Author Topic: 1st Autocross  (Read 2018 times)

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Offline rjbaren

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1st Autocross
« on: Friday,September 17, 2021, 04:16:49 AM »
Hello, I am signed up for my first parking lot type auto cross next weekend with the local BMW club.  I have a 73 Twin Cam Special.  My tires are stock size Barum and my suspension is stock except for Spax adjustable shocks and the stock front springs were changed because they were too tall.  I think a pair of 110 lb. springs were installed but I am not sure the poundage. 
Any tips on shock settings and tire pressures?  Any tips in general?  I have only auto crossed once about 30 years ago in a stock Sunbeam Alpine.
Thanks

Offline E Paul

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Re: 1st Autocross
« Reply #1 on: Friday,September 17, 2021, 10:40:08 AM »
It has been a long time since I autocrossed my stock suspension Europa Special. What I remember was that I needed to reduce the rear tire air pressures to help get the car to rotate. In the sharp corners of the parking lot events, there was just to much understeer.
Just have fun!

Offline Clifton

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Re: 1st Autocross
« Reply #2 on: Friday,September 17, 2021, 12:02:44 PM »
Awesome that you are doing it again. I autocross too. Start of our season is Sunday. My car is a little tail heavier than most Europas due to a different engine but I run 18 psi front and 24 psi rear.  If you don't have a pyrometer, you can get some chalk and chalk about an inch long and 1/2" wide of the edge where the tread and sidewall meet and you will know if you are too low and rolling it over. Lower is better until it's too low. If you have some tall, regular all seasons maybe a few more psi but don't follow what 3000lb + car guys run.  I run my shocks the same on the street as autocross. Just enough rebound without being uncomfortable. I don't change anything. Show up, run, and drive home. If you stiffened up the front springs only and not the rear it should understeer a little more. Easier to drive than loose but if it's excessive understeer, you can run the rear shock stiffer, if they increase bump and rebound and try to reduce the push. Only change one thing at a time.

Offline rjbaren

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Re: 1st Autocross
« Reply #3 on: Friday,September 17, 2021, 01:35:21 PM »
Hi and thanks Clifton.  I have all season tires I think they are currently set at 16lbs. front and 26 lbs. rear pressure.  Also I checked the shock adjustment and they are set all the way counter clockwise which I believe is the least stiff.   So would those pressures and basically no adjustment be a good place to start?   

Offline Clifton

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Re: 1st Autocross
« Reply #4 on: Friday,September 17, 2021, 02:23:56 PM »
I would run more front pressure and a little less rear. I run "street" autocross tires and have a pretty stiff sidewall plus I run stiffer springs, 250 lb front and a large bar. My 18/24  would probably too little for you with stock camber. I think 16 psi in the front with all seasons will roll over more than you want for autocross. Maybe 22 front 24 rear cold. I would bring a pressure gauge and if you aren't close to the edge on the first run, I would lower 2 psi then 2 psi on the next run. It's easier to lower then add in between runs. Your pressure will come up too after you get heat in them so don't go by the same numbers when hot. My 18/24 is cold.  I would just make sure the fronts are being used equally the same as the rear, both at the same point on the edge, whatever that ends up at. If you know they are being used equally the same and you find the limit of slip at one end, you can go from there rather than wonder if it's pressure or spring or both.

You don't have to chalk but it is easy to see and cheap.

First pic is chalk half wiped off so you can kind of see what it looks like before and after. It is a cleaner line after a run.

Second pic is how much roll over I have.

Third pic is wifes car, street use only. If hers looked like this pic after a run I would lower it 2 psi per run until it rolled over to match the second pic of mine.

Offline BDA

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Re: 1st Autocross
« Reply #5 on: Friday,September 17, 2021, 02:44:05 PM »
Ok, I'm a bit out of my depth, since it's been an age since I last autocrossed and I knew even less than I do now about shocks and springs but here are my thoughts (I welcome comments from those more knowledgeable):

Because you seem unsure about exactly what springs you have: How does your car handle now? Does handling on the highway seem vague or unstable? Do you have to intently watch steering inputs on the highway for fear of ending up in the next lane or even a ditch? That's likely a sign that your shocks are not stiff enough for your springs.

Is your ride rough or even bouncy where the car seems to follow every undulation of the road and absorbs none of them not enough of them? Then your shocks are likely too stiff for your springs.

Considering that you mentioned you might have 110# springs (probably on the rear since the stock front spring rate is 116#) and your shocks are set full soft, I'm guessing that you probably have your shocks, at the rear in that case, set too soft.

Assuming that your shocks are in a comfortable setting where steering and handling is precise in fast turns and the car is stable on the highway, you might turn your shocks up two clicks for your autocross. If that works well, you could experiment with a couple of more clicks.

Shocks can affect the balance, too. since they affect the roll resistance. Increasing the roll resistance in the front will give tend to give you more understeer and more roll resistance in the rear will tend to give you more oversteer. Tire pressures also affect roll resistance. I've attached some information about that.

