Super GT: Not Soft, but Compliant

How Honda Finally Got the Tyres Under Control. And Some Thoughts About 2027 Tyres.

Ahead of the Super GT round at SUGO, attention is once again fixed on the Honda Prelude-GT. Back in spring, the new car looked like a rather unconvincing experiment. Now it has two wins on the trot and four Hondas on the front rows at Suzuka. So what exactly changed over the summer break?

Naturally, with aerodynamic development supposedly frozen - well, “frozen”; there are holes in that restriction large enough to be used if one knows where to look, but that is another story - and the engine and chassis effectively locked down, the obvious place left to work was set-up.

After the second Fuji round, the cancellation of the Malaysian race gave Honda an unusually long stretch in which to sit down and think properly. The engineers worked the simulators, revisited suspension settings, studied how the car interacted with the tyres and, crucially, stopped asking merely which tyre was faster. The more useful question was which characteristic of the tyre the car itself was capable of exploiting.

In racing shorthand we constantly talk about “soft” and “hard” tyres. Unfortunately, those words are treacherous. A “soft” tyre does not necessarily have a soft carcass, just as a “hard” tyre is not necessarily a wooden cartwheel designed to survive until retirement age.

As usual, things are slightly more complicated than one would like if one dislikes digging into the machinery and expects every technical question to have a one-sentence answer.

You can have a soft tread compound over a stiff construction. You can keep the rubber relatively firm and change the way the carcass deforms. You can make a tyre build load more quickly but suffer over a long stint. Or you can surrender a little peak grip in exchange for a characteristic that remains usable lap after lap. Honda appears to have moved in precisely that direction.

Since the NSX-GT days, Honda engineers had generally favoured a stiffer tyre construction. There is perfectly sound physics behind that preference. A stiffer structure gives a sharper response and a higher peak in lateral force: on one qualifying lap it can look magnificent. But Super GT is not a sprint championship. The tyre has to survive sustained load, traffic, changing track temperature and a car whose balance changes as the fuel burns away.

For the Prelude, Bridgestone moved towards a different structural character. The key was not simply compound hardness, but the stiffness of the tyre’s internal structure - particularly belt shear stiffness.

A stiffer belt gives a sharper peak. A more compliant one may sacrifice a little at the top of the curve, but it deforms more progressively, spreads the usable range and makes the car less brutally dependent on hitting one narrow combination of load and temperature.

And by now we really ought to have learned one thing: an excessively soft tyre, without understanding how the car is going to work it, is not a racing weapon. It is merely an overheated hunk of rubber.

At Suzuka, the two Hondas went in different directions. The No. 100 Kunimitsu car used a higher-temperature window, while the No. 8 ARTA took another route. Early in the race the advantage lay with No. 100: the hotter track brought its tyre into range more quickly. Then the surface temperature changed, and the balance of power changed with it.

One car began to fall out of its optimum window while the other gradually entered its own. The gap disappeared not because one Prelude had suddenly become fundamentally faster than the other, but because one tyre was better matched to the conditions that existed at that particular moment.

There is another interesting point here. Japanese engineers link part of this to the maturity of the underlying car. The Supra has been evolving for years - although we still have an old issue of Autosport in front of us showing the first sketches of the car that was supposed to replace the Lexus. Time, would you mind slowing down a little? - and Toyota therefore understands its operating envelope and its preferred tyre family far better. The new Honda is still exploring the edges of its map, and that is precisely why it has been able to stumble upon a direction that surprised everyone else.

There is a certain irony in that. The car did not suddenly become quicker because somebody found more downforce or horsepower. It became quicker because the engineers stopped forcing it to do something for which it had never been well suited.

SUGO therefore puts the whole story in a different light. A short circuit with plenty of slower corners exposes how a car mechanically loads the tyre, how quickly the tyre reaches its working window and how long it can remain there. If the Prelude’s recent form really does come from understanding the mechanical character of its tyres, SUGO should provide a fairly brutal examination. And afterwards the No. 36 Supra can finally shake the wilted laurel leaves off its head and get back to a proper work.

At this point the inquisitive reader will immediately object: “Why spend all this time worrying about tyres if there is going to be a control supplier next year?”

And we answer: “Yes, quite right.”

“So we stop worrying about them?”

“No. We keep worrying.”

That is where the 2027 paradox begins. The variety will not disappear completely; it will change form. That does not alter the fact that the current Prelude story is valuable as one of the final snapshots of the old era. The oaks will not grow taller and the grass will not turn greener, but as a milestone visible from several parsecs away, it will do nicely.

