Gps Timers

[Starionturbo]

Im pretty new to these things, however was enquiring for a cousin of mine who does regular track days.

What we are after is a Cheap GPS lap timer - and not the kind used on the mobile phones. Have heard that the Driftbox is an option but im a bit skeptical - is there any other options that are around the price of the driftbox that have similar functions?

Hopefully you can help out!!

Regards

Kim

[Risking]

does it have to be a GPS timer??

They are known to have very varring results.

We use Ultra lap timers which are available on Ebay at a very good price. They use a beacon and receiver and we've proven them to be within .01 seconds to the actual dorian timing at various circuits.

[Angus Smart]

I have an Ultra lap timer for my kart and works great..

my car however i will be giving my N95 8gb a go for gps timing with www.racechrono.com on it. i did also buy an external bluetooth receiver for it. apparently alot more accurate than the internal phone item..

the software people are making for racechrono is great i'm pretty happy with it. and most of all its Free tho i think they deserve a little donation soon!

I'm doing a track day in march i will give both a go against the timing at ec also and see what the difference is.

[jonn]

They are known to have very varring results.

Care to elaborate?

I use the Performance Box, made by the same people as the Drift Box. They aren't cheap but are proven to be very accurate and reliable.

[scathing]

Care to elaborate?

It's accuracy is only as good as how it's measured, and with a GPS-based unit it's not particularly good.

Firstly, it depends on how fast the GPS unit refreshes its position. Most commercial GPS units (the ones built into satnav units and Bluetooth GPS receivers) refresh at approximately 1Hz (i.e. once a second). That means it only checks to see where you are once a second, so it can only be completely accurate to the second.

While that's good enough if you're trying to use it for tooling around town trying to work out how to get to the shops, on the track a second is a f**kload of distance and pretty much useless for measuring. In terms of accuracy, you may as well use a stopwatch taped to your steering wheel.

RaceLogic's two GPS timers (PerformanceBox and Driftbox) refresh at 10Hz. While that's better, it's still only really accurate to a tenth of a second.

They can apply formulae to interpolate values if you crossing the lap marker happens to fall somewhere between refreshes, but that's going to be a guess rather than an measured value.

Secondly, there's the accuracy of the receiver as well. If you have a read of the Wiki entry on GPS, you can see there's more information on how accurate GPS can be / is in practice than I can be bothered reading, let alone summarising. In short, there's plenty of scope for error.

I know that when I've taken my car for a spirited drive on a winding country road, my satnav unit has confidently told me that I was in a river or a field rather than the road I was actually on. That's without trees or buildings on a cloudless day with a reported 5 satellite connection, so interference was minimal.

Your traditional lap timer uses an infra-red beacon somewhere on the track, with a receiver in the car. Every time you go past the beacon (that is projecting in an approximately straight beam) the receiver marks a lap. Since the receiver reacts quite quickly as soon as the IR beam goes into it, it's a lot more accurate. It always measures in the same place, so the distance of a lap is constant rather than slightly variable due to the aforementioned refresh rate.

Edited January 6, 2009 by scathing

[jonn]

I do see your point and a good post there. Update rate of the GPS is only part of the story as far as timing goes. You have a dead reckoning algorithm based on speed, heading and acceleration to further reduce timing errors. It's a personal preference and I'm not in any way saying one is better than the other. For track days though I would think timing accurate to 0.1s is more than enough.

I guess the attraction of the GPS lap timers is the ability to log the whole lap, show sector times etc.

[scathing]

You have a dead reckoning algorithm based on speed, heading and acceleration to further reduce timing errors.

But that's just a guess. An educated guess, but a guess nonetheless.

With a beacon timer there's no running a "close enough" measured number through a series of equations to make it "closer enough". What you measured (i.e. what your timer observes in the real world) is what is true, or as true as the hardware can measure.

It's a personal preference and I'm not in any way saying one is better than the other. For track days though I would think timing accurate to 0.1s is more than enough.

That I'd agree with. Most people doing random track days in their daily driver street cars would struggle to be so consistent that hundredths matter.

I'm looking at getting a Driftbox eventually so I can log my line through a whole lap. I want to be able to overlay my lines onto a map to see how consistently wrong I am at them and how, as well as measuring sector times without having to buy an expensive timer and multiple beacons.

I definitely won't be getting one of those phone-based GPS timers, given their inaccuracy.

[some_cs_student]

Hey,

Just curious about this discussion, there are 5Hz external GPS receivers around, such as:

http://cgi.ebay.ca/NEW-Solar-Bluetooth-Gps...%3A1|240%3A1318

And the:

http://www8.garmin.com/products/gps16a/spec.html

Although obviously they cost a bit more, would that change your mind on GPS based timers at all? Or is every 200ms not accurate enough still?

Also, I haven't confirmed that phone based software would be fast enough to keep up with updates every 200ms, I'm just curious about this

[Duncan]

yeah I run the 5mhz receiver with the race chrono software and its great. very simple setup to give you good info like any lap vs best lap info which lets you see how mid corner speed affects your lap time.

I am just moving to a full data logging setup but that is ina racecar. for weekend use a 5mhz receiver and race chrono is excellent. and they have all nsw tracks now too, I submitted OP and WP they already had EC.