Tyre Pressures

[venomx]

MK2 Yaris SR

Tyres are 195/50r 16V

I used to run 35 at the front and 32 at the back ( as per Toyotas advice ) , but recently tried 35 all round. Seems to roll better but obviously more bumpy, is this normal ?

What PSI do you guys run your tyres ?

[TonyHSD]

Setting the correct tyre pressure is a very tricky job and requires patience, air compressor and pressure gauge. 
Most important of all is to do that in the morning before the sun sees the car and before you are driving off. If you take your car to the nearest petrol station you can’t set pressure correctly. 
Check your pressure values in the book or from the car label and adjust accordingly, no need to exceed the given pressures, just keep around these numbers. If the car becomes bumpy means you have over inflated, if it’s too squishy and soft means you have under inflated tyres. 👍

[venomx]

4 minutes ago, TonyHSD said:

Setting the correct tyre pressure is a very tricky job and requires patience, air compressor and pressure gauge. 
Most important of all is to do that in the morning before the sun sees the car and before you are driving off. If you take your car to the nearest petrol station you can’t set pressure correctly. 
Check your pressure values in the book or from the car label and adjust accordingly, no need to exceed the given pressures, just keep around these numbers. If the car becomes bumpy means you have over inflated, if it’s too squishy and soft means you have under inflated tyres. 👍

Thanks 🙂

Ever since getting a puncture a few weeks back I've been checking my tyres more regularly, sometimes a bit too often 🤣

I was driving around ( about a month go ) thinking, why am I having to accelerate constantly just to get up to speed on slow roads.  I then checked my tyres, and one of them was 20psi !!

Got it repaired and it's been fine since. I also got the tyres rotated whilst I was at it.

I know about how temperature affects PSI too. For every 5 degree drop or increase, the pressure can fluctuate by 1psi. So I always check cold, in the morning, when out of sunlight.

So even if you are a driving enthusiast like me, is it always best to run them at what Toyota say ? I was under the impression that over inflating the rears by 3psi might make the car roll nicer and feel more sporty 😁

 

 

[Catlover]

Increase tyre pressures and the ride will get harder and possibly wear more in the centre.  Lower tyre pressures soften the ride and you could get wear on both edges of the width of tyre. 
Increase your rear tyres by 1psi and see how it rides, then 1psi more and test again.  But as Tony says, do it when sun not shone on one side and not the other. 7am at this time of year should be ok.

[Rhymes with Paris]

I always try to kick all four tyres before the sun gets too strong.

If my foot bounces more than 6 inches then they are too hard ,if my foot sinks in a bit, they are too soft.

My 30 year old chrome mechanical gauge seems to work well too.

 

[Cyker]

IIRC I ran my Mk2 at 36 front 35 back, mainly for economy and motorway cruising. (The lower rolling resistance you get with higher pressures helps with the mpg!)

For comparison my Mk1 I ran at 38 front and 35 rear (That diesel lump is noticeably heavier than a petrol engine!) and the Mk4 at 40/38 (The narrower 15" rims definitely look less strained with higher pressures with that extra 200kg!)

 

TBH it's not that important a couple PSI either way as they will change naturally with temperature; I sometimes check the PSI's in the car (One advantage of the Mk4 is it can show the TPMS data on the dash!) and they will swing 5 PSI through the day!

[Rhymes with Paris]

IIRC, I know I have been around for a while on here, and seen this a few times.

But I don't know what it means, could some kind soul clue me in please ?

[Cyker]

Sorry, used to using all these acronyms from being born with a modem up me butt and also laziness  

IIRC - If I Recall Correctly

TBH - To Be Honest

 

[Roy124]

AIUI IMHO Cyker is right 😅

[Roy124]

Deleted 

[MikeSh]

8 hours ago, venomx said:

feel more sporty

That's a subjective idea, though it usually just means uncomfortably hard. The actual roadholding is likely to be worse because the suspension hasn't been set up to work with hard tyres.

I always use manufacturer's (car, or tyre if not OEM rubber) values. (If you have a serious crash and are well away from those then police and/or insurance may look dimly on you.)

I also don't see much point in raising pressure for motorway driving since sustained high speeds will raise the temperature and pressure anyway.

[PJR77]

When you consider that, for each tyre, the amount of tread in contact with the road is only about the size of the palm of your hand, why would you want to reduce the braking efficiency by over-inflating? It is not logical.

[TonyHSD]

3 hours ago, PJR77 said:

When you consider that, for each tyre, the amount of tread in contact with the road is only about the size of the palm of your hand, why would you want to reduce the braking efficiency by over-inflating? It is not logical.

Correct 👌👍

Many sport and rally cars are running low pressure tyres for that reason. The risk of low pressure tyres and high speeds otoh is that this can cause overheating and blow off the tyre. But here the numbers of pressure and speeds has to be extreme. Also the reason, why we see lorries with disintegrated tyres on the motorway, simply because they had been overloaded and speeding. 

[Rhymes with Paris]

17 hours ago, Cyker said:

Sorry, used to using all these acronyms from being born with a modem up me butt and also laziness  

IIRC - If I Recall Correctly

TBH - To Be Honest

 

Got it, thanks.

[Rhymes with Paris]

Different thing I know, but remembering back to going everywhere on a push bike,if the pressures were low the bike took a lot more strength (energy) to shift.

Also the tyres shifting about on the rims causing instability.

And a lot less when over pressured, and more stable.

Back to cars,if grip is reduced by increased pressures ( as recommended for higher speed by the door sticker) why would Toyota and other manufacturers recommend reducing grip while driving at high speed?