According to Japanese Autosport, GT500 is moving towards a standardised tyre system in 2027, with Bridgestone supplying three basic types: Soft, Medium and Hard. A range will be selected for each round, and compulsory use of two types during the race is also under discussion - because apparently somebody felt the urgent need to tow one of the World Championship’s less inspired “innovations” into Japan by the ears. Still, it is not quite a carbon copy of Formula 1.

In Formula 1, Pirelli normally selects three neighbouring compounds from its C1-to-C5 family and labels them Soft, Medium and Hard for the weekend. The basic tyre construction is essentially fixed; most of the distinction lies in the compound.

In Super GT, or at least as we hope we have correctly understood the Japanese, the idea was supposed to be considerably more interesting.

Indeed, the Japanese themselves did not seem entirely sure at first.

After Bando’s early-August press conference, the obvious interpretation was that even if everyone received only three names - Soft, Medium and Hard - there would still be room to vary the mechanics underneath them: carcass stiffness, belt stiffness, ply arrangement, contact-patch behaviour, temperature window, warm-up rate and the way performance changed with running.

Established Japanese outlets initially recorded the plan rather unambiguously: three different constructions for each Soft, Medium and Hard tyre. Nine physical specifications in total. Bando’s explanation was that current GT500 tyres have such narrow working windows that a Fuji tyre cannot simply be loaded into the truck and taken to Suzuka unchanged. The initial concept was therefore to begin 2027 with a 3×3 matrix and gradually reduce the number of constructions from three to two and eventually one as Bridgestone widened the operating range.

Then came the second test on 24 August, and Autosport described something different:

three compounds × three temperature ranges × one construction = nine tyres.

The October issue repeats the same, newer scheme. So somewhere during August, Bridgestone appears to have moved already towards the degree of standardisation that Bando had originally described as the eventual destination.

That makes the picture rather less cheerful - but still not completely Formula 1-ish.

It is already a major departure from current Super GT, where Bridgestone has openly explained that two tyres both called “Soft” for two different cars may differ not only in compound but in construction, and that a Soft for one car could even correspond physically to what another car calls its Hard.

Which means the current Prelude story - right down to changing belt shear stiffness - belongs to precisely the layer of engineering freedom that may disappear in 2027.

The three temperature ranges at least save the new system from complete Pirelli-isation. Taken literally, the Japanese description gives nine combinations:

Temperature range	Soft	Medium	Hard
Low	S-L	M-L	H-L
Medium	S-M	M-M	H-M
High	S-H	M-H	H-H

And that is considerably more interesting than the usual red-yellow-white exercise. In theory you could have a low-temperature Hard, a high-temperature Soft, a low-window Medium or a high-window Medium.

In other words, the word Hard would describe primarily where the tyre sits on the axis between peak grip and degradation/life, while a separate axis determines the temperature range in which that behaviour is meant to occur.

The Suzuka test also suggests that Bridgestone is genuinely trying to make S/M/H behave differently, rather than simply painting three colours on the sidewall. After long runs on the Soft and Medium, Sho Tsuboi noted that the Soft already had a very high peak, but in his view it still did not deteriorate sharply enough relative to the Medium; if that gap is too small, the Hard begins to lose its purpose altogether.

So Bridgestone is effectively tuning the shape of the grip-versus-distance curve. Soft needs the higher peak and steeper drop; Medium a lower peak and more stability; Hard still needs a sufficiently distinct operating territory of its own.

Put into the language of formulas, Bridgestone has a rather more difficult job than the one an averagely competent cook could manage:

μSoft > μMedium > μHard

is nowhere near enough.

They have to separate the tyres across several dependencies:

μ = f(T, N, slip, laps)

so that each specification differs not only in peak coefficient but also in sensitivity to temperature, load, slip state and accumulated work.

And naturally, we have no idea whether an imaginary Soft-L and Soft-H use genuinely different tread chemistry. They probably do - shifting the working temperature that far without changing the compound would be difficult - but Bridgestone is unlikely to hand that recipe even to Japanese journalists.

So, for the moment this does not look like a completely dreary Formula 1-style system. Nine meaningful combinations really are planned. But the most delicious layer of the present tyre war - the ability to alter the tyre’s mechanical personality through its construction and belt stiffness for a particular car - appears likely to be cut away in 2027 and sent to the museum.

Which leaves Honda in a wonderfully absurd position. They have finally learned how to win by exploiting a parameter that will be taken away from them in a matter of months.