Trail braking lockup
03-09-2018, 11:46 PM
#1
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Trail braking lockup
As my skills improve I find myself going deeper into the corners and trail braking more. The car seems to lockup inside rear as I am releasing the brakes. I am so far off them it’s hard for me to believe it’s brake related. On the other hand it could be related to pad compound. My driving coach noticed the same. He felt it was engine braking causing the lockup.
To solve the the problem I tried bumping the idle up to 1,200. It seemed to help a bit but not cure it 100%.
Previous brake pads were Cobalt XR2 front and XR3 rear. I plan to run Monday using XR4 rears and bumping idle sped to 1,500. I can’t see the downsides of trying this but just wanted to hear some of your thoughts. Maybe I am chasing the wrong thing? Could it be my OSG LSD? Maybe it’s the 15mm rear bar?
To solve the the problem I tried bumping the idle up to 1,200. It seemed to help a bit but not cure it 100%.
Previous brake pads were Cobalt XR2 front and XR3 rear. I plan to run Monday using XR4 rears and bumping idle sped to 1,500. I can’t see the downsides of trying this but just wanted to hear some of your thoughts. Maybe I am chasing the wrong thing? Could it be my OSG LSD? Maybe it’s the 15mm rear bar?
Last edited by k24madness; 03-10-2018 at 01:10 AM.
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03-10-2018, 02:21 AM
#2
ʎpunq qoq
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Ideally there shouldn't be that much load from the engine on the rear wheels. You mention dropping to idle which really isn't using compression braking correctly rather it sounds like you are slipping the clutch and using the MOI of the engine to brake. Really you should be trying to blip the throttle on down shifts to match revs with only a quick stab of the clutch to unload and engage gear. Then you use engine compression as the revs drop to assist in slowing... but even on my high compression engine at 8500 rpm it doesn't help slow the car _that_ much.
I used to find when trail braking into off camber corners that the rear inside wheel was lifting and locking. I changed rear suspension to have more droop and stiffened the front bar a bit and it doesn't happen as much now. I also changed to a brake balance bar with separate masters which is way more linear than a bias valve. Brake bias valves don't really give a constant ratio split of pressure but rather they have knee point in there dependant on total pressure.
I used to find when trail braking into off camber corners that the rear inside wheel was lifting and locking. I changed rear suspension to have more droop and stiffened the front bar a bit and it doesn't happen as much now. I also changed to a brake balance bar with separate masters which is way more linear than a bias valve. Brake bias valves don't really give a constant ratio split of pressure but rather they have knee point in there dependant on total pressure.
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03-10-2018, 03:44 AM
#3
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I'm starting to get the same thing, particularly on banked corners where I am pulling over 1.1 G on trail braking entry. I feel like it is related to my Torsen, as it always happens with a certain chattering tempo of lock/unlock. It seems more intermittent than a lockup due to running out of droop travel on the inside rear tire. I have considered disconnecting my rear bar as a test to see if it reduces the issue.
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03-10-2018, 07:47 AM
#4
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I'd love to see a video showing your tach and feet during such an incident. You're moving brake bias forward switching to the XR4s but my guess is it won't solve the problem, which is engine braking related. The biggest red flag is you trying to mess with idle to fix this...
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03-10-2018, 12:31 PM
#5
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@Madjak, I am not letting the engine drop to idle. I do blip when shifting. Turning up the idle speed was a way to lessen engine braking. Not convinced this is the cause but didnt think it hurt to bump it up anyway. It you’re setup doesn’t cause engine brake lockup then I doubt mine would be the cause too. I run same static compression but have much higher corrected compression due to my smaller cams. Still should not be enough to lockup insid rear with the wheel being off the ground.
I think you’re onto something with regards to rear suspension droop. I need to look closer at that.
I agree prop valve is not the best solution. As I fade off the brakes I am now outside of the effected area of the prop valve. This is why I want to move to XR4’s. I can dial bias back in under heavy braking to keep ideal balance under max braking conditions yet bias will be lower as I fade off (and outside prop valve knee point). The pads them self have better release.
