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      02-12-2015, 12:15 PM   #1
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Take a look at this slalom video. What can be improved?

My buddy took a video of my Z4M from the end of a slalom during autocross last summer. Looking at the behavior/balance of my car, what do you suggest should be done? Obviously coilovers would be ideal but I am sticking with stock suspension for the foreseeable future. Front sway bar maybe? Lower/higher tire pressures? Of course my driving could use some improvement but that's a work in progress!

Embedded video below but you'll have to skip to the slalom on your own (59 seconds).


Last edited by intoflatlines; 02-12-2015 at 02:00 PM..
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      02-12-2015, 01:29 PM   #2
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Cool video. Those tires are talking to you! It was hard to tell from the video, but were you experiencing understeer on turn-in on the faster sections? I also thought I saw a bit of oversteer when you were back on the gas at exit of some turns?
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      02-12-2015, 02:56 PM   #3
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Quote:
Originally Posted by jimmckendree View Post
Cool video. Those tires are talking to you! It was hard to tell from the video, but were you experiencing understeer on turn-in on the faster sections? I also thought I saw a bit of oversteer when you were back on the gas at exit of some turns?
Hi Jim. I do experience understeer on turn-in at faster sections. You can see/hear the understeer at the end of the slalom/beginning of the left hander for example. I do have a small bit of oversteer at exit at times, probably due to sloppy stomping on the gas though? Here's the helmet cam of the same run as the slalom clip.

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      02-12-2015, 03:16 PM   #4
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You should try to run less casterangle if you want more agressive turn in.
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      02-12-2015, 03:20 PM   #5
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Quote:
Originally Posted by intoflatlines View Post
Hi Jim. I do experience understeer on turn-in at faster sections. You can see/hear the understeer at the end of the slalom/beginning of the left hander for example. I do have a small bit of oversteer at exit at times, probably due to sloppy stomping on the gas though? Here's the helmet cam of the same run as the slalom clip.
What are you running for camber and toe on the stock setup?
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      02-12-2015, 03:35 PM   #6
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Quote:
Originally Posted by GuidoK View Post
You should try to run less casterangle if you want more agressive turn in.
That's not adjustable on stock suspension, right?

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Originally Posted by jimmckendree View Post
What are you running for camber and toe on the stock setup?
Front: -2.7*, 0 toe. Rear: -2.0*, 1/16-1/8 toe-in. Front has camber washers.
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      02-12-2015, 03:43 PM   #7
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Quote:
Originally Posted by intoflatlines View Post
That's not adjustable on stock suspension, right?
No (or at least not documented), but you could run the stock (non m) front control arm rear bushings. I don't know if that's allowed.

Or... you could rotate the z4m bushings (these are eccentric bushings, so by pressing them in another position in the lollypops)
If you rotate them 180 deg that might be too much.
I don't know if that has any other averse effects. The rear of the control arm sits maybe 1/2" lower then (or higher).
Then you're working with 100% z4m parts.
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      02-13-2015, 01:21 AM   #8
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the car looks like a mini muscle car hustling around that narrow circuit!! awesome,

what suspension is on the car, body roll looks quite high, if not on a proper set of coilovers i think this would do wonders for you as you pick up a decent amount of adjustability too.
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      02-13-2015, 10:00 AM   #9
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Quote:
Originally Posted by intoflatlines View Post
Hi Jim. I do experience understeer on turn-in at faster sections. You can see/hear the understeer at the end of the slalom/beginning of the left hander for example. I do have a small bit of oversteer at exit at times, probably due to sloppy stomping on the gas though? Here's the helmet cam of the same run as the slalom clip.

The body roll on the stock suspension tends to put a high loading on the outside front tire. A stiffer front sway will help the under steer a little bit as it will reduce the loading on the outside tire. It won't be dramatic, but it will help. Installing some Koni or Bilstein struts/shocks will give increased damping and will reduce transient body roll also. Outside that, it's tuning the driver.

That being said, you can make any car under steer if you're hard on the brakes and turning sharply at the same time. A little bit of braking to load the front end to help with initial turn in is good, but then you have to be off the brakes and try to roll through the turn so both ends of the car can share the load. It takes some experimenting (and some cone killing) but once you get it down you'll see your times drop. A more advanced technique is to use throttle cut over steer to get the car to start rotating before you get to the turn so there's less turning to do in the turn. This takes a LOT of practice, and you'll have some awesome spins during the learning process, but once you get it down you should see a dramatic drop in your times.
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      02-13-2015, 10:54 AM   #10
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Quote:
Originally Posted by GuidoK View Post
No (or at least not documented), but you could run the stock (non m) front control arm rear bushings. I don't know if that's allowed.

