Tyre Pressures

[captain chaos]

I am new to Sevens and my new car came with ACB10's (7.0-21.0 Front & 8.0-22.0 Rear).

The previous owner advised running 12psi. Today I shod the vehicle with some new boots (same ACB10's again) which were fitted by BMTR in Birmingham. They put 20psi in each tyre. I have only done about 80 miles in the car so i do not have much experience for the purposes of comparison.

Comments please. (poss giving both road and track pressures would be helpfull).

Thanks.

[Peter Carmichael]

20psi is almost certainly too high.

 

Because they are crossplies, they have poor control of the large tread width. A high pressure will tend to balloon the tyre. This has nasty consequences, with snap oversteer in the wet being a favourite.

 

So less than 20.

 

12 seems to be a good lower bracketing value.

 

Get a pyrometer and try them out on track.

 

(On 6in ACB10s, you aim for ~21psi hot running pressure which equates to ~16-18psi cold depending on front/back, cold/hot day etc.)

[captain chaos]

Thanks for your reply Peter.

[Mike Biddle]

I have been running the same tyres for a year now, and would agree with peter, 20 psi cold is too much. After a lot of testing / adjusting including the use of a pyrometer, I found that 18 psi cold works best for my vauxhaul engined car. You don't mention which variant you have, and a lighter car may require less.

Anyway, you can have a lot of fun finding out!

[Red SLR]

any idea what psi a 1600 superlight should be at?

 

X777CAT

[captain chaos]

Mike,

Thanks for our comments. The car is a Vx 2L running wide track and bilsteins with adj platforms.

I am going to run 16 psi at the ring in a couple of weeks and take it from there.

 

[Alex Wong1697456877]

By all means start with 16psi all round. If you find it understeers too much, try upping the fronts to 18 and/or dropping the rears to 14. 18/14 seems to give my car a good balance for sprints. I do have different tyre sizes than you (6inch front, 7inch rear ACB's) but the chassis/dampers/springs and engine weight are the same.

 

Alex

[old captain slow]

What about AO32's. My son's Cat has them (13x185x60) and he's running them at 20psi all round.

[Dave H]

Whatever tyres you are running, there is a certain amount of trial & error to get the setting which best suites your driving in your car. I don't think any 2 Sevens are the same, so find a good bit of road (or a track) and get out there and try it, adjust it & try again.

20 psi sounds a little high - I ran mine at between 15 & 18 psi, depending on temperature/load/what I was doing/how the car felt on the day.

[Nick]

The following is a copy of one of Mike Bees' old posts, (thanks Mike!) and may help. I run the AO32Rs on my 1.6K ss at about 18 psi cold. Apparently, 23 front and 24 rear are good pressures to aim for when hot - so say Hyperion and a few others.

 

Quote:

A rather obvious but possibly overlooked point about tyre pressures: For a given situation there will be a tyre pressure that gives the optimum amount of grip from the tyre. Adding more pressure than this will reduce the grip, reducing the pressure from this will reduce grip.

 

Ergo you can't have a general rule which says "increasing the front tyre pressures will cause less/more understeer" because it depends (a) on where you're starting from and (b) on loads of other factors.

 

The best way to determine the tyre pressure for optimum grip (or more correctly to make optimum use of the tyre's width) is to use a pyrometer. Measure the temperature of the inside, centre and outside of the tyre. Plot them on a graph with temperature on the Y axis and position (inside, centre, outside) on the X axis. If the 3 points form a horizontal line then bingo you're heating the whole width of the tyre evenly. If they form a straight line but with the inside hotter than the outside or vice versa then the pressure is optimised but the camber and/or toe is causing a temperature gradient across the tyre. If they don't form a straight line then draw a straight line between the inside and outside temperature points - if the centre point is below this line then the middle of the tyre isn't doing enough work so more pressure required, if the centre point is above the line then the middle of the tyre is doing too much work hence less pressure is required.

 

The requirements will change from venue to venue and day to day (even hour to hour) at the same venue.

 

[Peter Carmichael]

er...

 

corner to corner

 

According to Buddy Fey's book 'Data Power', a twenty second straight is sufficient time for tyres to lose their temperature resulting in a 15% (from memory...) reduction in peak cornering force in the following corner. You can also go twenty seconds in a sequence of right hand bends between left handers and this will mean that your RHS tyres will cool off and you will have reduced grip in the second left hander.

 

When you take tyre temperatures they will only tell you about what they have done most recently. Before relying entirely on the pyrometer think about what the last corner before the pits was. Adjusting pressures/camber etc. must take into account the balance between being resonably set up for all corners and being optimised for the corner which has the greatest single consequence for the stopwatch.

 

Which isn't exactly relevant to most of us.

 

This is the sort of stuff that is relevant to race compound tyres (A032Rs, ACB10s) which develop most of their heat from hysteresis in the tread compound. I'd go with Dave's try lots of things and see what feels right.

[Andy N]

If you are new to sevens get a copy of a book called Lotus Seven-restoraton,preparation,maintenance by Tony Weale.

 

Has just about all you need to know to get you going.....

[Alan Clark]

What tyre pressure do Caterham recommend for the road use of a 270s fitted with Avon ZT7 185/60 R14 supplied as standard with the kit a couple of years ago?

[Jonathan Kay]

2021 Owners Handbook "all models":

Jonathan

PS: If you'd like a copy of the Assembly Guide and Handbook please send me a Private Message with your email address.

[7 wonders of the world]

Blimey they are high for cold pressures .......

[SM25T]

That's what I thought

 

[StevehS3]

I am going to try 23 in my 13" ZZS. I wonder if the good old 18 PSI is too low. 

[7 wonders of the world]

I run 185/60/13 & 205/60/13 Toyo R88R and run them at 16psi cold

 

[Toby S]

Same set up and tyres  as 7 wonders - 18 psi all round on 1400 Supersport for what its worth

[Guy Lowe]

This is like the oil threads 

FWIW I have 185/60/13 & 205/60/13 Toyo R88R and run them at 18 fronts & 15 rears cold for the road but warm when on track.

[7 wonders of the world]

https://www.lotus7.club/sites/default/files/images/docs/guides/Avon%20Tyres%20Motorsport%20-%20Lotus%20Seven%20Club%20-%202021%20Edition.pdf

See pages 12 & 13

[bahf]

#21 - it's a brave person who argues with the manufacturer's recommendation. Note that they refer to HOT temperatures which is what their operating environment will be on track. I wonder what engineering metric or unit is used to define "hot"? "smiley"

[aerobod - near CYYC]

"Hot" for a track suitable tyre typically is from 60 to 110 C, so if 85C is taken as the average, the Ideal Gas Law for a hot tyre pressure of 1.6 bar (23PSI) / 2.6 bar absolute would indicate a cold pressure of 1.1 bar (16PSI) at 20C ambient temp. A more road-tyre orientated hot temp of 60C would correlate with 1.3 bar (19PSI) cold pressure.