Skip to main content

Wintertime boredom, but then I remembered that I had two stopwatches, and what could be more fun than running them against each other to see if they read the same after an hour or so?  Well, they showed different times at the end.  Enough to matter?  YES!  Using either watch, you could be more than 0.1 second off at the finish in just about any speed class.

They're both Robic SC-500, kind of the standard ORR stopwatch.  So what to do about the difference?  Being too technically challenged to do a GPS time comparison, I set up a highly accurate 10 MHz temperature-compensated crystal oscillator and tested each watch against that standard.  One is fast and one is slow.

I sure would appreciate any of you sharing your stopwatch results if you test two against each other.  Maybe I just have two not-so-good units.  Of course, if I had Simon Gibbons' "big bag-o-stopwatches" I could just pick the best one.  Or maybe if I put in fresh batteries.... Oh well, Meme and I use the Silkrip computer, so the watch is just for backup.  But still...

Is it May yet?

Tom King
Life Member
Car # 111

Original Post

Replies sorted oldest to newest

So Tom.... where were those stopwatches made?

Have you tried any non-electronic stopwatches?  You know, those antique analog wound-spring types?  It would be interesting to see how far off they were from your temperature-compensated crystal standard.

Currently I use GPS timing, in a "hope" that it's really close to the same GPS timing being used in the race. 


Robic watches are made by MBI ("Master Builders Inc.") in Fort Collins, Colorado, but the watches themselves are, of course, assembled in China.  AJ, I think your strategy of using GPS timing is really good, especially that I now know how far off the electronic stopwatches can be.  As for mechanical, well, I think they'd be worse unless it was something from Rolex maybe - but then that would be worth more than my car ;-).  Mr. Bauer probably has us all beat with his egg timer... he's definitely faster than I am, so there must be something to it!

OK, I made the same test. The watches are different brands (Robic and Acutrak) but both look identical and are both SC-500 models so I'm pretty sure that they are the "same".  I forgot them and let them run for 2 hrs and 21 min. The error between them after that time was .06 sec. This error could easily be due to my starting and stopping. Who knows. At any rate pretty darn close.

I don't think any mechanical stopwatch would ever be a good choice since I do think that temperature would affect their accuracy.

Gee, Dave, you're pretty smart for an Unlimited guy!  Of course I calculated the % error and applied it to the target times for the lower speeds - and that's how I determined that depending on either one of my stopwatches would  probably leave me out of first place and wishing I had one of Charlie's super-accurate watches.  Or else wishing I had your courage so that precise timing didn't matter ;-)

Tom, how did you start and stop the watches?  I have done a similar test with my two electronic digital watches and there was always a slight difference, but I assume the difference was caused by me.  Possible variables: left hand verses right hand. the button mechanism, one thumb fatter than the other, etc.   The variable causing error may be magnified or cancel out during the start and stop sequence. Unless you can start and stop the watches with the same mechanism, you are introducing other variables.

Hey Ken and Dave, thanks.  Here's my procedure: I put one watch in each hand and start each with the respective index finger.  I was worried about different starting times, so I performed the test a number of times and got very consistent results even when I switched hands.  The ratio between the two watches is 0.9999356 within a very small range.  Now that may not seem like much of a difference, but if you multiply it by the number of seconds in your target time (e.g., 2160 seconds for 150 mph), you get more than 0.1 second.  That generally puts you out of first place.  And then, when I compared each watch in turn against my precision clock source, I also got very consistent results, with one watch slow and one watch fast.  

Now you know how boring my life is.

Tom - Kudos on a great pot-stirring topic to shake off the wintertime blues!

Fewer subjects may spark more furious debate on any ORR forum than stopwatch variance. It's got to be right up there with asking what brand of oil enthusiasts prefer, or which brand of tires are best to run. No matter who you ask, their answer will always be a bit different. Yet, according to a book I read, the answer to all questions in the universe should clearly be 42. Of course!

In my humble opinion, the answer to finding the variance between your two watches can most likely be found within Ken's answer. Of course, some may not accept that as likely, or even possible. Yep, and that's probably for the best.

However, should anyone desire to skip the fun of trying to solve the ultimate space/time question and simply prefer to buy some magic beans, I'm open to trading the stopwatch I use for the right price. Say, clear title to a C6 Z06? Yeah, I could go for that. Any takers?

Berry "Ricky Bobby" Lowman

Tom - I like my chances on picking the correct 6 numbers a bit better than anyone swapping a C6 Z06 for my stopwatch, but either is worth a shot, eh?

What could ever be more exciting than tires, oil, or a good ol' fashioned timing discussion? Probably nothing!

That said, something I bet even more folks would love to hear about is the difference between traditional stopwatch button pushing vs the King Compumaster 3000R? Now THAT could make for some stimulating conversation!

Looking forward to seeing you again in May! Maybe in the 155 class? #DoTheDew

Berry "I WANNA GO FAST" Lowman


Hey Berry, you want a job as a marketing whiz at Silkrip?  I mean wow, the "Compumaster 3000"!  

