Limiting understeering and tire wear on BMW 135i

I just started the 2014 lapping season April 23 at ICAR. Was eager to test my new endurance brake pads (Carbotech XP10), Quaife LSD, and new alignment. Temperature was cold (8 degrees Celcius/46 degrees Fahrenheit). I am quite happy with the braking performance, with no or very little fade towards the end of my session. After four 20 minute sessions, the front pads already are thinner than the rear pads. Im a bit surprised by that. I now understand why people get a more robust pad in the front than in the rear (XP12f and XP10r).

I have the M3 front control arms, and Camber plates set to max, and my alignment is -3.2. I have RE-11 tires (225/40R18f and 255/35R18r), maintain my hot pressure below 40PSI, but still get outside edge wear and some understeer. My RE-11 front tire outside edge tread is almost gone (looks like slicks). Maybe the sidewalls of this tire is too soft, even with correct pressure in them.

I need advice on next modification on my 135i to increase my cornering turn-in speeds. Turn out speed is much better with my LSD, but still overwhelming my rear tires (could use a little more traction). I consider myself an intermediate/advanced driver.

To those who track their 135i on stock suspension, how much does the E92/M3 and E93/M3 Front Sway Bar help with reduction of understeer?

I wonder what the next logical and affordable modification I should invest in to reduce understeer even more, without sacrificing rear traction. Budget is $1000-$1500.

Scenario #1: Try different alignment in the front , more toe out for better turn-in.

Scenario #2 : Get E93/M3 or thicker Front Sway Bar. Evaluate results on the track, and move on to the next logical upgrade if required (Swift springs and Koni sport struts).

Scenario #3 - On my stock style 261 RIMs (18x7.5f and 18x8.5r), get wider/stickier Extreme Performance tires in the front (235/40R18), or narrower in back (245/35R18, to approach square setup. I would hate to do that because I want best traction possible to have my Quaife LSD (with 3.46 final drive ratio) put the power down. This is already a challenge in second gear with RE-11 tires.

Scenario #4 - On my stock style 261 RIMs, get R-compound/streetable tires (front = 225/40R18, and rear=245/35R18, or 255/35R18). I wonder if stickier tires and stiffer sidewalls would help my situation.

Scenario #5 - Get a dedicated set of 17" style 68 M RIMS (7.5x17 and 8.5x17) shod with R-compound 225/45R17f and 255/40R17r. Keep my my stock style 261 RIMs for street use, or sell them altogether as I use the car mainly for lapping events (70%) and street (30%). Besides the major advantage of lighter RIMS and reduced unsprung weight, I wonder if this choice would result in better or worse traction than scenario #4.

Open to your suggestions. Thanks for your help!

The tire pressures (40+ psi) seem very high. Tire pressures vary with tires, car, suspension geometry, suspension stiffness, car weight, downforce, etc. But still, 40 PSI seems a bit high. Perhaps someone who has run the same tires on the same car could give you an indication of a ballpark figure that works ?

1.
The alignment in the front is a good idea, 0 toe or even toe-out will improve your turn in. If you still use the car on the street, toe-out is not recommended as it will make the car twitchy and somewhat scary to drive in the rain and on rough roads (Montreal has plenty of those :p )

Toe-in will increase stability, but turn-in will be worse. Also, since (almost) ALL modern car suspensions toe-in under compression, running toe-in in the front will result in even more toe-in after the weight transfer has occured under braking.

2.
A stiffer front swaybar will actually increase understeer. What you could try for 1 session only is to disconnect the swaybar you currently have. It'll take you 10 minutes and if you don't like it you can always connect it back. You can do this by disconnecting just 1 endlink (left or right). This will allow you to know if a softer setup is better (no swaybar) or if a stiffer setup would be the way to go.

I would really start with tire pressures, honestly pressures and temperatures can do such a big difference. On the cars I work on, we adjust pressures within 0.1 psi, at some tracks a 1 psi difference is worth seconds, not tenths, SECONDS.

When we go testing with the race team, we have dedicated days to try different pressures and see the effect on tire temps / lap times & tire wear. We we work with engineers from the tire manufacturer and change NOTHING else on the car except tire pressures for a whole day (300+ laps) to find the right setup and compromise between tire temperature/wear/laptimes.
 