@curly, With the XR4’s I was going to dial in more rear bias to keep balance the same under heavy braking. I don’t know for sure if it’s engine related. It’s just suspect given I am just about off the brakes when it happens. I would have never considered it if not for my driving coach.
@tankrust, I plan to play with rear sway bar as well Monday. I ended up with a 15mm rear by mistake and need to swap the 14mm back in to see if that helps.
I think you’re onto something with regards to rear suspension droop. I need to look closer at that.
I agree prop valve is not the best solution. As I fade off the brakes I am now outside of the effected area of the prop valve. This is why I want to move to XR4’s. I can dial bias back in under heavy braking to keep ideal balance under max braking conditions yet bias will be lower as I fade off (and outside prop valve knee point). The pads them self have better release.
@curly, With the XR4’s I was going to dial in more rear bias to keep balance the same under heavy braking. I don’t know for sure if it’s engine related. It’s just suspect given I am just about off the brakes when it happens. I would have never considered it if not for my driving coach.
@tankrust, I plan to play with rear sway bar as well Monday. I ended up with a 15mm rear by mistake and need to swap the 14mm back in to see if that helps.
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03-10-2018, 06:44 PM
#6
Agreed that it's basic brake bias from fiction or hydraulic ratios so either playing with caliper piston bore diameters or Mu in the pads.
What tire size/model and spring rates?
Also which shock?
With enough grip, soft enough Springs and lack of droop travel inside rear locking will be a problem that cannot be fixed by messing with the brakes.
What tire size/model and spring rates?
Also which shock?
With enough grip, soft enough Springs and lack of droop travel inside rear locking will be a problem that cannot be fixed by messing with the brakes.
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03-11-2018, 01:33 AM
#7
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This is my initial reaction. Is this an IR lock that you can feel, or just one that's being witnessed externally? If the latter, then perhaps that wheel isn't touching the ground anyway.
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03-11-2018, 12:27 PM
#8
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My car does the same trail braking into Oak Tree @ VIR sometimes. There is so little weight on that wheel that it doesn't cause me to oversteer or flat spot the tire. I only know because I've seen it in other people's videos.
1000/400, stock sways, Xidas with helpers on SM7s.
1000/400, stock sways, Xidas with helpers on SM7s.
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03-11-2018, 01:42 PM
#9
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Agreed that it's basic brake bias from fiction or hydraulic ratios so either playing with caliper piston bore diameters or Mu in the pads.
What tire size/model and spring rates?
Also which shock?
With enough grip, soft enough Springs and lack of droop travel inside rear locking will be a problem that cannot be fixed by messing with the brakes.
What tire size/model and spring rates?
Also which shock?
With enough grip, soft enough Springs and lack of droop travel inside rear locking will be a problem that cannot be fixed by messing with the brakes.
AST 5220, 1000/550 1.25”/15mm sways
Brakes are Stoptech 11.75” fronts with 38/36 piston (3.34” total area), Rears are Sport calipers on 10.90” rotors. 1” master.
I suppose it it could be droop related. It does seem like I have a lot though. Hard to image the back coming up that high in the air. I could be fighting that 15mm rear sway. That maybe limiting droop.
Monday is kinda a test and tune day. I swapped the XR3 for XR4 in the rear. Not sure the MU but you can see from the chart it’s a good deal less torque. I kept idle the same as before at 1,200. After resettting prop valve if I still have the issue I am going to swap in the 14mm rear sway. Even if it fixes it I want to try it anyway. For some reason Tim was against this during last test day. Maybe because it felt so good as it was.
Thanks for all the feedback everyone!!!
Last edited by k24madness; 03-11-2018 at 02:48 PM.
