either when used on the track or when stopping hard on the highway from very high speeds. The rotors are not warped because, as you note, there is no vibration in normal street use. The vibration occurs as a result of pad material deposited on the surface of the rotor and/or "hot spots" meaning that when the rotor gets hot, it expands unevenly, so that some spots expand more than others. Either of these factors can cause the vibration. You can read more about it here.

You could live with the shudder for the occasional time when you have to stop hard. However, if you're going to make a habit of it and/or take your car on the track, you can eliminate the shudder entirely by buying new rotors and breaking them in properly (and breaking in new pads properly) as noted on that website. However, you may still find that this happens over time, if you make a habit of getting your brakes very hot periodically. I have had this vibration develop on my rotors due to track use. But I don't worry about it; as I see it, rotors are a wear item, and when they develop shudder or cracks, I just replace them. You'll see that this won't happen when the rotors are new.

Incidentally, this is ONLY with the front rotors. Replace the front rotors, and it will be fine. Even with all my track driving, I have never had to do anything with the rear rotors.



[Modified by nsxtasy, 12:11 PM 3/3/2003]

Dirt

On my way to the Dec. dyno meet, my dad brought his mobile GPS unit. 205/50/15 Kumho 712s on OEM wheels and when I showed ~99mph indicated, the actual was ~92 mph. ~85mph indicated was ~78mph actual. ~75mph indicated was ~69 actual. Granted, 205/50/15 is 1.6% smaller diameter than 195/55/15 and measurements from manufacturers can vary but it's still not very accurate. Not a big deal though, since I now know what the difference is.

I wonder what 195/50s will do to my difference. -shrug-

AamirCWITR

GPS takes time to send signal, process, receive signal. I would say it has a slight time lag so your speedo is more accurate.

Aamir

io_burn

Thanks a ton, nsxtasy... I remembered reading something about that, I figure I won't really bother with new rotors/pads and breaking them in properly until I start tracking my car this summer.

jond

Are you running stock size rims and tires?
In a recent post about top speeds, I think someone posted a gps reading of their car and told what the speedo read. I think it was a 5% differnce at 140mph. 14 off at only 100 sound way off.

Dirt

In my case above, I held each speed for a few seconds and the GPS would update itself every full second.


[Modified by BrewCityR, 12:39 PM 3/3/2003]

TypeR 01 886

I like your avatar... I love Zelda....

Chris N

I had that loose feeling at gingerman last october.... it happened towards the end of the day when I had pretty much overheated my tires, rotors, pads and fluid. It was VERY unsettling one time in particular, at the end of the front straight at close to 100. I took it easy the rest of the day (except for that one warmup lap....)

Good info nsxtasy.

About the GPS and tach, what size tires do you have?
15mph difference is very odd. If a stock tach was 3-5% off, at 100mph your tach would read 103-105 or so, not 115.

But, if you have 195/50's or 205/50, your tach will be off to start with, then add in the 3-5% (that's an assumption on my part) inaccuracy of the tach....

ajchen

the GPS may or may not be more accurate..... not sure about lag times and what not... but i've found the stock tach to be very inaccurate (as compared to the digital RPM readings on my VAFC) so i suspect the stock speedo may not be very accurate as well.

Bbasso

OK here is what I have noticed in the past using GPS, They are a few seconds behind real world time and are very close to being accuate if not perfect.

Second the vib feeling is most likely the brakes and you should take off your rotors (disks) and have them slightly cut at a shop for a amount no larger then 15.oo per rotor. and that should get you a solid feeling again. and will be better if you put new pads in to.

nsxtasy

Turning (cutting) rotors works if the rotors are truly warped - that is, if they are giving you a vibration at all speeds, even when they're relatively cold.

As noted above, what you are experiencing is due to hot spots and/or pad material. Turning the rotors may help initially but it will also make them more susceptible to developing shudder again even quicker in the future.

SonicGSR

Is this GPS unit Portable?

If so try it with different cars?