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post #41 of 70 (permalink) Old 08-26-2014, 04:46 AM
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One more anecdotal story regarding brakes.... For my track day this last Saturday I ran this setup:

Front: Carbotech XP10
Rear: Carbotech XP12

Front: Hoosier R6 225/45 R15
Rear: Hoosier R6 245/40 R17

EBC 4000 series rotors all around

The brakes were very, very good. Yes, I was activating the ABS on every corner. This is a braking intensive course with 4 100+ mph straights ending in 45ish mph corners. For reference, I was running in the instructor group with lap times basically the same as a couple of Z06 Vettes in the advanced group. NO ice mode events all day! It was really nice.

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post #42 of 70 (permalink) Old 09-12-2014, 07:44 AM
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This doesn't play to the OP's desire to calculate what is needed, but there is one other option I know about that might still be available. The rear brakes on this car are the same as on a Gen I viper w/o abs, and there was a man (tom hayden) who set out to improve the performance of these parts.

These calipers can be bored out from 36mm pistons to 38mm or 40mm, and Tom was doing that. The additional rear bias in the viper resulted in braking forces in the viper from .8g -> 1.0g if I remember right.

Some people relocate the front caliper to the rear, but another option would be to bore it out a little more as well. If you want to contact Tom to ask if he's still doing this, send me a PM.

"Give a man a match, and he'll be warm for a minute, but set him on fire, and he'll be warm for the rest of his life."
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post #43 of 70 (permalink) Old 09-12-2014, 08:24 AM Thread Starter
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Cool. I'm trying the brake pad bias thing with the CL pads right now, and will be autoxing it on sunday to try it out under duress. If it looks like it hasn't really solved the problem, you'll prolly get a PM.

David de Regt - Bellevue, WA
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post #44 of 70 (permalink) Old 09-13-2014, 02:00 PM
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codingparadox,

Cheapest and easiest thing you can try is to reduce your rebound damping setting on your Penske double adjustables! Reduce the rebound damping! Try it. I'll say it again. Too much rebound damping can keep the tires from maintaining contact with the pavement, that can cause the ABS to go into ice mode.

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post #45 of 70 (permalink) Old 09-13-2014, 02:14 PM Thread Starter
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I did that at an autox (to try to reduce some lift-off oversteer) and it had no effect on the ice moding.

I'm trying the different pads. I don't like the RC6 since it squeals like hell on the street, but the brake balance is much better this way. I'll likely keep these on for autox.

David de Regt - Bellevue, WA
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post #46 of 70 (permalink) Old 09-13-2014, 02:17 PM
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Quote:
Originally Posted by codingparadox View Post
I did that at an autox (to try to reduce some lift-off oversteer) and it had no effect on the ice moding.

I'm trying the different pads. I don't like the RC6 since it squeals like hell on the street, but the brake balance is much better this way. I'll likely keep these on for autox.
How much? one click, ten clicks? front and rear? what settings are your f&r reabound at? And your compression settings too?

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post #47 of 70 (permalink) Old 09-13-2014, 02:23 PM Thread Starter
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Only rear. 10 clicks.

This has custom valving, so there's no way to map to what yours are. It's stiffer than the valving he's given anyone else since it's going to be an SSM car.

To be clear, this is the same exact problem I had on my original super stock elise, which had much less damping all around, especially rebound (I ran very little on that car, mostly compression.)

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post #48 of 70 (permalink) Old 09-13-2014, 09:06 PM
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A couple things:

- Reducing front rebound MAY help with icemode.

- Adding rebound in the rear makes the car more stable on lift/entry. If you take away rebound, the car will be looser as the shocks are allowing the springs to push up on the car faster and take weight off the rear tires by transferring it to the fronts.
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post #49 of 70 (permalink) Old 09-13-2014, 09:10 PM Thread Starter
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Someday maybe I'll figure out how shocks work...

My understanding with taking out rear rebound was that it let the inside rear stay on the ground better on corner entry instead of hanging in the air providing no grip. I take it that's incorrect?

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post #50 of 70 (permalink) Old 09-13-2014, 11:59 PM
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Quote:
Originally Posted by codingparadox View Post
Someday maybe I'll figure out how shocks work...

My understanding with taking out rear rebound was that it let the inside rear stay on the ground better on corner entry instead of hanging in the air providing no grip. I take it that's incorrect?
That was correct.

If you wheels/tires are more offset, (like 15/17) then the rears may seem to be locking(or not spinning fast enough).

Secondly the bias will have moved forewords, which may not be too desirable.
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post #51 of 70 (permalink) Old 09-14-2014, 07:44 AM
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Originally Posted by codingparadox View Post
Someday maybe I'll figure out how shocks work...

My understanding with taking out rear rebound was that it let the inside rear stay on the ground better on corner entry instead of hanging in the air providing no grip. I take it that's incorrect?
You are correct if there is a TON of slow speed rebound in the shocks already. It takes a fair amount of lowspeed rebound to hang up a tire. If you have that much rebound, you have bigger problems. And here's the important part: cars are heavy and will stay on the ground if not lifted by springs.