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03-11-2018, 02:46 PM
#10
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03-11-2018, 10:22 PM
#11
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My setup is extremely similar:
245/580/15 Hoosier R80
AST 5220, 800/500 22mm Mazdaspeed solid / 12mm rear
Brakes are Wilwood 11.75" / Stock caliper wilwood 11.44" with Ferodo DS1.11 all round
Front master is 0.625" / rear master 0.7"
I run a lighter spring rate and less bars because the tracks I drive on are bumpy as hell. I also run torsen rear diff (I'll upgrade to an OS Giken if the Aussie dollar gets a bit better)
Most of my issues are with lifting the rear inside over crests or kerbs and loosing drive through the torsen. I have one corner where I'm trail braking on and off camber sweeper which is pretty hard on that inside tyre but I don't find I lock it under brakes.
I would highly recommend my race pads. Next time I'd upgrade to the DS Uno pad but these endurance Ferodos are extremely friendly on the rotors and work from cold. Since I do half hilclimb events I need them to work on the first corner as well as 6-7 laps at pace on a track. I also highly recommend moving to a balance bar over a biasing valve. I am constantly tweaking my bias during events, especially on cold wet tracks vs hot grippy surfaces. I tend to be able to push the bias more rearward on the stickier surfaces even if the car squirms more under brakes. But then on bumpy road surfaces or tracks where I need to be more aggressively trailbraking I can wind in a bit more front. I often find I tweak it a click here or there throughout the day as the track changes.
245/580/15 Hoosier R80
AST 5220, 800/500 22mm Mazdaspeed solid / 12mm rear
Brakes are Wilwood 11.75" / Stock caliper wilwood 11.44" with Ferodo DS1.11 all round
Front master is 0.625" / rear master 0.7"
I run a lighter spring rate and less bars because the tracks I drive on are bumpy as hell. I also run torsen rear diff (I'll upgrade to an OS Giken if the Aussie dollar gets a bit better)
Most of my issues are with lifting the rear inside over crests or kerbs and loosing drive through the torsen. I have one corner where I'm trail braking on and off camber sweeper which is pretty hard on that inside tyre but I don't find I lock it under brakes.
I would highly recommend my race pads. Next time I'd upgrade to the DS Uno pad but these endurance Ferodos are extremely friendly on the rotors and work from cold. Since I do half hilclimb events I need them to work on the first corner as well as 6-7 laps at pace on a track. I also highly recommend moving to a balance bar over a biasing valve. I am constantly tweaking my bias during events, especially on cold wet tracks vs hot grippy surfaces. I tend to be able to push the bias more rearward on the stickier surfaces even if the car squirms more under brakes. But then on bumpy road surfaces or tracks where I need to be more aggressively trailbraking I can wind in a bit more front. I often find I tweak it a click here or there throughout the day as the track changes.
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03-11-2018, 10:38 PM
#12
Well I can tell you one thing that is completely irrelevant: messing with the idle. Put it back to normal, stop touching it. Achieves absolutely nothing. Unless you are one of those people that have the clutch pedal on the floor through an entire braking zone. If so you need to go back and learn proper brake technique and how to heel-toe downshift.
If you're not one of those people, next step is to check corner balance and try to get some exterior pictures of the car while it's doing this so you can figure out if maybe droop travel is the problem here like others have suggested after you test out the prop valve thing.
If you're not one of those people, next step is to check corner balance and try to get some exterior pictures of the car while it's doing this so you can figure out if maybe droop travel is the problem here like others have suggested after you test out the prop valve thing.
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03-11-2018, 10:56 PM
#13
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I know this is unlikely but sometimes the easy solution can be overlooked.
Your not just trail braking too late and the final/extra steering input is jacking the car?
Change down later/late in the braking process, complete braking sooner for those corners..?
Your not just trail braking too late and the final/extra steering input is jacking the car?
Change down later/late in the braking process, complete braking sooner for those corners..?
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03-11-2018, 10:58 PM
#14
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Are you getting the car to rotate at the end of the trail brake then going to gas pedal to maintain slip? If your just autox max braking the inside front or rear can puff some light smoke.
