Car Limits Day - North Weald

[Crunchie Gears]

I went to a Car Limits day at North Weald yesterday in the Zenos and had a great day out with plenty of driving. 

The day was organised by Southern Kit Car Club and had 20 cars.  For some it is competitive as they use radar traps and stopwatches to record and publish them on leader boards.  The banter and excuses add to the day.

The day is split into sections starting with a high speed bend and a radar trap to measure success.  Then two sprint circuits which are done in two groups and then a change over.  An hours lunch and then a big sprint circuit (about a mile and a quarter) until 4.00pm then debrief search for the guilty, praise and honours for the best spin and so forth.

[Crunchie Gears]

So how did the Zenos go?

I have to say that I do not get the high speed bend (see the excuses) but did OK. About mid-field with a 72 mph best agains the winner at 81.

Then we went on to the sprints and I found the car. Was locking up quite easily and was understeering through slow bend in particular.  It would then snap to oversteer and I was about mid-field again.  I did come out of the fast bend foot planted and the back end snapped out.  I then did a tank slapper with power still on until I lost it completely and had a big 270 spin.  My passenger jumped out quick!

I should point out that I was running factory tyre pressures.  After lunch a sprinter who visits N. Weald often to compete in sprints,  He had his car there yesterday and hence the 81mph above,  helped me make some changes.  So we set a base lap time and to check it he did two laps too.  And set an average of 87 seconds.  Which was only beaten by 3 cars on the first ru n but the quickest was a GBS Zero with 200+ bhp in 80.6 seconds.

So we dropped the tyre pressure to 22 all round and went out again.  83 seconds.  The car was still locking up a little and the understeer was highly reduced.  The car was now much improved.

We took the temperature across the Tyres and the centre of the fronts was a little hotter than the inside.  So either, or both, the Tyres were still to hard or the car needs more negative camber.

Anyway we dropped the fronts to 20.  First run out 80.63.  Them 79.94 then got a heavy passenger (see more excuses) and a 80.69.

My best time was good for second place as Rob's Zero had moved to a 79.5.  Jeff's pheonix sprint car had a miss fire and would not Rev over 5500rpm of a max of 9500 and still managed a 79.97.

The car is much improved and I am very happy it is in the mix with some very quick sevens.

On the way home I found the car tramlines all over the road LOL maybe the Tyres have dropped pressure as they cooled.  Maybe the factory have a point

[Moleman]

I have never done a sprint as I rather do track days where you get more time on the track. I'm told there is a lot of hanging about but I could be wrong. 

[Facial Hair Optional]

You sure Colin? The last sprint I was at all I hear people say was "he's not hanging about is he"!! 

I don't know what the factory pressures were on your car CG but most kits/sports cars at kit car type weights (say 500 - 700kgs) are running anything from about 16 to about 20psi as standard when not on a track. It's mostly about the weight of the car I think?

There is a formula online somewhere I found ages ago that tells you exactly how to work it out. I will see if I can find/remember it sometime.

[Crunchie Gears]

It is a different type of day to a trackday.

1.  It costs £45
2.  Timing is Ok
3.  You can take it past the limit as there is nothing to hit.
4.  You can try different driving techniques and see what difference it makes on the time.
5.  With 20 cars there is about 1 hour 20 minutes driving in the day which is OK.

I prefer a trackday but IMO they are different but both have something to add and are good fun.

As for the tyre pressures I agree FHO but they have done so much testing you have to wonder.  The Westfield works at 14 all round.

[Facial Hair Optional]

I'm sure they have done tons of testing but unless the tyres are radically different from any others made the old time tested and proven formula should still hold true I would think. I cannot see any reason why one car would be different than all the others.

The formula is (I think) something like this... On the side of each tyre is a rating, it'll be something like 950kg at 50psi. Obviously it's different for every tyre depending on the make, size, speed rating etc but every tyre has this written on it I think. That particular rating was for the tyres on a BMW X5.

The formula is this... the weight of your car in kg divided by 4 times the 1st figure on the sidewall, ie in this case 950kg x 4 (cos you have 4 tyres over which the weight is distributed), then multiplied by the 2nd figure (in this case 50 psi)

So for example for an X5 it might be 2,400kgs (inlcuding the driver) divided by (4x950)kgs = 0.6316 x 50psi = 31.58psi

Try it for your cars and you will see it is pretty much spot on! That is why on car tyre pressure diagrams (inside the door etc) they show a higher pressure for 5 people plus luggage, more weight (and also high speed motorway driving of course). The same reason track work means you have to change pressures, cos the tyres are under more load.

