Hey everyone,
I recently pulled my S2000 out of winter storage to get ready for the season. After giving the car a once over (pulling wheels and inspecting everything, replacing all fluids) I took it for a drive. As part of these test drives, I typically try a panic brake to make sure the brake system is working correctly. Typically this is a 50mph-0 stop. ABS kicks in and everything tracks straight and true until the end. When I tried a panic brake this year & it was dramatically worse. The steering wheel swings +/- 20 degrees in each direction. It isn't a pulse, or a buzz, but a large movement to the left and right. Sometimes the car generally tracks straight while this is occuring, sometimes is pulls slightly to the right while this is occurring. Has anyone seen this before?

Some additional details:
- I`ve driven the car (let's say over the speed limit) on the highway, including bumps, potholes, and other real world conditions. It tracks straight and true and I haven't heard any unusual clunks or thumps.
- The steering wheel movement described above happens consistently every time, I`ve done in at least 30 different times in 30 different places. I don't think this is attributable to the road or local conditions.
- The car feels fine under low effort, casual braking. The problem only occurs in a panic stop situation.
- When I turn the key on the ABS light turns on (bulb works). When the car is started, the ABS light turns off.
- I`ve already replaced the front rotors, front pads (carbotech xp10), and front calipers with all new parts.
- I`ve rotated the wheels front to rear, no change in the behaviour.
- I recently replaced all of the brake fluid using Billmans gravity bleed method, and used a clamp to firmly depress the brake petal overnight. New fluid was motul 660.
- I track the car. All bushings were replaced last couple years with poly bushings (new OEM compliance bushings). Suspension is Ohline DFV with 12/10 springs. I tracked the car on this setup last year and did not have this issue. No changes have been made to the setup or suspension geometry.
- Brake system is largely stock - new pads, lines, and fluid, but stock calipers, rotors, master cylinder, abs, etc.

Has anyone seen this before? I have a track day scheduled for this Saturday, but I`ll need to cancel if I can't get this sorted out.
The car is currently at my local shop, but they`re a general shop and don't have specific S2000 experience. They`re on day 2 of checking things out, but I get the impression they don't have any strong leads on the cause.
I`ll try anything! I`m stumped!

 

Have you checked front end components well? Balljoints, tie rods?

 

I gave everything a visual inspection last week. All the nuts were still tight, all cotter pins still in place.
I wasn't suspecting any suspension related issues because it drives and turns beautifully, but 100% I`ll follow up on this tonight, if the shop hasn't already.
Thanks for the suggestion.

 

You're not using stock wheels, are you?

 

You may need to have your rotors turned. Often corrosion builds up on the rotors when sitting for long period of time except the area where the pads are resting. This could create an uneven surface that can lead to pulsation or steering wheel shaking under braking.
you could try driving it for a bit drag your brakes and maybe here and there. it will skim the corrosion off and might make it less noticeable.

 

That fits the title of the post, but not the content of the post. Not sure why he used such a misleading title

 

Ball joints take a bit of pressure to check properly. You can look up various ways to do it or your shop will know how. Visual inspection will not show you much. Tie rods need an inspection by feel as well vs a visual. It only takes a minute amount of play in there to make a noticeable symptom when driving and braking.

You mentioned too that you recently replaced rotors and pads. When you bedded them in, if you allowed the car to come to a complete stop with your foot on the brake for a bit, it can cause a buildup of material (a ridge so to speak) that can cause a pulsing or vibration feeling under braking. If you are unsure you can rebed them. Typically though if you pay attention you can also feel this in the pedal when it is happening.

 

Currently I`m on a set of square wedsport 105n with 245/40/17 Falken 660s. The tires have 1 season on them. I forget the backspacing (+49?) but they started life a group guy on this site years ago.
I have a set of old RS4s on OEM wheels out in the shed, also 245/40/17 (rims are bored for a square set up). This feels like a front end problem to me, so I dropped off 2 of these at the shop this morning. They`re going to install and test drive to eliminate a wheels/tires as the problem spot.

New rotors have already been installed. In the past, I`ve experienced warped/cracked rotors as a "pulse" in the wheel. This doesn't feel the same. I`ll try to grab a video to illustrate how much movement I`m experiencing.

 

The description indicates uneven brake force, left vs right.

A sticking caliper, or sticking slide pins are primary suspect.

Caliper pins are often either neglected, or wrong type or wrong amount (too much) grease used.

You don't want to use caliper pad grease. Its too thick. Pins will not slide easily enough. Too much grease can also cause pins to bind. Basically they hydrolock.

That this happened after sitting makes me think its gummed up caliper pin grease.

Get some brush on silicone grease. Remove pins, clean them up. If corroded, replace, or for now, put them in drill, use scotch pad, clean them up.

Use brake clean to clean out old grease from caliper pin holes.

Apply a nice coat of grease to pin. Don't overdo it, don't apply a ball of grease at tip.

Insert pins, make sure they slide easily.

When you first take it apart, if you need to really use effort to remove the pins, you know this is likely the issue (or at least an issue that needs to be addressed). If not next check caliper pistons.

Open bleeder screw, attach hose into catch bottle. Use clamp to push piston in, see how force required.

You want to open bleeders so you don't push fluid backwards through ABS system.

Lastly, of its not any of those issues, could be ABS thinking one wheel is locking up, and reducing brake force to that caliper. Hence uneven braking, and some amount of ABS pulsing in pedal. That could be bad ABS sensor, or just spider nest in ABS sensor hole. Remove ABS sensor, look through passage where it was removed from. Clean out.

 

This problem started with the old pads and old rotors. These were properly bedded, heated, and used on track last season.
I replaced everything with new parts last night in an effort to track down this problem. It didn't have an effect on the vibrations.
The carbotech xp10s were "prebedded" before I bought them. Honestly, I haven't put any effort into getting them hot because they didn't have any impact on the steering wheel motion.