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03-11-2018, 11:39 PM
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I would highly recommend my race pads. Next time I'd upgrade to the DS Uno pad but these endurance Ferodos are extremely friendly on the rotors and work from cold. Since I do half hilclimb events I need them to work on the first corner as well as 6-7 laps at pace on a track. I also highly recommend moving to a balance bar over a biasing valve. I am constantly tweaking my bias during events, especially on cold wet tracks vs hot grippy surfaces. I tend to be able to push the bias more rearward on the stickier surfaces even if the car squirms more under brakes. But then on bumpy road surfaces or tracks where I need to be more aggressively trailbraking I can wind in a bit more front. I often find I tweak it a click here or there throughout the day as the track changes.
Manual brake conversion is on the radar. I am just trying to get more seat time before I take on the next upgrade. Kinda burnt out on development and just want to drive the wheels off it.
Looking back I did not have this problem with the old Wilwood 11.75/1.8 rear calipers. By swapping to XR4 rear it looks like I will be close to the splits on my old setup.
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03-11-2018, 11:45 PM
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Well I can tell you one thing that is completely irrelevant: messing with the idle. Put it back to normal, stop touching it. Achieves absolutely nothing. Unless you are one of those people that have the clutch pedal on the floor through an entire braking zone. If so you need to go back and learn proper brake technique and how to heel-toe downshift.
If you're not one of those people, next step is to check corner balance and try to get some exterior pictures of the car while it's doing this so you can figure out if maybe droop travel is the problem here like others have suggested after you test out the prop valve thing.
If you're not one of those people, next step is to check corner balance and try to get some exterior pictures of the car while it's doing this so you can figure out if maybe droop travel is the problem here like others have suggested after you test out the prop valve thing.
With regards to blipping and shifting. Got it down. It's not driver error. My driving coach (Tim @ TFB) noticed the same thing.
Corner balance has been checked/adjusted several times without change. I doubt that's it.
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03-11-2018, 11:48 PM
#17
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This could be something interesting. I do run a ton (5.5-6) of castor. I would have thought castor would jack outside front wheel thus pushing down inside rear. If the opposite is true then.........hmmm
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03-11-2018, 11:50 PM
#18
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It was a mission to fit the Ferodo to the stock pad shape. I had to cut them by hand with a cutoff disk which is a messy task. In our timeattack series most of the top 20 cars would be running them the DS Uno or DS1.11 / DS2.11... they are that good. I'm not sure they are the best option for track only work but for anything where the brakes need to work from cold there is nothing better. They also are super nice on the rotors and don't dust up much. I'm looking at fitting Wilwood powerlites to the rear so that I don't have to cut them by hand again!
Why not try doing the dual masters? It was such an easy mod you could do it in a few nights. The hardest part is getting the pedal box in and out of the car but if you pull out the seat first it's not so bad. It means you can run any master combination to balance out whatever calipers you have and then tweak the bias on track. It's far better than switching calipers and running mixed pad combinations which I think isn't ideal because the different compounds can behave differently at different times adding to the variables.
Why not try doing the dual masters? It was such an easy mod you could do it in a few nights. The hardest part is getting the pedal box in and out of the car but if you pull out the seat first it's not so bad. It means you can run any master combination to balance out whatever calipers you have and then tweak the bias on track. It's far better than switching calipers and running mixed pad combinations which I think isn't ideal because the different compounds can behave differently at different times adding to the variables.
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03-11-2018, 11:52 PM
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I am not quite back to gas when it happens. It's during the lifting of the last half of the brake pedal. Just about near the end really. Wheels rotate freely etc.
Again guys thanks for all the good ideas. Hitting the track in the morning. I'll get back to you all tomorrow night with some answers.
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03-12-2018, 12:12 AM
#20
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I've found that the ELBJ have helped if anything being able to get decent caster with max camber, although I'd never be anywhere near full lock.
Oh last thing... are you running a brake booster? Taking that off made a massive difference on my car as the vacuum was so variable due to my crazy cams. I could feel through the brake pedal the change in assist as I blipped through the gears and my brake pressure logs showed fluctuations throughout... with manual brakes the logs are smooth. The Honda cams are pretty extreme compared to a stock Miata.
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