Tyre question: performance

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Geektastic

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#261651 8-Dec-2019 21:44

I have Goodyear Eagle F1 SUV tyres on my Range Rover in 250 55 20.

The OEM tyre is Continental SportContact in 250 50 20. I actually have one unused (was the spare) but if it is an original (it may well be) then it's 11 years old and never been driven on, so I do not know if it would still be safe to use.

I recently did a 1500km trip in the car and the tyre were, generally, fine. However, I did notice that on occasion at what you might call mid speeds, the tyre would seem to grip suddenly during a turn to a greater extent than it had been. It was not violent or a cause of loss of car control or anything but just felt a bit strange.

The tyres are at the correct pressure (36).

The car is heavy at approx 2500kg and also reasonably powerful with 400BHP.

I know lots of people here know a lot about cars and tyres, so

1) does the fact that the tyres fitted are 55 instead of 50 have much impact on this (or anything else)

2) Having never driven the car on other tyres, I do not know if the steering behaviour is a car dynamic or a tyre dynamic; any ideas?

3) If I replace them with the OEM tyre at some stage, is the one I took off the spare, which still has the delivery paint on it, likely to be safe to use?

nzkc

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#2370724 8-Dec-2019 22:15

I'm no expert here, but I can tell you this... (and apologies if you know this already)

The 250/55/20 will have a higher/larger wall than the 250/50/20. Thats because the middle number represents the wall height as a percentage (I think) of the width (which is 250 in your numbers). The 20 is the rim size in inches - Im always amused its mm width and inches diameter.

What impact will this have? Well definitely a potential for a change in driving characteristics and thats what brought you here. Also it'll have an impact on the speed you are travelling at vs what your speedometer says. This is because your overall tyre size is now larger.

Ok...its not a huge amount of change 125mm to 137.5mm. I cant say how much of an impact or effect that would have. But I imagine it could be what you are noticing. Also - it could just be the different make and model of tyre.

What would be bad is adding the old (spare) tyre on as its a different size to the other 3! I'd definitely be getting them to all be the same size.

Edit: Oh and there are other numbers on the tyre too which provide info about things like maximum speed etc.

Scott3

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#2370738 8-Dec-2019 22:31

Regarding the increased aspect ratio From OEM (change from 50 to 55), What this means is the diameter (and circumference of the of the tire is 3.4% greater than OEM). This is below the threshold (5% Change) where you need to have the change certified, so is legal to drive on.

The "55" tires have a sidewall being 13mm taller than OEM (an increase of 10.2%).

This will have some impact on the characteristics of the car.

13mm more ride height - Extra ground clearance, slightly taller CoG, Seats a touch higher when getting into the vehicle, vehicle will be a little taller when checking clearance in low car-parks.
Actual speed will be 3.4% more than the car thinks it is going (impacting odometer, and speed readout)
Increased sidewall height will give more ride comfort, but looser handling
Effective gear ratio will be changed by 3.4%, so RPM will be a touch lower at open road speeds than with OEM tires
Clearances around tires will be decreased, May no longer have enough clearance for snow chains.
Contact patch size may be increased, you could try a touch more air pressure than the vehicles specifies to compensate, - will also give slightly tighter handling characteristics.

I can't really comment on the particular handling characteristic you identified.

Regarding point two, short of finding somebody with the same car, and swapping wheels, this would be hard to diagnose. You could take it to a Range Range rover specialist, or a tire specialist, but I think both would try to push you to buy OEM size tires.

Regarding point three, (the spare tire). The key issue here is tire age. There will be a manufactured date code molded into the side of the tire, from which you can establish it's age. The tire manufacturer recommends tires be removed from service 10 years from the date of manufacturer regardless of tread wear. (unless the vehicle manufacturer recommends something else)

https://www.continentaltire.com/news/how-long-does-tire-last

In reality, aspects like UV exposure impact tire life, implying if the spare positions is somewhere dark, life time may be longer. Also, much of the NZ fleet has old and/or very worn tires as spares. Many NZ trailers also have quite old tires on them (although based on holiday travels trailer tires seem to fail way more than car tires).

If you ever need to use you spare with your current set of tire's, treat it like a spacesaver (diff's will be working overtime to balance wheel RPM), and be aware the car's traction control etc may freak out.