Of course, I understand the difference between increased pressures and over inflation, but the logic still applies to my limited understanding 🤔

[Stivino]

A lot of door stickers don't have a reference to speed, just load.

[MikeSh]

1 hour ago, Stivino said:

A lot of door stickers don't have a reference to speed, just load.

It's been many years since I had a car with a speed note for tyre pressures. I wonder if it was a cross-ply thing? Tyres are so different these days.

[MikeSh]

1 hour ago, Rhymes with Paris said:

why would Toyota and other manufacturers recommend reducing grip while driving at high speed?

It's a compromise. Trains are an extreme example - really hard tyres (solid steel) giving great load bearing and economy of movement but atrocious grip.

[baxuz]

39 minutes ago, MikeSh said:

It's been many years since I had a car with a speed note for tyre pressures. I wonder if it was a cross-ply thing? Tyres are so different these days.

The Mk4 Yaris with 15" tyres has a sticker. It's 2.3 front and 2.2 back for < 160km/h, and 2.4/2.3 for over 160km/h.

 

[MikeSh]

2 hours ago, baxuz said:

The Mk4 Yaris with 15" tyres has a sticker. It's 2.3 front and 2.2 back for < 160km/h, and 2.4/2.3 for over 160km/h.

 

Ah, that makes sense then. They probably don't supply that info in markets where that speed is nowhere near legal.

Only 0.1 bar (1.5psi) difference too. That surprises me, but does suggest that at UK motorway speeds the required extra would be miniscule.

Can the MK4 sustain 160kph (100mph)?

[TonyHSD]

5 minutes ago, MikeSh said:

Ah, that makes sense then. They probably don't supply that info in markets where that speed is nowhere near legal.

Only 0.1 bar (1.5psi) difference too. That surprises me, but does suggest that at UK motorway speeds the required extra would be miniscule.

Can the MK4 sustain 160kph (100mph)?

Yes they can , judging by the speeds I have been overtaken by Yarises mk4 on the motorway 🏁🚗👍

[Cyker]

What's even scarier is the speed of the Aygos overtaking the Mk4's!!  

So, the idea of increasing pressures at high speeds is to stop them overheating; At lower pressures, the tyre will flex a lot more which can cause them to overheat faster. At normal road speeds this is unlikely to happen, but I imagine it's more likely at higher speeds.

When the Highways 4x4 made me drive my dad's Verso on the flat tyre, it was literally smoking by the time we got to the lay-by because it was flexing so much and rubbing against itself, and this was at an idle-crawl in 1st gear! (It also shredded the sidewalls, with what I can only describe as a curious chevron/herringbone tear pattern. I'm still wondering why that formed and was so even!)

The higher pressure reduces that flex, meaning the tyre will not heat up as quickly, and also increases stability at the expense of some lateral grip. As someone who routinely runs higher pressures, I find effect on straight-line braking is negligible; If anything it's better as it's more stable and doesn't flop about. However, braking while turning has more tendency to trigger understeer.

 

I'm surprised at what that sticker says - In previous Yarisusueusuis the pressure difference was much higher, 5-6psi IIRC. For 1.5 differenc it isn't even worth changing them as the heat from driving will make more difference than that!!

I'm also wondering why it has the pressures so low; My manual says 41psi!   I think if I had them at 33psi the tyres would overheat very quickly on mine as there is a large amount of bulge and flex at that sort of psi...!

[Rhymes with Paris]

Further thoughts on this, I do understand the compromise between rolling resistance and straight line and lateral grip.

And the logic of reduced contact patch of a tyre.

But as anyone who has experimented with grip in wet or icy or soft snow conditions(where safe or off road) will know, the amount of grip is surprisingly high when the conditions are such that it is difficult to stay on your feet.

And not forgetting the slick tyres in dry racing situations being far superior to treaded tyres.

Maybe a Michelin tech is on here  somewhere to enlighten 🙂

Oh , and IIRC,see getting the hang of it now 😁

I had an old Talbot autosleeper once, and that ran tyre pressures around 60-70 psi, which I did not believe in the handbook, but checked and that was correct.

[TonyHSD]

My story with the tyres and my current car. I switched from 215/45 17 down to 205/55 16 . The manufacturer recommended pressures at oem size 17”  were 2.3 and 2.2 bar. Since I swapped the tyres the new size obviously are softer with higher side walls I inflated them to the same values 2.3-2.2 front and rear and the car felt right but very soft , then I inflated to 2.4-2.3 bar and I had a bit of harder drive. Then I thought what was the point of having different size wheels and tyres if I have the same or worse comfort., I set the tyres at 2.35-2.25 bar front and rear on cold and since then I found my sweet spot, the perfect balance between comfort, rolling resistance, road noise and tyre wear. I do regularly check how they are wearing and tbh they do very equally front and rear on all 4 corners. 👌

[Cyker]

I find the narrower the tyre, often the higher the PSI so it's not that strange - I think my spacesaver is rated at 60-70psi too! (Maybe space-savers are really reclaimed Talbot tyres!  )

My brother's old racing bike, before someone nicked it, was even more ridiculous, needed 120psi!  

He thought he could get away with a lower pressure as none of us had a pump that would go that high, but he was constantly getting punctures (I'm feel at one point the inner tube was more patch than inner tube  ). He eventually bought a pump that could get to 120+psi and was fine after that, although was a bit of a bugger finding a pump that could go that high that he could carry with him!!  

Bizarrely, narrow tyres also have better grip than wider tyres, everything else being equal, but overheat faster. This is more noticeable with sub-optimal surfaces, e.g. rain and snow. However with wider tyres you can use a stickier compound as there's more surface for it to wear out; The same compound on a narrow tyre will likely wear out too quickly.