Rear end wiggles under hard braking
08-02-2010, 08:39 PM
#31
Rennlist Member
08-02-2010, 08:41 PM
#32
Burning Brakes
Thread Starter
08-02-2010, 08:44 PM
#33
Rennlist Member
08-02-2010, 08:51 PM
#35
Rennlist Member
08-02-2010, 09:58 PM
#37
Instructor
Guys,
This season I had a major tank slapper at the end of a long straight, this after 3 years of doing the same thing in the same spot at 146 / 150mph with no issues. I switched the stock 993 turbo rears for RS units years ago to move some of the front bias to the rear with no ill effects. The LSD is the stock Porsche unit with Guard internals. The only change I made from the previous year was a RUF nose with a spoiler lip and an turbo S tail. There is no doubt that the front is creating some down force and taking some weight off the rear under heavy weight transfer braking.
My previous style was a very hard initial brake application. I found after the "incident" that I had to do a softer application and then increase the pedal force. This let the car settle for a bit and I used this over the rest of the weekend with no ill effects and also longer stopping distances. Again 3 years of the same stuff and then the subtle aero change causing big moments. Never thought I was that close to any type of limit during the braking of the car until this.
Solution will be two fold. More rear wing, actually a rear wing that actually works and switching to an LSD that has ramp options that allow 100% decel lock up. This will take changing the stock LSD housing for a Guard race unit.
The old saying about " that's why they put the weight at the front of the arrow" rings true.
Rick
This season I had a major tank slapper at the end of a long straight, this after 3 years of doing the same thing in the same spot at 146 / 150mph with no issues. I switched the stock 993 turbo rears for RS units years ago to move some of the front bias to the rear with no ill effects. The LSD is the stock Porsche unit with Guard internals. The only change I made from the previous year was a RUF nose with a spoiler lip and an turbo S tail. There is no doubt that the front is creating some down force and taking some weight off the rear under heavy weight transfer braking.
My previous style was a very hard initial brake application. I found after the "incident" that I had to do a softer application and then increase the pedal force. This let the car settle for a bit and I used this over the rest of the weekend with no ill effects and also longer stopping distances. Again 3 years of the same stuff and then the subtle aero change causing big moments. Never thought I was that close to any type of limit during the braking of the car until this.
Solution will be two fold. More rear wing, actually a rear wing that actually works and switching to an LSD that has ramp options that allow 100% decel lock up. This will take changing the stock LSD housing for a Guard race unit.
The old saying about " that's why they put the weight at the front of the arrow" rings true.
Rick
08-02-2010, 10:06 PM
#38
Burning Brakes
Thread Starter
Tank slapper, nice, another motorcyclist. 
I'm gonna do a little fine tuning. I changed sooooo much recently that I think I need to start tweaking it. I went from stock to completely hard.
I'm gonna do a little fine tuning. I changed sooooo much recently that I think I need to start tweaking it. I went from stock to completely hard.
08-02-2010, 10:15 PM
#39
Racer
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Actually, the weight transfer will be a function of the deceleration rate (and ride height). the rate of weight transfer can be altered, but the ride height can be altered creating some instability as well, especially with driver input) So, there is some instantaneous forces that could be helped or hurt by shock bump and rebound settings , f/r. I think one of the posters had it best, when asking the question, "when is this a problem in the braking cycle?" All these solutios are knee jerk answers and may make the problem worse depending on its cause. If its a real fast straight in to a very hard consistant braking zone, the more than likely answer might be found in the alignment or tires. If its on initial brake, too much rebound might the issue as VR has suggested, but if its late, or initially in the braking zone at a slower speed, it could be rear braking bias. Driver input has a play here as well. I had a friend that had this issue into the hairpin at Laguna. I was behind him on some practice laps, and his issue was a suspension linkage washer installed incorrectly. that rear wheel was doing the "dance " under braking.
It will be interesting to see the fix on this one.
It will be interesting to see the fix on this one.
The only way to know is to test, test, test.
08-02-2010, 10:34 PM
#40
Race Director
Adam,
the back of my car wiggles going into Jacques. Although I think my LSD is shot I think that going downhill and braking is putting a lot of weight on the front tires despite my RS rear wing.
the back of my car wiggles going into Jacques. Although I think my LSD is shot I think that going downhill and braking is putting a lot of weight on the front tires despite my RS rear wing.
08-02-2010, 10:46 PM
#41
Rennlist Member
08-03-2010, 12:54 AM
#42
Burning Brakes
Thread Starter
08-03-2010, 10:22 AM
#43
Race Director
It's early in the brake and messes with my ability trail brake. It only happens on really fast straights ending in hard braking zones. There are three particular spots on my home track.
It feels like an unweighting that doesn't settle in time to turn in comfortably.
I'm still running quite quickly, just want to nip this in the butt as I'm sure that even though it isn't rearing it's head on all braking zones, I'm sure it's playing it's part.
It feels like an unweighting that doesn't settle in time to turn in comfortably.
I'm still running quite quickly, just want to nip this in the butt as I'm sure that even though it isn't rearing it's head on all braking zones, I'm sure it's playing it's part.
08-03-2010, 10:30 AM
#44
Burning Brakes
IMHO, when testing this type of setup issues, make only one change at a time. There are a lot of great suggestions here, but if try all of them or several at the same time, you won't know what works for your car with your style of driving.
08-09-2010, 10:27 AM
#45
Burning Brakes
Thread Starter
Follow up:
I did two things. I increased compression (bump) damping 2 clicks front and rear and more importantly, I experimented with braking technique.
The latter made the greatest difference and fixed my slight fade issue. I think that so many drivers forget that the brake pedal can make the car faster than the gas and like to stomp on the gas and ease on the brake. I had the most success loading the front and then getting on the brakes hard and evenly. Many zones didn't requires anywhere near the size braking box I used to use and combine this with adjusting corner entry speeds (faster) made for a much smoother overall lap.
Setting initial and final braking box points allowed me to start adjusting my technique and lap after lap the boxes got smaller until ABS started to kick in (BAD!). Probably cut 2 seconds off my lap times and gave me exponentially more confidence in my car.
I have to admit, the wiggle wasn't completely gone but it isn't scary at all anymore, just controlled.
I did two things. I increased compression (bump) damping 2 clicks front and rear and more importantly, I experimented with braking technique.
The latter made the greatest difference and fixed my slight fade issue. I think that so many drivers forget that the brake pedal can make the car faster than the gas and like to stomp on the gas and ease on the brake. I had the most success loading the front and then getting on the brakes hard and evenly. Many zones didn't requires anywhere near the size braking box I used to use and combine this with adjusting corner entry speeds (faster) made for a much smoother overall lap.
Setting initial and final braking box points allowed me to start adjusting my technique and lap after lap the boxes got smaller until ABS started to kick in (BAD!). Probably cut 2 seconds off my lap times and gave me exponentially more confidence in my car.
I have to admit, the wiggle wasn't completely gone but it isn't scary at all anymore, just controlled.