Batman

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#2370758 9-Dec-2019 06:52

tyre: no issues. 55 is more comfort and more squirmy during corners.

sudden increase in grip: hard to know what speed and what corner. apart from wanted/unwanted computer controlled/mechanical (LSD) input to your axles i think the most logical explanation is what i call tyre squirm.

you see the tyre sidewall is holding up the entire weight of that car's front quarter. the bbigger the number (55) the taller the sidewall the less strength. when you turn left, the right front tyre has huge amount of load put on it. this load is not static but dynamic. the tyre actually deforms and warps.

so it is effectively wobbling as your 2 tonne car is bouncing around the undulating camber (forces are constantly going very high and very low from your suspension trying hard to respond 10 milisecond too late to the car bouncing, when it's completed the bounce by then and starting on the next bump). to add to this the tyre isn't perfectly built and isnt 100% uniform all round, increasing the wobble esp at resonant frequency. (f1 tyres are as close to 100% uniform as possible, but i doubt your goodyear eagle are anywhere near that!)

is it all the tyre? i suspect a Disco's suspension is made for offroading and not so much maximum damping for the highest bank speed. maybe swap the fronts and back around, get it rebalanced. i'd also get your suspension looked at but i suspect the dynamic forces on non perfectly flat roads is the reason.

Batman

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#2370760 9-Dec-2019 06:54

Azzura

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#2370846 9-Dec-2019 07:22

This may or may no be related to your situation.

All I can tell you is in the past on my Mitsubishi Triton. Put the 2 new ones (same model/part as the tyres being replaced) on and left the older ones on (still had some life on them) ---man did I get a surprise on a couple corners on the way home....the anti skid or the abs feature was kicking in a little ---pretty scary!

Got it home and started to think wth? --started to wonder if the difference in circumference between new and old tyres was enough to trigger the anti skid or abs feature during certain cornering conditions. Took the Triton in the next day (driving tentatively) replaced the older tyres...problem was gone. Ever since I get all fours replaced when new ones are needed.

Batman

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#2370852 9-Dec-2019 08:02

Re Q3. Use 4 tyres from the same batch as different batches may have differences that could do what azzura describes

ShinyChrome

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#2370855 9-Dec-2019 08:08

Unless you can find a 11 year equivalent (no, don't even do that, I am being facetious), do not mix the spare tire with new tires. That is a recipe for a bad time. If you want to do something useful with it, turn it into a garden planter.

I'm inferring this issue has only started since you fit these tires?

Does your Range Rover have any form of electronic stability control?

When you say it seems to have more grip suddenly in a turn, are you meaning something like it suddenly starts turning tighter for the same amount of steering angle/driver input?

Does it happen consistently on a corner of the same angle or at a range of different angles?

Does it happen on a variety of different road seals ie. smooth, rough etc?

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Geektastic

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#2370897 9-Dec-2019 09:44

ShinyChrome:
Unless you can find a 11 year equivalent (no, don't even do that, I am being facetious), do not mix the spare tire with new tires. That is a recipe for a bad time. If you want to do something useful with it, turn it into a garden planter.

I'm inferring this issue has only started since you fit these tires?

Does your Range Rover have any form of electronic stability control?

When you say it seems to have more grip suddenly in a turn, are you meaning something like it suddenly starts turning tighter for the same amount of steering angle/driver input?

Does it happen consistently on a corner of the same angle or at a range of different angles?

Does it happen on a variety of different road seals ie. smooth, rough etc?

Yes the car has DSC, ETC, ABS and all that stuff.

Your description of the characteristics is quite accurate. It felt like the car was turning at 50% then suddenly at 60% if that makes sense.

It didn't happen consistently as far as I could tell. There was no noise or chatter from the breaking system, which there usually it's when the ETC or HDC are engaging. I suppose the DSC might have been doing something, but I was well within the normal performance envelope, the roads were as normal as they get here, not wet either.

The tyres were fitted by the dealer last year when the car was purchased. The size change probably came because whoever fits their Tues for them got that size cheaper....

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#2370934 9-Dec-2019 10:43

There's a new gimmick called torque vectoring that could explain your issue but I didn't think your vehicle had it.

Geektastic

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#2371004 9-Dec-2019 11:22

I spoke with my tyre guy.

He reckons I ought to try running the tyres softer. He suggested calling by when I next head his way and he'll reset all the tyres for me to 34. He said that if I know that I will be doing a long drive on gravel roads (such as the Acheron Road which is a 5 or 6 hour drive on gravel) then deflating to 30 would be helpful as well.

He suggested returning to OEM size when the tyres are worn but that it wasn't a reason to be concerned enough about that they should be changed early.

Bung

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#2371060 9-Dec-2019 11:55

2psi seems a bit subtle. Were there any waves in the seal that may have resulted in your rear wheels doing a little skip?