The tire pressures (40+ psi) seem very high. Tire pressures vary with tires, car, suspension geometry, suspension stiffness, car weight, downforce, etc. But still, 40 PSI seems a bit high. Perhaps someone who has run the same tires on the same car could give you an indication of a ballpark figure that works.

I would really start with tire pressures, honestly pressures and temperatures can do such a big difference. On the cars I work on, we adjust pressures within 0.1 psi, at some tracks a 1 psi difference is worth seconds, not tenths, SECONDS.

When we go testing with the race team, we have dedicated days to try different pressures and see the effect on tire temps / lap times & tire wear. We we work with engineers from the tire manufacturer and change NOTHING else on the car except tire pressures for a whole day (300+ laps) to find the right setup and compromise between tire temperature/wear/laptimes.

Most of us start with a cold pressure of about 30 psi, and try to stay clear of 40+PSI. The way I do it is start off my 1st session with OE recommended pressures of 36f/38r, just to save my tires from rolling over too much, and wasting 2-3 laps at slow speeds to warm up my tires and thus increase pressure.

After 1st session and pit-in, I adjust my hot pressure to about 38PSI, and they stay at that pressure for the remaining sessions, typically at 20 minute intervals. Yesterday was exceptional, and we averaged pit-in times of under 15 minutes, because there was very little beginner run group participants.


The alignment in the front is a good idea, 0 toe or even toe-out will improve your turn in. If you still use the car on the street, toe-out is not recommended as it will make the car twitchy and somewhat scary to drive in the rain and on rough roads (Montreal has plenty of those :p )
Toe-in will increase stability, but turn-in will be worse. Also, since (almost) ALL modern car suspensions toe-in under compression, running toe-in in the front will result in even more toe-in after the weight transfer has occured under braking.
Please see my current alignment in post #31, and provide your opinion of what tweaks could be made to the front.

A stiffer front swaybar will actually increase understeer. What you could try for 1 session only is to disconnect the swaybar you currently have. It'll take you 10 minutes and if you don't like it you can always connect it back. You can do this by disconnecting just 1 endlink (left or right). This will allow you to know if a softer setup is better (no swaybar) or if a stiffer setup would be the way to go.
Will think about this. Most E8x and E9x owners seem to state that a stiffer FSB helps reduce understeer, for some crazy reason. Will try to find the a reference post ...
 
Je ne connais pas ton expérience, mais as-tu pensé à faire embarquer un très bon instructeur avec toi ? Je pense surtout à des coach de l'ASE qui roulent avec des RWD depuis un moment.
 
I really don't get the bigger swaybar = less under steer on bimmers. I actually don't believe it.

I would lower front pressure also.
 
Je ne connais pas ton expérience, mais as-tu pensé à faire embarquer un très bon instructeur avec toi ? Je pense surtout à des coach de l'ASE qui roulent avec des RWD depuis un moment.

Je suis dans le groupe vert a ICAR depuis la fin de l’été passée. Je suis la formation de AISA a l'ASE ce week-end. Il y a toujours quelque chose a apprendre, mais la formation ne reduiras pas mon sous-virage, a part me faire dire de réduire ma vitesse au TURN-IN, il y a des limites a réduire ma vitesse
 
I really don't get the bigger swaybar = less under steer on bimmers. I actually don't believe it.

I would lower front pressure also.
What pressure do you recommend, to avoid rolling over on my tire sidewalls? I aim for mid thirties to 40 MAX ....
 
http://www.1addicts.com/forums/showthread.php?t=944858&highlight=understeer

it's common with any MacPherson strut front car, either FWD or RWD. The camber change due to roll, stock, is so unfavorable that reducing roll increases traction on the outside front tire more than enough to compensate for the increased weight transfer to that tire caused by the Sway bar.

Unlike double wishbone type suspension, the MacPherson strut has an unfavorable camber curve, in that the camber does not increase linearly as the suspension is compressed. So as the car takes a corner and sets, the outside suspension is going to compress past the roll center and will go from a favorable increase in negative camber to start to lose the negative camber. When that happens the front end will lose grip rapidly. One of the solutions to alleviate this problem, is to deploy thicker front anti-roll bars to prevent the outside suspension component from compressing PAST the roll center and start to lose camber.

