FH5, Dirt vs. off-road race tyres

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With a semi-slick excursion.

A comment in a recent video by Alex “FailRace” Day had me thinking that I ought to see whether off-road race tyres are all they’re cracked up to be in dirt races.

I started by doing five trail races on dirt tyres followed by the same five on off-road race tyres to see whether the latter had a significant effect on the times.

I used my Fiat Abarth 131. PI S1 820 (dirt tyres) or S1 808 (off-road). 422hp, 915kg, PWR 0.461. Front 56%. Times in […] are my lead over the drivatars at the end of the race – where I remembered to note them.

Baja California

  • Dirt: 02:21.817 [+2]
  • Off-road: 02:17.020 [+4]

Barranca

  • Dirt: 02:24.321 [+8]
  • Off-road: 02:25.668 [+7; scrappy race somewhat skews this result]

Tulum

  • Dirt: 02:55.251 [+6]
  • Off-road: 02:54.474 [+4; not such a great improvement]

Cordillera

  • Dirt: 02:54.062 [+4]
  • Off-road: 02:50.844 [+5]

Tapalpa

  • Dirt: 02:15.293 [+8]
  • Off-road: 02:15.983 [+8]

The results are variable with off-road races tyres sometimes having a clear advantage and sometimes offering little, it seems. I decided to try the Baja Scramble over five laps because that’s all dirt.

Baja Scramble

  • Dirt: 06:13.677; best lap 01:13.082
  • Off-road: 06:03.590 [+24]; best lap 01:10.723

And that confirmed the truth of the matter: off-road race tyres are almost certainly best in all-dirt races, but probably less advantageous in mixed-surface races. They probably have the greatest impact where the dirt sectors comprise most of the race.

I was also curious to try both types of tyres in some road racing and headed to the Horizon Festival Circuit for some five-lap races. I picked a blueprint which was on a clear-post-rain track, which I raced out of curiosity to see whether off-road race tyres might also be advantageous in wet conditions. I basically left the build as it was, but inflated the tyres to 1.85 bar.

Horizon Festival Circuit (wet track)

  • Dirt: 05:51.751 [+14]; best lap 01:08.877
  • Off-road: 05:57.547 [+15]; best lap 01:09.960

And thus it seems that off-road race tyres offer no advantage on a wet tarmac.

I then made my own blueprint so that I didn’t have to waste time searching for the race conditions I wanted. Five laps again and same tuning as before.

Horizon Festival Circuit (dry track)

  • Dirt: 05:45.023 [+22]; best lap 01:07.429
  • Off-road: 05:50.721 [+13]; best lap 01:08.616

Very roughly speaking, the difference in time remains the same whether the track is wet or dry.

One more race. I wanted to see what difference a race build made while the engine remained the same. On semi-slick tyres the Abarth 131 ended up being S1 839 even though the PWR was only 0.462 because the weight dropped to 914kg.

Horizon Festival Circuit (dry track; race build)

  • Semi-slicks: 05:37.374 [+13]; best lap 01:06.138

Conclusions

Obviously, for off-road races and the Baja Scramble, off-road race tyres are the best choice. For mixed-surface races, off-road race tyres are probably the best choice because there’s not enough tarmac in any of them to give an advantage to dirt tyres.

It rather raises the question of the point of dirt tyres if off-road tyres are also better for a PI advantage. To put it a slightly different way, dirt tyres would seem to be a choice for the budget-conscious road racer, being more like semi-slicks, but adding less to the overall PI, which would allow for more power perhaps at the expense of handling.

In testing, the PWR remained unchanged. With off-road race tyres, if I’d built the car back to S1 820, it would’ve been more powerful. It rather shows the ridiculousness of the PI system and how unbalanced it can be when the same car with different tyres produces different PI’s and times without altering anything else especially when the build with the lower PI has the better times.

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