If you plan on doing more autocrossing, it would be good to know what springs you have. Quite often springs are marked with their rate. Sometimes with paint, or engraving on the end of the spring (the flat part). Sometimes there is a band that is heat shrunk on the coils with the rate. Who knows how it might be noted on the spring (or not). Look all over the spring for some identifying marks and if you can't find any, you can use a bathroom scale, some 2x4s, threaded rod and a ruler to determine the number of pounds force is required to compress the spring one inch. It's linear so you don't have to start at 0 and go to 1 inch, you could start at 3/4" and go to 1 3/4 inch for example. Knowing how your spring rates compare with the stock rates would be helpful.

Because I only know a little about this (and because I already have more springs than I need), I would try to stay close to the same multiple for spring stiffness. For example, you mentioned you might have 110# springs. If those were on the rear, they are 46% stiffer than the stock 75# springs. I would want a similar increase in stiffness in the front springs to maintain the balance that Lotus designed into the car. Of course, if you are an experienced performance driver you might have a different idea of how you want your car to behave and thus might want a relatively stiffer rear spring for more oversteer or a relatively stiffer front for more understeer, but I think most of us "amateurs" just want what Lotus designed or maybe "a bit more" of what Lotus designed.

A lot of this is personal taste. I've made my suggestions based on my experience with new shocks and not actual racing or autocrossing experience. I'm still experimenting and I readily admit that I am walking in the dark to some degree. I hope if someone more knowledgeable thinks I've steered you in the wrong direction, he will set me straight.

Offline rjbaren

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Re: 1st Autocross
« Reply #6 on: Sunday,September 19, 2021, 10:02:52 AM »
I checked my pressures this morning and the fronts are at 14psi and the rears are at 24 psi.  I also added 8 clicks to the firmer side on the rear shocks.  I read if you adjust all four shocks equally than you haven't really made a change.  So I will leave the fronts alone and then maybe even drop back on the rear.
The car felt fine in a straight line.  Then I made a couple of quicker turns and the tires were squealing quite a bit.  I am thinking of adding more air to the fronts, like 2 lbs., and trying it again.   

Offline BDA

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Re: 1st Autocross
« Reply #7 on: Sunday,September 19, 2021, 04:55:38 PM »
Tires squealing just means they’re working hard. Two more pounds in the front might be good. Use Clifton’s trick about the chalk (when I was autocrossing an age ago, we used white shoe polish - can you still get that?) to see if your tires are rolling over to help decide if you need more or less air in your tires.

I think what you read about adjusting all your shocks the same amount is about the balance. If you have too much or too little damping, it will make a difference.

I seem to remember that the SPAX folks told me that the adjustments are not linear - i.e. the difference between click one and click two is less than the difference between click two and three. I also seem to remember they suggested increments of two clicks.

Just some thoughts…

Have you run your autocross? Let us know how it went!

Offline rjbaren

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Re: 1st Autocross
« Reply #8 on: Sunday,September 19, 2021, 08:16:27 PM »
Autocross is the 26th, unless its rains.  Then I will take my BMW wagon instead.  I don't think the Europa is water tight.  I bought some chalk and I want to find a cheap 12 volt compressor with cables to connect at the battery because the car doesn't have a cigarette lighter.

Offline Clifton

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Re: 1st Autocross
« Reply #9 on: Monday,September 20, 2021, 05:02:14 AM »
Bike pumps work good. I cut the end off a female cig plug and stick the wires in a 18v Makita battery for portability. Pumps fast with more volts.

Offline rjbaren

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Re: 1st Autocross
« Reply #10 on: Monday,September 20, 2021, 08:28:53 AM »
They make electric bicycle tire pumps?  Sounds interesting.

Offline BDA

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Re: 1st Autocross
« Reply #11 on: Monday,September 20, 2021, 08:35:45 AM »

Offline Clifton

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Re: 1st Autocross
« Reply #12 on: Monday,September 20, 2021, 11:40:30 AM »
They make electric bicycle tire pumps?  Sounds interesting.

Idk but I meant hand pump bicycle pumps.

Offline rjbaren

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Re: 1st Autocross
« Reply #13 on: Tuesday,September 21, 2021, 03:58:59 AM »
I found a very small 12 volt compressor by Slime.  I bought the cheapest one for $17.99.  I spliced in the necessary wires to be able  to connect to a 12 volt Bosch drill battery and I left the cigarette light plug for other cars if necessary.  It is supposed to inflate a tire from flat in 12 minutes.  It seems a bit slow but it works and it should be fine for the autocross.

What a great idea to connect to a portable drill type battery.  Thank you very much.

Offline rjbaren

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Re: 1st Autocross
« Reply #14 on: Monday,October 04, 2021, 08:10:12 PM »
Well, I got the results and I came in 68th of 70 cars.  I ran with the Windy City chapter of the BMWCCA.  The winning car was 2005 Subaru Sti.  There were 6 Porsches and a slew of BMWs.  It was fun that's for sure.  The only mechanical problem I had I noticed at the end.  My ignition light came on and the battery wire to the alternator came uncrimped.  Overall I am happy with the result, I drove there, ran the course 11 times and drove home.
Clifton, your car looks incredible in your photo.  Is there a post where you have more photos and a description of what you have done with the car?