Tyre Rubber Classifications

Question

I was looking through a bike catalogue at one of the high-end road bikes (a $6000 Giant Propel) and I couldn't help but notice that this bike is advertised with specific tyres for front and rear wheels.

This was a new one to me, I've not known any other bike to have this and, as far as I can see, tyre manufacturers don't make front and rear tyres - they're just tyres! So my next question was, "what's the difference?"

The two tyres are the same size (700x23) and it turns out that the difference is in the tread patterns [edit: I now realise this is incorrect]. The front tyre is described as sticky 60/64/60 durometer. The rear described as durable and fast 64/70/64 durometer.

I was just wondering if anyone knew what these values actually mean? (left/centre/right thickness?) Or is it something proprietary to Giant?

Just to be clear, I understand why you might put different tyres on front and back, I understand that the difference between these two might be unnoticeable to all but the best cyclists, its those numbers I'm interested in understanding.

Answer 1

The technical term is, I believe, "woo". Or marketing, if you're an economist. By using nigh-on-infinitesimal differences between their product and everyone else's they can add profit. At least in this case the difference appears to be real, although they don't tell us the uncertainty in the output so for all we know the rubber is 60 +/- 10, making 64/60 effectively identical. It's likely to be much more closely controlled than that, but we don't know.

Their website says:

HIGH-PERFORMANCE TREAD COMPOUNDS
High-performance front (60/64/60) and rear (64/70/64) specific tread compounds using sticky rubber for the front, and fast, durable rubber for the rear. And dual-compound center-to-side rubber ensures maximum speed plus cornering traction.

They use a harder compound on the rear to give lower rolling resistance at the expense of traction. Which, while technically true, is unlikely to be detectable by the rider.

For comparison, Kenda use 68/60 dual compound in their road tyres:

R2C COMPOUND.Kenda's finest road-specific competition compound that blends the best attributes of traction, speed and durability. A grippy (60 shore A durometer) compound is featured on the tire's shoulder for cornering assurance during extreme lean angles and is paired with a fast-rolling (68 shore A durometer) center compound rubber for straight-line speed. Both high-performance durometers are blended on the same casing for the ultimate in performance.

They both use a centreline strip of harder rubber to give better wear in a straight line. Note that if the difference is detectable, that will most likely be under hard front braking where the harder rubber will lose traction earlier. If you do test this, please post video.

Maxxis go so far as to actually define their terms but none of them seem to have published anything about the relative performance of different tyres, let alone of tiny variations in rubber hardness. Bicycling magazine has a shorter definition using smaller words.

Answer 2

Even though I don't have specific knowledge on road bike tires, nor specific numeric durometer designations, I have some expertise on MTB tires. The given specification numbers mean that the bike has a slightly harder rubber compound on the rear tire.

The reason for having different tires front and rear is that the dynamic loads that each tire has to cope with are different, all depending on bike geometry, rider technique and skill level and type / usage of the bike.

The rear tire copes mainly with traction load, that is pushing the bike forward, also side load from turning but less than the front tire and a little fraction of the braking forces. Also during acceleration and rolling on flat, it bears a bigger fraction of the rider + bike weight.

Front tire on the other hand bears with almost all the braking force, specially on road bikes. It also copes with most of the force needed to steer the bike, which means greater side loads than the rear tire. Also, in strong braking the front tire bears with a bigger fraction of the weight.

When a forward moving wheeled vehicle brakes, a weight transfer occurs from the static center of mass towards the front of the vehicle. This is easily observed on cars, motorcycles and full suspension bicycles. That's why front tire works harder during braking, because the rear wheel has lesser downward force applied to it, so it is very limited on the friction it can create.

In the particular case shown on the question, the bike has a slightly harder rubber compound on the rear tire. This means that it will last a little longer and will create a little less rolling resistance (Compared to a tire with the same structure but using the softer compound and keeping all other variables equal). Conversely, the front tire, being softer will have more grip and will do a better job braking and steering.

In my opinion that makes sense because on a road bike, you spend more effort pedaling on flat or climbing, and during such events, the rear wheel has a grater fraction of the weight applied to, that is, the bike has less rolling resistance than if it had both tires made of the softer compound, but a little more grip than with both tires made of the harder compound. It's like trying to get the best of both worlds.

In MTB where my experience comes from, different threads is common practice. It is usual to pick tires independently, even different brands. Some manufacturers even have front and rear specific tires with the same model name. Others design a rolling direction for front use and the other for rear use.

On my particular case I have bikes with different tires front and rear but also bikes with the same tire model. While comparing wear on different tires is very difficult, on the bikes I have with the same tire front and rear, there are different wear patterns.

On and old road bike that I have with cheap identical tires front and rear, I use a little more pressure on the rear tire, for the reasons explained 3 paragraphs before.

Answer 3

To expand on @Mσᶎ's answer, the Bicycling Magazine article that he cites contains the following:

Durometer A tread compound's give is typically measured in Shore A (sA) Hardness (a scale used within the tire/rubber industry), in a range of 0 to 90. For bicycle tires, 70sA is usually considered a harder, more-durable compound and 40sA is a supersoft DH compound. Most (but not all) manufacturers will label durometer on the tire sidewall and packaging—and call out available options on their websites. Several manufacturers use a mix of different tread rubbers on a single tire—harder compounds for the center knobs for fast rolling, softer on the sides for cornering grip.

So this at least explains what each of those numbers means. Googling the Shore A scale was also quite informative, yielding a number of results from rubber-related businesses. This scale is clearly quite common in the world of rubber although as I say in the question I'd never heard of it regarding cycling before.

Giant told me that the x/y/z referred to shoulder/centre/shoulder hardness - so you get more grip as you move away from the centre.