Jumped on my bike the other day and started off down the trail only to notice that my tyres were a bit flat. Got me thinking, in the perfect world should MTB bike tyres not be pumped up really hard and then the tread design do all the work at generating grip. Why do we run our tyres slightly deflated to generate grip (I know its to increase surface area)? Do we just have to do this because of bad tread design? If the tyres were pumped up hard then they would roller better/faster and all the grip issue would be left to the tread to sort out.

I’m ignoring the suspension benefit of soft tyres as I am on a full sus bike. Im not saying the tyres should be rock hard but just as hard as say road bike tyres. Are we being fleeced by bad tyre design?

Am I missing something

Why do we run our tyres slightly deflated to generate grip (I know its to increase surface area)?

And, perhaps more importantly, to allow the tread to conform to the shape of the trail directly under it. Rock hard tyres don’t allow this, so no matter how good the tread is it won’t work as well (…m’be…)

I don’t think we are being fleeced.

Softness in tyres allow the wheel to roll more smoothly – important not just for the mentioned suspension benefit [for the rider].

Yes, we don’t run them slightly deflated, we run them inflated to the pressure we need to get the balance of contact patch and compliance we need. Same with cars, trucks, road bikes, motorbikes, etc…

Theres 2 types of friction involved (one technicaly isnt friction).

The compound grips the surface of the trail

The tread digs into the surface and then relies on the surface not giving way.

On a road bike only the first one is relavent as the surface doesn’t deform. And friction is independant of area (if you go into detail it involves pressure x area, but pressure is force/area so you end up with just the force).

In mud only the second is relavent so you can use a harder compound (at the risk of flying off on roots/rocks etc which obviously dont deform and hence winter tyres are classicaly bad at anything but mud). Obviously the more surface area you present to the mud, the more nobbles are digging in and the less likely it is that the mud will break away from the mud underneath it.

Ohhh, and the suspension benifit generates grip too, as static friction is generaly higher than dynamic friction. So if the tyre isnt slipping then its got more grip, so make the force cosistent with a bit of suspension to even it out and your less likely to slip (pun intended) into dynamic friction.

Yep you are….

As far as I understand it the tire needs to deform to match the profile of the ground under it, if it is rock hard it is not able to do this.

Edit: Far to slow…

See I knew you guys would know the reasons.

Using Nobby Nic’s at the moment and they are a bitch to get the pressure right. When the pressure is spot on they are great but all others times they are crap and either love to slide of the rims or just don’t grip. As I was pumping them up I thought why don’t they publish the correct tyre pressures or have sag indicators like suspension forks/shocks

publish the correct tyre pressures

This is entirely subjective and down to you/your setup/your preferences, hence all they can suggest is a recommend range and a max pressure for safety.

Is is all subjective, its not for suspension forks and shocks as the manufacturers go to great lengths to make it easy to setup the right sag and pressures. Should it not be the same to tyres?

Granted a lot of people adjust suspension setting away from what is recommended. If a tyres is designed for a certain purpose, mud, hard pack, etc should there not be a recommended pressure or sag. What did the designer use when testing and perfecting the design.

Using Nobby Nic’s at the moment and they sometimes dont suit my riding right. When my riding is spot on they are great but all others times i`m crap and either the tyres love to slide of the rims or just don’t grip.

fixed that. 😉

Granted a lot of people adjust suspension setting away from what is recommended.

It may be recommended setting, but the fact that so many people use it as starting point and then adjust away from it to find THEIR PERSONAL CORRECT setting is the key point.

Even with road tyres, where the variable range is arguably reduced compared to off-road, my ‘correct’ tyre pressure my be between 90 and 120 psi, depending on conditions.

What did the designer use when testing and perfecting the design.

Ultimately, I suspect they used a lot of pro riders and averaged out all of their opinions – design is compromise process within the original brief.

Some interesting thoughts OP:

I know its to increase surface area?

The size of the contact patch is one factor for sure, and many people run larger volume tyres. But that’s a lot of extra rubber and weight when the same effect can be achieved with a tyre that deforms to make a larger contact patch

Do we just have to do this because of bad tread design?

I don’t think so. If a tyre deforms then it tends to shed mud better IMO. Big knobs are good on soft ground but they weigh a lot and make the ride bobbly and slow. Personally I like a shallower centre tread with sipes for smooth, grippy straight lines and the bigger knobs on the edge to dig in when cornering.

If the tyres were pumped up hard then they would roller better/faster and all the grip issue would be left to the tread to sort out.

Quite the opposite. A tyre that deforms rolls quickly through rough ground, while hard tyres get deflected up and down so are slower.

I’m ignoring the suspension benefit of soft tyres

The rebound speed of hard tyres is fast and feels skittery. More rubbery tyres at lower pressure seem to have a lower rebound speed.

For me, the main issue with lower pressures is pinch punctures and being pushed of the rims in hard cornering. But that what UST/LUST tyres are all about.