[Closed] Tyre pressures and speed... Corellation ?

Tyre compound is far more important, and how smooth/bumpy the road/trail is.

Lets assume this limit is influenced by rider rather than actual grip

What's the point is this assumption? Grip is integral to carrying speed.

What’s the point is this assumption? Grip is integral to carrying speed.

BEcause IMO the rider is likely to be at the same limit irrespective of what PSI, if the riders ultimate speed is say 16.5kph in a particular corner, that's likely to be their speed irrespective of what pressures ? within certain limits of course... Or do we think that rider A will corner 16.5kph with 1 pressure, but 17kph with it different ?

Loads of variables even on road bikes, more on MTB.

On a perfectly smooth surface with a smooth tyre, the harder the tyre the less it deforms as you ride along, so the lower the rolling resistance. However on anything rougher than the very smoothest roads, the vibrations transmitted through a rock hard tyre actually slow you down. So you want to find the sweet spot between rolling resistance and

On an off-road descent though with a rider as you describe, how fast you can go depends on how secure you feel, which depends on how much grip you have. So in that case the softer the better. But too soft and your tyre squirms, which isn't great, or you **** the rim on rocks and roots. So if you are using a typical 2.5" tyre you're probably looking at 18 ish as a minimum pressure.

The lowest pressure you can run seems to depend on how much sidewall support your tyre gives, and this seems to vary across brands and models.

Or do we think that rider A will corner 16.5kph with 1 pressure, but 17kph with it different ?

Yes, because if you have more grip you feel more secure and you can go faster, even if you aren't at the actual limit of grip.

To give a bit of clarity on this for myself.

I'm racing in a few weekends... I generally run 20psi both ends... but for a fast blue sweepy trail should i be running high like 30+ to get faster rolling between corners, or low to give higher grip in corners ? Would i even notice the reduction in grip from 30+, but maybe get better rolling speed...

15% sag on typical (ie not smooth) roads.

More off road.

Roughly. As a start point.

That will give different pressures for different tyre widths and wheel sizes and rider weights.

how on earth are you measuring sag on a tyre ?

Or do we think that rider A will corner 16.5kph with 1 pressure, but 17kph with it different ?

Yes.

I'll frequently go round in the same argument in my head about draggy tyres, aye my 2.6 Mary might drag on tarmac roads, but the grip in corners will mean I'll be quicker on the trails.

but for a fast blue sweepy trail should i be running high like 30+ to get faster rolling between corners, or low to give higher grip in corners ? Would i even notice the reduction in grip from 30+, but maybe get better rolling speed…

Gravelly surface? trade off between skittishness and rolling speed will be the factor. Tbh you'll probably make more gains riding smooth and keeping momentum than you will with tyre pressure.

how on earth are you measuring sag on a tyre ?

With a vernier height gauge, doesn't everyone? 🙂

@weeksy good thread mate, much better than the same old tripe about covid, independence or roadies being squashed. 🙂

how on earth are you measuring sag on a tyre ?

Amusingly I have a 15% sag chart that gives tyre pressure for a a given weight (on a single wheel) with different plots for different tyre sizes.

And no idea how to post it.

but for a fast blue sweepy trail should i be running high like 30+ to get faster rolling between corners, or low to give higher grip in corners ?

25 front 30 rear

I generally run 20psi both ends… but for a fast blue sweepy trail should i be running high like 30+ to get faster rolling between corners, or low to give higher grip in corners ?

Are you pedalling up or along it? Or just down?

trade off between skittishness and rolling speed will be the factor. Tbh you’ll probably make more gains riding smooth and keeping momentum than you will with tyre pressure.

Smooth is easy... just go slowly... i'm smooth as silk... just not bloody fast !

At the end of the day this whole discussion is mostly pointless, looking at the riders and times from last weekend, if i get within 2-3s of LAST place i'll be doing OK, first place is a mile ahead of me...

But of course as with anyone, we want to do our best. The only reason this came up in my head is because i was watching a local youngster sticking about 45psi in their tyre at the weekend... It was someone who has ridden there tonnes and i was thinking "do they know something i don't" and that led me onto the question here.

It's something i'll try when i shoot to 417 on the 12th, but won't be scientific as i'll be hopefully getting faster throughout the day as i learn the track anyway.

Are you pedalling up or along it? Or just down?

In this instance it's 70% downhill with a couple of pedally flattish sections in the middle, course run will be 2mins approx...

weeksy
Full Member

Lets assume this limit is influenced by rider rather than actual grip…

So, with all that in mind… would a rider likely go quicker with 18psi (tubeless of course), 25psi, 30psi, 35psi ?

Whatever pressure the rider likes most. Because it's the rider limit that's setting the speed and the rider will go best on tyres that feel good to them, not necessarily tyres that are fastest or grippiest. So if it were me, that'd be "probably too soft", if it were ChrisL of this forum it'd be rock solid, and so on

Then definitely go lower.