Or... you could rotate the z4m bushings (these are eccentric bushings, so by pressing them in another position in the lollypops)
If you rotate them 180 deg that might be too much.
I don't know if that has any other averse effects. The rear of the control arm sits maybe 1/2" lower then (or higher).
Then you're working with 100% z4m parts.
Interesting, I will look into that. Thanks!

Quote:
Originally Posted by Beedub View Post
the car looks like a mini muscle car hustling around that narrow circuit!! awesome,

what suspension is on the car, body roll looks quite high, if not on a proper set of coilovers i think this would do wonders for you as you pick up a decent amount of adjustability too.
It is quite fun! Suspension is 100% stock right now. Don't want move to coilovers because it will change me to a different class with my club, which will put me up against race-prepped cars.

Quote:
Originally Posted by dc_wright View Post
The body roll on the stock suspension tends to put a high loading on the outside front tire. A stiffer front sway will help the under steer a little bit as it will reduce the loading on the outside tire. It won't be dramatic, but it will help. Installing some Koni or Bilstein struts/shocks will give increased damping and will reduce transient body roll also. Outside that, it's tuning the driver.

That being said, you can make any car under steer if you're hard on the brakes and turning sharply at the same time. A little bit of braking to load the front end to help with initial turn in is good, but then you have to be off the brakes and try to roll through the turn so both ends of the car can share the load. It takes some experimenting (and some cone killing) but once you get it down you'll see your times drop. A more advanced technique is to use throttle cut over steer to get the car to start rotating before you get to the turn so there's less turning to do in the turn. This takes a LOT of practice, and you'll have some awesome spins during the learning process, but once you get it down you should see a dramatic drop in your times.
Interesting. I thought that a stiffer bar decreases roll but gives you less grip on that end? For example when FWD guys put on a stiff rear bar, they have less roll but less grip at the rear to give them more oversteer. Or do I have that mixed up?

I'm still developing my skills for sure. I was able to take 1st place in my class for the 2014 season so I'm hopefully heading in the right direction. This season there are some faster guys joining the class so I have a lot of work to do.
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      02-13-2015, 12:51 PM   #11
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Quote:
Originally Posted by intoflatlines View Post
Interesting. I thought that a stiffer bar decreases roll but gives you less grip on that end? For example when FWD guys put on a stiff rear bar, they have less roll but less grip at the rear to give them more oversteer. Or do I have that mixed up?

I'm still developing my skills for sure. I was able to take 1st place in my class for the 2014 season so I'm hopefully heading in the right direction. This season there are some faster guys joining the class so I have a lot of work to do.
It's a delicate balance on stiffening the suspension. The effect on a given car is dependent on the suspension of a given car, and comparing effects of suspension changes on a FWD car vs a RWD car can be incorrect. On the Z4 if you go really stiff on the front, you'll get a lot more under steer because the suspension can't do it's job. Reducing body roll somewhat helps distribute the load on the front tires a bit so you get some reductions in under steer. As I indicated earlier, it's nothing dramatic, but it's an improvement. One other thing you can try which will help with the under steer is change from the stock 225/40 x 18 front tires to 255/35 x 18 size. It's a slight pinch fit, but you do gain some added contact patch area which will increase front grip a bit.
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      02-13-2015, 01:30 PM   #12
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Quote:
Originally Posted by dc_wright View Post
It's a delicate balance on stiffening the suspension. The effect on a given car is dependent on the suspension of a given car, and comparing effects of suspension changes on a FWD car vs a RWD car can be incorrect. On the Z4 if you go really stiff on the front, you'll get a lot more under steer because the suspension can't do it's job. Reducing body roll somewhat helps distribute the load on the front tires a bit so you get some reductions in under steer. As I indicated earlier, it's nothing dramatic, but it's an improvement. One other thing you can try which will help with the under steer is change from the stock 225/40 x 18 front tires to 255/35 x 18 size. It's a slight pinch fit, but you do gain some added contact patch area which will increase front grip a bit.
Sounds like it's just a matter of testing and seeing what size works. So it's a trade off between body roll and lateral grip to a certain extent? Don't some RWD guys run with no RSB to give them more grip in the rear at the expense of increased body roll?

And I should have mentioned, I have different wheels/tires. 245/40/18x9 up front and 275/35/18x10 in the rear. Tires are BFG Rival. I should have gotten a square setup but am worried about the limited amount of tire that can be fit up front.
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      02-13-2015, 02:08 PM   #13
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Need stiffer rebound up front.
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      02-13-2015, 02:40 PM   #14
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Quote:
Originally Posted by intoflatlines View Post
Sounds like it's just a matter of testing and seeing what size works. So it's a trade off between body roll and lateral grip to a certain extent? Don't some RWD guys run with no RSB to give them more grip in the rear at the expense of increased body roll?