You can even make fine adjustments to include driver, passenger and fuel weight or weight over each axle as it is unlikely to be 50/50 in most cases. Of course how much difference it will make in the real world is anyone's guess but it's a good place to start if you are not sure what tyre pressures you should have on your car. All you need is the tyre ratings (on the sidewall) and the weight of your car and off you go!

When I first got my Tiger (before I was a BKCC member) and someone told me I should have about 18 psi in the tyres I didnt believe them. The car came to me running 32 at the front and 35 at the rear!!!!!! 

Because I did not believe 18psi was right I researched that formula and did the calculation, guess what?? Yep it came out at 18 psi ! My Fury being about 100kgs lighter worked out at 16.5psi, again I found that to be just about perfect for the car. I have yet to do the calculation for the ASBO but I will be doing it now as it is so light it probably needs less that it has in it at the moment!!

As you found out Paul they can run at crazy low psi and I think the ASBO may well turn out to need 14 along the lines of your Westie. The formula also explains why small changes in weight in a lightweight car can mean a significant change could be needed in tyre pressures. Plus or minus 50kgs in an BMW X5 will make sod all difference in tyre pressure requirements (try the formula and you will see!) but in a 450kg kit car can make quite a difference indeed.

If you found the Zenos improved on track as you lowered the psi then that kinda says a lot, also if it is tracking all over the road cos the pressures have dropped when the tyres cooled wouldn't it mean yiou have to just put them back to the level where they were good again NOT necessarily the factory settings? Also the tracking may be suspension/camber/castor or whatever set up - best ask Dave Gallop about that one cos it's a black art as far as I am concerned!! My head is spinning now!!!

[Crunchie Gears]

Thanks FHO I will try that and see what the results are. 

[Facial Hair Optional]

It'll be interesting and keep you busy for ten mins if nowt else CG!!   

[sanzomat]

The part of all that tyre pressure stuff that I've never understood is why car manufacturers specify a higher pressure for high speed use. I would have thought high speed use would mean higher temperatures so the pressure would go up all by itself so if you set them higher at cold then surely the'll be massively over when they are hot on a fast run?

[Moleman]

Dave at the start of every track day I just set them to what I want them to be do a few laps come in once they have warmed up & reset them again. ( to which you are right once the tyre is hot it does increase the pressure. 

[Facial Hair Optional]

Yeah I dont really get that either Dave but maybe we are confusing track use and road use? Maybe it's a different protocol for some reason? 

[Crunchie Gears]

Interesting as some of the cars at V-max run the pressures very high!  I do not get this but some say it is better.

[Facial Hair Optional]

For (some of) us to relate to that CG, you'll have to explain what V-Max is!

[Crunchie Gears]

v-max is a day out to a long runway and 200+ mostly supercars get invited to go and try to Velosity max out their cars.  Some of the cars run their Tyres quite hard.  A lot of the cars  manage 200 mph +

There is a thread on pistonheads for it but they normally go not post the date so they do not attract attention but there was about 50 people with cameras outside the venue this year.

[Facial Hair Optional]

That explains the high pressures then, I suspect it's because the tyres don't get chance to get too hot as they are not doing lap after lap and the firmer tyre gets you off the line better as there is less deformation/deflection? No cornering either so less lateral grip needed and hence no need for lower pressures maybe? I'm just speculating here cos I don't actually know for sure, just trying to figure out why it might be. Anyone else reading this know why??

[Facial Hair Optional]

Just found this.....

Because of the weight they bear, pneumatic tires' sidewalls bulge and their treads flatten as they roll into contact with the road. This results in dimensional difference between the tire's "unloaded" radius (i.e., between the center of the axle and the top of the tire) and its "loaded" radius (between the center of the axle and the road). The engineer's call the difference between the two radii "deflection." Increasing vehicle speed will cause the tires to deflect quicker and increasing vehicle load will cause the tires to deflect farther (if tire pressure isn't increased).

The European Tyre and Rim Technical Organization (ETRTO) establishes the standards for tires sold in Europe, and recognizes that the tire's deflection must be minimized and controlled in order to surpass high speed driving stresses. In order to accomplish this, the tire inflation pressure recommendations and the tire's rated load capacities are customized when speeds exceed 160 km/h (99 mph) for all tires up to and including a V-speed rating, and when speeds exceed 190 km/h (118 mph) for all tires that are Z-speed rated and above.

Full blurb here.... http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=72

Obviously on a race track top speeds are not as high AND you need cornering grip, hence the lower pressures I suspect

[Crunchie Gears]

FHO that is a great bit of research I am impressed.  The article makes sense and explains the issues.  I do not think most. Of the people putting air into their Tyres have anything more than instinct to go by.

Thanks again.

[Facial Hair Optional]

What would we do without Google eh?! Algorithms rule man!!!