Whatever pressure the rider likes most.

Is a very good point. Certainly how we play things on motorbikes on track... We run Mrs Weeksys at the pressure she wants, which is only admittedly 2psi different than some recommend, but she does like things where she likes them.... i'm less fussy.

I also think i'm less fussy on trails, but i do plan on testing this theory throughout the day and seeing how i do on a wide range of pressures.

you cant corner hard on soft tyres. At least i cant and i dont even corner that hard.

that said 18psi in a DH casing is very different in feel to 18PSI in a singleply casing.

how many ews pros use 18psi? not many. have a look at ews bike checks.

I certainly find that lower pressures (<20psi) will roll noticeably slower. So if it's a smooth-ish downhill section where maintaining speed is important, then I'd run harder tyres.

If it's steep enough that you will roll as fast as you need, particularly if it's rocky, then I'd go for lower pressures.

So to answer your question: Yes it'll make a difference.

Is there an opportunity for multiple runs? It seems to me the best way of checking this out is go as low as you dare, try that, and go as high as you want, try that, some where between those is probably a sweet spot that balances speed, grip, squirm, rebound...

Never thought about this. I just go a low as I can without hitting rim or the tyre feeling like it's going to roll or burp. So 18/20psi. I might go higher by a psi or 2 if its super dry and fast, but any lower is trouble for me.

Is there an opportunity for multiple runs?

There certainly will be on the day i go to practice, i'll start with my usual low 20s though until i start getting to know the track and i'll go up to 35ish i think in testing on the day as well... Will be an interesting experiment.

Of course, come race day the conditions may change completely, but as with most trailcenter trails they're quite good in terms of weather.

Can anyone explain to me how this all tracks with rider weight?

Is it as simple as I think in that if we have a minimum "recommended" pressure for a given tire of ~18 PSI for a 70 kg rider, someone weighing in at 105 kg (150 % of the little fella) would need a minimum of ~27 psi to get the same effective sag/grip/feel/squirm/resistance to pinch flats etc. from their tyres?

how many ews pros use 18psi? not many. have a look at ews bike checks.

Good question... i don't know the answer... What do EWS pros run ? clearly there's many sections on EWS type stuff that are fast flowy trails.

Isabeau Courdurier runs 18/21psi. She is 52kg.

What do EWS pros run ?

Probably the minimum they can get away with, without risking blowing the tyre off the rim.

But I think typically 23-28 ish depending on weight.

EDIT: an actual answer, albeit from 3 years ago: https://www.perpetualdisappointment.co.uk/what-tyre-pressures-to-top-ews-enduro-and-world-cup-downhill-riders-use/

That chart is interesting. I should be up by 2psi front and rear going by the average ratios....if I could ride as hard as they do. But I don't.

Richie Rude is a beast & he's 26/29 or 27/30 on full DH casings front & rear. But the man can certainly chop a corner pretty hard!

I would say my sweet spot is ~24 front & 28 rear with DD casings. Any less and if I square a corner off, or get too excited doing a cuttie, I can burp a tyre. I don't run thinner tyres, but if I did, I would up the pressure more, as they squirm like crazy when you load the bike up in corners.

You’ve assumed that tyres roll faster at higher pressure but that’s only the case if you’re on a surface as smooth and hard as a steel rail.

Twice your weight in stone in psi does work pretty well as a starting point - higher if you really rip turns and burp tyres or slam through rock gardens, lower if you’re smoother or it’s greasier. Foam inserts and heavier casings allow lower pressures without pinching/burping. I like inserts a lot!

'Lets also assume we’re at our limit…

Lets assume this limit is influenced by rider rather than actual grip…'

This is the rub.. what are you most comfortable at?

I’d add that referencing a specific psi is pointless unless using a calibrated pressure gauge.

Is the trail actually a conveyor belt?

From experience, it won't be tyre pressures slowing you down, more brake levers.

From experience, it won’t be tyre pressures slowing you down, more brake levers.

Of that there's no doubt

My experience of this is to go as low as you can without the tyres feeling like they're rolling or squirming in corners. As you gain confidence from the grip that low pressure gives you, you'll start cornering harder and need more pressure to prevent this feeling. What that actual pressure is will depend on your carcass and how hard you corner (human and tyre!).

how many ews pros use 18psi? not many. have a look at ews bike checks

I'm not sure it's relevant comparing pros to us tbh, on most things. Some DH riders are riding 25mm carbon rims with poke tension backed right off, similarly I don't think that would suit yer average Jo(e)

I find that either low pressure leads to squirming and therefore slow cornering and pinch punctures.

I find high pressure leads to lack of grip.