Just so you know, Bunny is thinking of picking a charity in Ely to raffle off a Silkrip ORR Computer at NORC 2017.  So you have a chance to find out first-hand the difference between your stopwatch and the CompuThingy at very little cost!

And yes, Berry, we're thinking of trying the new 155 mph class -  let's see, lower the car a bit, screw up my courage more than a bit, reprogram the CompuBomb, and go for it.

Tom - Having you in the 155 will be awesome! I always say, the greater the talent in the class, the sweeter the thrill of victory will be to whomever stands atop the podium! Plus, who doesn't want a shot at a FREE case of Mountain Dew?!? #DoTheDew

Funny enough, marketing & promotion is what I do. That said, these days, I only agree to pitch things I know I like, so if the Compumaster 3000R needs a spokesperson, I'd love to give it a whirl sometime to see how it fares!

Dave - Join us in the 155! If the level of heavy hitters sign up for it that we anticipate, I've got a good feeling it's going to be THE class to win this year!

Before my navigator ( yes I know I let him drive last September, he won his class and I have created a monster) gets too carried away with the challenges, as Dave said, I am the Driver and the only challenge from the 466 car will be for everyone to have a safe and enjoyable trip down 318 no matter which class you decide to run in or which "Egg Timer" or "Mountain Dew and Snickers" you decide to use.    BTW, count me in for some of those Raffle Tickets for the CompuBomb, Compumaster 3000R (Silkrip).  I heard that Mike Davis from TX will be trying one out down 385 in April if he gets into the race :-))))


Last edited by Lanny Lowman

Looked at current Robic offerings - the SC-505W which used to read to the millisecond now only reads to the hundredth of a second - with no change in model number or price or even, in some cases, description.  Buyers beware!

Now to modern technology.  I looked at the iPhone standard clock application, and it reads to the 0.01 second - but it at least can be periodically synced with the GPS atomic clock if you ensure that Settings - General - Date & Time - "Set Automatically" is turned on.  Then I got a pretty good stopwatch app that reads to the nearest 0.001 (i.e., millisecond) by downloading it from the App Store.  It's called, appropriately enough, "Stopwatch", and it's by Haim Benshimol.  Here's a link:

I used it to verify the accuracy of my two Robic watches - got the same results as I did when I measured them against my 10 MHz temperature-compensated crystal time standard; i.e., one watch is 0.00227% fast and the other is 0.00096% slow.  So I have some confidence that the Stopwatch app that takes advantage of GPS clock correction is pretty darn good.

Hope Owen Green doesn't read about this and beat us when he enters one of the 318 events.


You and Aaron are a class act.  I always enjoy seeing you both at the races.

As for timing in these TSD rallies, in my opinion, the whole run is determined at the start line ie: reaction time to start your watch. I have done hours of practice to determine my reaction time and have a range of 0.05 to 0.175 seconds. I therefore try to cross the finish line 0.1 secs early. I got lucky at SSCC. Probably will never happen again!!




Jim Burnett has it right - aim to be a bit early to compensate for the delay time at the start.  And he did it amazingly well in September, driving solo, using a stopwatch, and achieving 0.0054 secs off perfect.  

And Dave, the stopwatch you referenced is just 0.01 second resolution, so Jim's time would be well inside that.  Scary.

In any case, there should be a separate class for hand timers and maybe a separate one yet for Jim.

Jim is definitely an awesome competitor and great value to the ORR community !

We still laugh about the story all the time when we got the grid sheet in Sept. and only had 3 entries in our class, then I think we both looked at the other names and had the same OH SHIT! moment at the same time, when we saw it was Jim and Angel ... FUN STUFF !



yeah gonna beat the high dollar computers and stopwatches with a $38 timex. oops too late Tom  Owen has read it . wound up with a 67 merc cat in the back yard ,with a 70 351C with 71,2v heads now to come up with the money to reassemble and put the large plumbing inside gotta find a front clip and pass door. then eye might have a black cat to play with..    see ya all soon   O           ps. I must be confused a Mopar fan that drives Chevys and winds up working with Ford race teams I must be confused I wind up being All Makes Combined, AMC?  

Last edited by Owen Green

Owen, we'll be looking for that Cougar with the big engine and the Timex watch strapped to the rear view mirror!  Good luck with the time and money thing!

By the way, I did buy a new stopwatch, and it seems to be very accurate (unlike my other two).  I'm still not in the same league with Simon in terms of number of watches in the Big Bag, but at least I have one that is not embarrassingly far off.

Yesterday at the awards we learned that our time scoring was 1.2590 seconds off target/late. But our stop watch indicated 0.46 secs off/late at the finish line.
We really don't understand how it became 1.2590 seconds off. That's a big difference IMO.
Are we starting getting slow here, or can an old stopwatch battery simply slow down the Robic stopwatch?
Any wise heads here that can explain this mismatch?

Add Reply

Link copied to your clipboard.