And I should have mentioned, I have different wheels/tires. 245/40/18x9 up front and 275/35/18x10 in the rear. Tires are BFG Rival. I should have gotten a square setup but am worried about the limited amount of tire that can be fit up front.
I was running Rivals at the start of the season due to back order on RS-3s.
There were a number of people in the local autocross groups forced into the same thing, and all of us have gone back to the Hankooks. If you get the Rivals hot enough, they're really good on concrete surfaces. I don't know anyone who's tried them on asphalt that's been truly happy with them. If you get them too hot they go away rapidly. I'm running 255/35 x 18 Hankook RS-3 up front and same size as you in the rear, also RS-3's. I think I could fit 275's up front with a 5 mm spacer. I've got my 275 Rivals still and I might experiment with this at some point. I'd be stuffing them onto some 8" rims to stay legal for SCCA, and I'm looking at then getting them shaved flat to get as much tread area as possible.
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      02-13-2015, 03:48 PM   #15
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Interesting. I thought that a stiffer bar decreases roll but gives you less grip on that end? For example when FWD guys put on a stiff rear bar, they have less roll but less grip at the rear to give them more oversteer. Or do I have that mixed up



Herb Adams of Pontiac GTO and Trans Am fame in the 60s swore by stiffer sway bars and soft springs. He also added rigidity throughout the chassis to help control body roll.
He wrote a very good book where he explains in laymans terms his engineering philosophy.
http://www.amazon.com/Chassis-Engine.../dp/1557880557
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      02-14-2015, 10:50 AM   #16
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Not too different from his slalom.

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      02-14-2015, 12:07 PM   #17
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      03-17-2015, 05:50 PM   #18
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Sounds like a bit too much tire squeal to me. Tires grip the most right before they start sliding. But if you were 1st in class you must be doing something right.
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      03-17-2015, 10:22 PM   #19
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Quote:
Originally Posted by The HACK View Post
Need stiffer rebound up front.


Quote:
Originally Posted by dc_wright View Post
I was running Rivals at the start of the season due to back order on RS-3s.
There were a number of people in the local autocross groups forced into the same thing, and all of us have gone back to the Hankooks. If you get the Rivals hot enough, they're really good on concrete surfaces. I don't know anyone who's tried them on asphalt that's been truly happy with them. If you get them too hot they go away rapidly. I'm running 255/35 x 18 Hankook RS-3 up front and same size as you in the rear, also RS-3's. I think I could fit 275's up front with a 5 mm spacer. I've got my 275 Rivals still and I might experiment with this at some point. I'd be stuffing them onto some 8" rims to stay legal for SCCA, and I'm looking at then getting them shaved flat to get as much tread area as possible.
275s up front with a spacer? What's your suspension/alignment at? I think next tires I will try RS3 or Star Specs but having a staggered setup means no rotation on those. If I could go 275 square that'd be... impressive.

Quote:
Originally Posted by Sprint57 View Post
Sounds like a bit too much tire squeal to me. Tires grip the most right before they start sliding. But if you were 1st in class you must be doing something right.
Pressures were probably a little high for that run but I can agree that I have much to learn. I did very well in my class last year but I'm in trouble this season because some seriously fast guys with more modded cars plus r-comps are moving into my class.
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      03-20-2015, 09:11 PM   #20
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It's tough to tell from the front, but it look like you're slightly behind on setting the car. It looks like you're just a hair behind on the first turn-in when the car sets, and the car is behind then through all the cones.
I'm not talking from a expert position, just I know that's one of my problems is the time between my input, and the car moving into the loaded position to it tracking in the directions I place the tires is always a struggle. Initiating the turn-in earlier so the car takes the set at the place you want probably gets you in a more controlled slalom.

Although, stiffening your suspension will probably reduce your wait time between neutral to left loaded or right loaded set, and surely between left to right and vice versa sets that the slaloms induce.

again, tough to tell from the front.
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      03-21-2015, 09:07 PM   #21
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Quote:
Originally Posted by intoflatlines View Post


275s up front with a spacer? What's your suspension/alignment at? I think next tires I will try RS3 or Star Specs but having a staggered setup means no rotation on those. If I could go 275 square that'd be... impressive.

Pressures were probably a little high for that run but I can agree that I have much to learn. I did very well in my class last year but I'm in trouble this season because some seriously fast guys with more modded cars plus r-comps are moving into my class.
I'm at -1.4 neg camber in the front. Right now I've got 12mm gap between the inside of the tire and the strut tube with the 255's. In theory the 275's should only use up 10mm of that and will likely use less since I'm stuffing them onto the 8" rims, but all things considered 2mm isn't much clearance to plan on. Having to stay with the 8" rims is the main reason I haven't done it yet. I don't think I'll get the full benefit of the tire width change and may not get much at all, just some diameter growth.
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      03-21-2015, 11:37 PM   #22
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You're on OEM wheels right?
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