What the pressure that avoids the above is a constant judgement about so many factors - from tyre brand, tread and carcass, to rims, wheel tension, my weight, with kit or without, the exact trail and how hard I'm riding.

So I would say about this pressure.

🤷

https://road.cc/content/feature/rolling-resistance-how-choose-right-tyres-279289

During lock down Rob Warner replayed a few big world Cup races with people in the races talking about them

3 female XC riders disagreed with their teams over tyres pressure. They were all worried about pinch flats so pumped there tyres up harder than the teams wanted. The teams wanted lower pressure as it would roll faster.

The Pressure the riders went upto was 20 psi.

Looking at that link, they're more 25+ than low 20s

I'm going to throw 25/28 in for the weekend, although admittedly this weekend will be non tech xc riding

Or do we think that rider A will corner 16.5kph with 1 pressure, but 17kph with it different ?

Absolutely. For any given combination of tyres, bike, trail, conditions, rider, etc. there will be an optimum pressure. A 1 PSI change probably won't make much difference, but going from 18 PSI to 35 will make a massive difference.

‘Lets also assume we’re at our limit…

Lets assume this limit is influenced by rider rather than actual grip…’

But then what dictates the limit in terms of tyre pressure if not grip?

Bottoming out in a massive berm?
Reaction speed to getting pinged off line?
The Fear?

If it's the last one, then tyre pressure will become less relevant because you're not at the point where it will either slip or not slip, and neither at the point where rolling resistance is so high that you stop comfort braking. Just rolling along and going round corners like normal people.

There's a couple of corners like that at Swinley that I just have a complete mental block about. R15, the left hander at the end of the eroded rooty bit, I just can't hit it without dragging the brakes. And the one on the last but one blue section. Up the helter skelter, then the trail drops down and there's a final tabletop into a steep LH berm before a series of berms and a short sprint upto the fire road. I always slam on the brakes before the jump, my brain just cant compute how to make it stick at full speed, which according to STRAVA is where I lose all my time (and subsequent speed) against the top 10 (I'm down in the thousands). So it's doable, but in my case and sounds like yours, tyre pressure isn't the limiting factor.

Other than that, I just set the pressure based on the surface of the fastest corners. If a ride is almost all natural muddy winter trails, then low pressures work great. As soon as you throw in typical hard surfaces of trail centres with relatively massive amounts of grip then you need more pressure just to keep the tyre from rolling over. It's even more obvious on a fat bike where 5psi (or less) works in the snow, but anything less than 12psi becomes unmanageable at trail centres.

Whats the trail in question?
A 2 minute race at a trail centre downhill in the South? Is it Launchpad at FoD?

I'd suggest that more grip = better cornering speed and exit speed* = faster time, unless we are into the realms of prolonged straights where you are traveling at speeds where you are unable to practically pedal.

Where that limit is is going to vary based on the rider and trail surface and conditions etc but I dont think you will exceed it.

*of course its highly likely that this is going to be limited by skill and/or bravery rather than the actual limit of the equipment, but more grip is still going to get you going faster.

Whats the trail in question?
A 2 minute race at a trail centre downhill in the South? Is it Launchpad at FoD?

Blue Raccoon at 417Flyup.

But then what dictates the limit in terms of tyre pressure if not grip?

courage i guess... IMO a person has a limit and that's percieved grip rather than actual grip and even increasing the ACTUAL grip may not enable them to corner quicker due to the fear factor and their being at the limit of their ability.

Blue Raccoon at 417Flyup.

I've done that.

I'd go for most grip possible - finding out where that is, will be dependent on what tyre and your body weight.

I'm by no means good at competitive bike riding, but on that trail I'm riding at my percieved limit in corners, and in some or most cases braking before them.
There are very few prolonged straight sections where a reduction in rolling resistance would be beneficial to me.

I've never gone under 25psi front and keep the rear close to 30psi using 2.4 tyres and weighing 77kg, I rarely flat or suffer rim dings, I think going as low as you can get away with is the wrong direction, it's interesting that Danny Hart's mechanic upped his psi without telling him for his win at Snowshoe.

Don't think anyone has posted this link yet:

https://axs.sram.com/guides/tire/pressure

Helpful to get you in the right ballpark, but don't be afraid to adjust if you feel the need.

We have to join a mixed bunch of roadies/mtbers tomorrow riding up and down mountain roads. No offroad. Pollution here spiked today so didn't want to be outside doing exercise.

As one poster says (15% sag), and some reading of websites, the max tyre pressure printed on the side of tyres is max pressure, not optimal.

Whilst offroad riding I know what I need. On road not so much.

We have tyres that say 60 or 65psi on them. What do you put in for road riding where there may be dust and mud on the roads, and occasional cracks from earthquake damage? 55 and 60, or 50 and 55?

Thread title should say "tyre pressures, speed, and grip"