It is quite possible that my front tires are wearing their outside edges because my 135i is rolling too much and even lots of static camber is not enough to keep the outside edges off the pavement. I might go with a tire that had a stiffer sidewall...like Dunlop ZII's which can be used quite effectively on a camber challenged car.

To a certain extent, a Front Sway Bar helps to limit the weight transfer and thus prevent the tire from rolling over onto the sidewall and off the contact patch. However, too much bar reduces the independent action of your suspension. There's a reason the car manufacturers don't just put massive ARBs on every car. A reasonably stiffer sway will reduce the total amount of weight transferred (due to spring effect), therefore helping reduce understeer.
 
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What pressure do you recommend, to avoid rolling over on my tire sidewalls? I aim for mid thirties to 40 MAX ....

What pressures are you running hot?
First I would get a pyrometer and read tire temps.
Tires tell soooo much with temps!
I like the Longacre Racing Accutech stick-in type (Perry). Infrared works but you have to be careful how you read. Sure beats nothing!
It's the only way to really know how your alignment needs to be based on how your tires are working.
Since your under steering seems bad I would lower 2 psi and then adjust by 1 psi increments.
 
Thanks for informative link.

Even without measuring temp, I can relate to this: "Out edge hotter than inner - Not enough negative camber or too much toe-in.". Rest of my front tires have normal wear (middle and inner edge).


Following symptoms apply too, due to my staggered stock wheel setup. Disagree with the FSB statement on the 1-series, simply because, 1-series owners have tried stiffer FSB, and reduced tire and front end roll, and understeer: " Front tires hotter than rear - Car is under steering (pushing). Too much front spring/sway bar, not enough rear spring/sway bar, front pressure too low, rear pressure too high, front tires too narrow, rear tires too wide."
 
First I would get a pyrometer and read tire temps. Tires tell soooo much with temps!
I like the Longacre Racing Accutech stick-in type (Perry). Infrared works but you have to be careful how you read. Sure beats nothing!
It's the only way to really know how your alignment needs to be based on how your tires are working.
Since your under steering seems bad I would lower 2 psi and then adjust by 1 psi increments.
Thanks for these tips. Will try this out.
Do you think I can borrow someone's pyrometer at the ASE track this week-end?
 
Makes NO SENSE at all ! On ANY car

That's what i told him also...

Unlike double wishbone type suspension, the MacPherson strut has an unfavorable camber curve, in that the camber does not increase linearly as the suspension is compressed. So as the car takes a corner and sets, the outside suspension is going to compress past the roll center and will go from a favorable increase in negative camber to start to lose the negative camber. When that happens the front end will lose grip rapidly. One of the solutions to alleviate this problem, is to deploy thicker front anti-roll bars to prevent the outside suspension component from compressing PAST the roll center and start to lose camber.
 
Had the same MacPherson issue with my p.o.s. 7th gen. Civic.
Reducing front sway and big ass rear sway still helped.
 
Unlike double wishbone type suspension, the MacPherson strut has an unfavorable camber curve, in that the camber does not increase linearly as the suspension is compressed. So as the car takes a corner and sets, the outside suspension is going to compress past the roll center and will go from a favorable increase in negative camber to start to lose the negative camber. When that happens the front end will lose grip rapidly. One of the solutions to alleviate this problem, is to deploy thicker front anti-roll bars to prevent the outside suspension component from compressing PAST the roll center and start to lose camber.

Well, this actually makes sense, for large lateral accelerations on a production based car, the camber curve could go over-center and camber could actually decrease if the roll center is really high. Mazda MX-5 Cup cars had this problem, if you ran the car too high or even or if you completely removed the swaybar in the front because it would cause the camber curve to go over-center, reducing grip. So the front swaybar couldn't be completely removed because the front would become so soft that the increased wheel travel would make the tires lose camber and grip...

What you could do on a McPherson strut car like yours is lower the front which would lower the roll-center and prevent the camber curve from going over center. Another solution could be to move the suspension bottom pick up point on the upright (wheel carrier) upwards, that would involve a lot of modifications so it wouldn't be recommended except for a full on race car...

