Where is the best place to mount a transponder?

If it is not specified in your racing class rules, where is the best place to mount a transponder on your racing car?

People will often say it doesn’t matter. But does it?

Surely putting it out front has got to be better?

Let’s do a bit of investigation, a bit of Maths and find out for sure.

Not sure what a transponder is?

A picture of a racing car transponder
This is a Transponder

The transponder is a device used to precisely time your racing car lap times round a track.

It is critical.

Without it your car is a ghost on track! No times, no race positions, nothing.

The transponder records when you cross the start-finish line.

There are various types but the aim is the same – precision lap times, typically down to 1/100th of a second.

You need to rigidly mount the transponder somewhere on your racing car (or kart).

The best place to mount a transponder is so that the tracks central timing system can reliably pickup your car and record your times.

Does it matter where you mount your transponder?

The context is important here.

In qualifying, when it is just your racing car against the clock, then it is true, the location of the transponder is not important.

This is because your transponder is crossing the line at the same point (within your car) as where it started.

Unless you move the transponder during the lap (humm … 😉 ) then it does not matter where it is located.

In races, however, the place to mount a transponder does become important.

Take this close racing finish:

A close finish in motorsports
Close Race finish. Worth a watch … https://www.youtube.com/watch?v=Wp1klmtsWQA

As you can see, the red car (at the top) crosses the line slightly ahead and is (rightly) declared the winner.

The cars are all the same and they have their transponders mounted in the same place.

Therefore, the timing system concurred with the visual result.

But what if the transponders were in different places?

I’ve annotated the image with 1 and 2 in yellow circles (see zoomed in below:)

Annotated close finish for article on transponders

What if the top red car had its transponder at the back (1) and the white car had its transponder at the front (2) ?

Well in this case, the timing screen would (wrongly) declare the white car crossed the line first.

Eek.

It would not have been by much – maybe less than 0.03 seconds – but it would not be correct.

Real race results are effected by this.

Yes they typically get visually corrected but why risk loosing the win to what people subjectively see?

Therefore the transponder location is probably more important than people realise.

The best place to mount a transponder IS at the front (but …)

When I bought my first racing car, the guy was well aware of this.

So much so that he put the transponder right at the front of the racecar – actually IN the nose cone!

A somewhat “all-in” approach I thought.

All motorsports is officially non-contact, however, in practise, bodywork can get damaged and the nose is especially vulnerable.

If it came off, along with my transponder, then I’d be a ghost as far as the race timing system was concerned.

Not good.

Quantifying the compromise

What we want is to mount the transponder safely and securely – on something more solid than a fibreglass nose cone!

At the same time, we want to have it as far forward as possible to minimise the relative negative time loss on any run to the line.

Time for a bit of simple maths to really understand the lap time implications.

For subscribers, I’ve put together a little spreadsheet, that automatically calculates everything I’m about to describe. If you’d like it (for free) just fill in the form below to get it, or, as you’ll see it is quite easy to create yourself.

How sensitive is the transponder location?

If we can calculate the time delta effect of placing the transponder in different positions on the car the we will know if it is worth building out a special bracket for the transponder or whether mounting on the first solid bit of car will be ok.

The time delta is effected by two things:

  • the distance from the very front of your racing car, to where the transponder is located ( (a) in the cartoon below ), and
  • the speed with which you cross the start-finish line.
Best place to mount a transponder

Below is a heat map I’ve put together (that is in the spreadsheet) to help you quantify the time delta for different positions of transponder location for your racing car.

Time delta to help work out best place to mount a transponder
Time delta for different transponder positions verses speed

3 Steps To Using The Table

  • Measure the distance from nose to your transponder (a).
  • Determine the speed you cross the start-finish line.
  • Then read along to find the time delta of that mounting place, verses having it right at the very front of the car.

For example, say you mount your transponder 0.7 metres from the front of the racing car, and you cross the finish line at 100 mph.

In the spreadsheet you just need to input these two values to get the answer but the Maths is pretty simply and goes like this:

  • Convert speed into metres / second (100mph = 44.4 m/s)
  • Take the inverse (1 over) of this value to get the time taken to travel 1 metre in seconds (1 / 44.4 = 0.023 seconds to travel 1 metre)
  • Multiply this by (a) the distance your of transponder from the front of the racecar (0.023 * 0.7 = 0.02 seconds time loss difference)

0.02 seconds – is not a lot of time.

However, 0.02 seconds IS within the timing tolerance of the transponder.

It can be detected.

It would be enough to put you behind.

Looking at the table though. If you were able to mount your transponder within 0.2 metres (20 cm) of the front of the racing car, then the time loss would be basically ZERO seconds.

What this is telling us is that we can mount the transponder anywhere up to 20 cm behind the front of the car, and it will have little or no difference to the time.

This knowledge is really useful.

It gives us the freedom to find a much more secure and solid location to mount the transducer without loosing out to someone who risks putting it right out front.

Great news.

What else is the data telling us?

Slower speeds makes this critical

Interestingly, the slower you cross the line the more important the placement of the transponder.

The below image is the same data from the table but presented as a 3D surface.

It is not very often that a 3D graph is useful (data viz 101) but in this case it is really quite powerful.

What it is saying that the speed becomes exponentially more important the slower you cross the start-finish line.

If your series is fairly low speed then the placement of the transponder will have a bigger difference.

Have a check on the table again or in the spreadsheet to see how sensitive your series is.

If you have a very slow series (kart??) now is the time you might consider a special bracket to mount the transponder further forward?

In Summary

Many series specify a location for the transponder to avoid any issues.

Furthermore, a finish as close as in the picture above is also typically called visually – although that might only done for the winner …

In all circumstances, it is a pain to have to deal with the uncertainty and disappointment this could throw up – especially if you are the one to lose out on the win or podium etc …

I’d therefore recommend, that if you have freedom of placement, that the best place to mount a transponder is as follows:

  • Get your transponder as far forward as possible, but in a secure place.
  • Check on table above (or precisely in the spreadsheet) for the time delta trade-off you are making placing it further back.
  • If you are in a slower class, consider what you could do to get more solid structure further forward.

Getting this right really could mean the difference between your car winning and loosing.


Want to take the guess work out of setting your tyre pressures? Try this article including a free calculator: https://www.yourdatadriven.com/how-to-set-your-racing-car-tyre-pressures-perfectly-every-time/

Wondering what the tyres are actually doing? This article should help your visualisation:https://www.yourdatadriven.com/tyre-slip-angle-explained/