However, if you lower the car some more, watch out for bottoming out. You'll also have to run a stiffer anti roll bar in the front if you lower it quite a lot because lowering the car will lower the roll center and increase geometric weight transfer (the load transfer caused by the lower roll center)

Hope that helps !
 
I think I might have figured out part of the problem after my 2-day AISA driving school last week-end at Autodrome St Eustache.

It rained all day on the 1st day, which helped work on the race line, and smoothness of inputs to the car. My RE-11's were beyond the wear bar in the front (no tread left on out half of the tire), and at the wear bar in the rear. It made it more challenging to control the car and DSC kicked in often. I was humbled by letting every participant pass me that day.

The car was hydroplaning and needed more traction, but handling was neutral. At these lower speeds, and with lack of traction, understeering was not as much noticeable, but oversteer was. I had a chance to discuss about my issue with different instructors that sat in the car, and mentioned I was running -3.2* camber in the front. He mentioned that with that much camber, and little traction in the wet, I was effectively reducing the front tire contact patch, and that I should try reducing my camber, or lowering my front+rear tire pressure to 30 PSI hot. I lowered the pressure, did not change the camber, but did not notice a big improvement.

The next day, was an eye opener. Conditions were much better and the track got dry. This was my first track event on asphalt pavement with this car, and RE-11 tires (prior to this, all my lapping was done at ICAR's cement track. I did not adjust my tire pressure from the previous day, and was getting serious grip, and no understeer, with slight oversteer under power, to rotate the car - just the way I like it. I started getting more traction and bite than I ever had, built up my confidence level, and passed a lot of cars that day. What a great feeling that was.

The damn tires stuck like glue on the dry ... What the heck was that about. Could the asphalt track be the solution to my understeer or was it lower tire pressure? I dont see the pressing need for suspension mods anymore. Im still perplexed by the massive gain of traction and control of the car. I am still trying to determine whether the asphalt pavement made that much difference, or the lowered tire pressure did.

Will monitor tire pressure, and traction on cement pavement when I hit ICAR tomorrow evening if it doesn't rain.
 
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C'est excellent que tu réalises tout cela.

Maintenant, je pense que chaque dollars de ton budget de pièce devront aller sur du temps de piste et de l'instruction exclusivement.

Ton évolution est parfaitement normale...au début, c'est toujours une question de machine, même quand on se fait dire le contraire... jusqu'à ce qu'on réalise soi-même qu'une voiture, c'est un tas de compromis avec lesquels il faut jongler.

Crois moi, tu as beaucoup moins de problème d'understeer que 90% des gens à la track.
 
C'est excellent que tu réalises tout cela.

Maintenant, je pense que chaque dollars de ton budget de pièce devront aller sur du temps de piste et de l'instruction exclusivement.

Ton évolution est parfaitement normale...au début, c'est toujours une question de machine, même quand on se fait dire le contraire... jusqu'à ce qu'on réalise soi-même qu'une voiture, c'est un tas de compromis avec lesquels il faut jongler.

Crois moi, tu as beaucoup moins de problème d'understeer que 90% des gens à la track.
Comme tu dis - je blâmais la machine, mais la, c'est un peu la chaussée que je blâme (asphalte versus ciment);:D
A suivre, lors de mon prochain lapping a ICAR, avec les même pression des pneus, même settings d'alignement ...
 
Hi Daniel !

Good to meet you on Sunday for a bit! Sweet car and good set up!
30 hot for a dry day seems low...but if it works..

You will have the perfect opportunity to test the pressures and the camber adjustments on the ASE LC test and tune day in May. Run a video camera for all of your sessions to record times.
Record hot tire pressures and complete tire temps. (Interior/ exterior and middle)

ASE is a technical track and small tire pressure adjustments will pay off on grip and tire wear.
In my case at ASE I am running different pressures for all four tires.Front right is the lowest , because of the carousel and hairpin.

LCMT, ICAR, Mosport and Tremblant tire pressures are paired F/R with F lowered for grip.

As you may have already seen, strong braking will remove that understeer.
You will experiance this at Tremblant. T4-esses, T8-carousel... And 14 - namerow.
With your gobs of low end torque and LSD You will quickly recover ! Have fun!


Dave S
 
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