Imagine your car had no springs what so ever. Imagine braking. There's no way the back wheels are going to lift off the ground.

Now that's where springs change the equation. They will allow the chassis to move around and build up a momentum that's comprised of the cg height and the mass of the car and the lateral forces introduced by the tires. If there were no dampers, the springs could have their way with the car as illustrated by softly sprung and damped stock cars. They could even flip over.

In the instance of a braking event, the rear springs are exerting their pushing force up on the back of the car, helping LIFT it on to the nose. When the car lifts up like that, less weight is maintained on the back wheels after suspension extension is complete. If you increase the rear rebound force, it limits the energy in the springs and allows the rear of the car to stay low and heavy... ie, with more grip and thus becoming more stable on trail braking.

Adding compression to the front shocks does the same thing.

Suspension tuning is all relative. One end of the car has a relationship with the other....
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post #52 of 70 (permalink) Old 09-14-2014, 08:00 AM Thread Starter
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Hm. Cool explanation. Okay. I'm at an event right now. I'll try cranking up rear rebound after a run or two and see how it changes the rear stick on lift. Thanks much!


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post #53 of 70 (permalink) Old 09-14-2014, 01:59 PM Thread Starter
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Didn't seem to have much effect. If anything it made it worse. I actually spun on my last run when the car started penduluming badly through some offsets. Sigh.

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post #54 of 70 (permalink) Old 09-14-2014, 02:19 PM
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Quote:
Originally Posted by glagola1 View Post
...
In the instance of a braking event, the rear springs are exerting their pushing force up on the back of the car, helping LIFT it on to the nose. When the car lifts up like that, less weight is maintained on the back wheels after suspension extension is complete. If you increase the rear rebound force resistence, it limits the energy in the springs and allows the rear of the car to stay low and heavy... ie, with more grip and thus becoming more stable on trail braking.

Adding compression to the front shocks does the same thing.

Suspension tuning is all relative. One end of the car has a relationship with the other....
^This^ is not quite correct.
Under static conditions there is, say 1000 pounds on the rear.
Under braking it might be 600 pounds.
The car will be lower with 1000 pounds than under braking with only 600 pounds.
The springs force on the rear wheels will linearly change exactly the same as the rear height changes.
The dampers either increase or decrease the springs force.
When the high rebound is set correctly (for braking) the weight/force of the rear moves in more of a step function versus time, than in a ramp. So even thought the rear is initially low, as soon as the brakes are jammed on the weight it off the rears even though it is low.
With no dampers the weight is on until the car rocks forward in pitch.

I forgot the OP's question, but his interpretation was correct.
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post #55 of 70 (permalink) Old 09-14-2014, 02:30 PM
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Note that I did not say crank up the rear rebound...

I said lower the rebound... referring to all the rebound, front and rear.

Have you read this? Penske shock manual. page 17
https://www.google.com/url?sa=t&rct=...-ytz_GE4MRU02g

Quote:
Basic Start-up Procedure

The following setup procedures are basic recommendations for reaching an initial starting point using double adjustable Penske Racing Shocks. This procedure is ideal for use on an open test schedule. A race weekend
may not allow enough time. Start by making the compression adjustments as described below, until it feels right, then move to the rebound adjustments.

COMPRESSION
The idea is to set the compression damping forces to suit the bumps in critical areas, such as corners, corner exits and braking zones.
Increasing or lowering cannister pressure (range 150 to 300 psi) can have an influence on support under brak- ing, acceleration, and tire loading on turn in, and on mid-corner grip.
Step 1 - Set the rebound adjuster at full soft.
Step 2 - Starting with the compression setting at full soft, drive a lap then return to increase the bump settings. Continue this process of adding bump control to minimize the upsets until the car becomes harsh, loses tire compliance and traction. At this point you know that you
have gone too far on the compression settings; back off one click.

REBOUND
The idea is to tighten up the car, stabilize the platform and eliminate the floating "Cadillac feeling". This will also reduce the rate of body roll.
Step 1 - With the rebound setting at full soft, add clicks of rebound adjustment, then return to continue the process until the car becomes "skittish" or the rear wheels hop under braking. At this point you know you have gone too far on the rebound settings, back off
one flat at a time for final balance.

Once again, this is a basic procedure for finding your initial setup for a given track.

Rebound adjustments are usually indicated by the driver asking for more stability. By increasing low speed damping, stability will be enhanced; decreasing damping will allow more movement in the car, but will result in a
little better tire wear. Also, the amount of rebound can have a great influence on weight transfer. Less front rebound allows weight transfer to the rear under acceleration. Less rebound in the rear allows for a greater amount of weight transfer to the front under braking and turn in.
When a car is over damped in rebound it can pack down in a series of bumps and a driver will recognize this as too stiff and usually will think it is compression damping. Too much rebound can cause lack of grip on cornering. When making a large spring change keep in mind where the rebound adjuster is and do you have enough range to compensate. Sometimes a spring change will bring a better balance to the damping values after the spring change. If the spring/shock combination was balanced, the rule of thumb is a stiffer spring requires lower compression and higher rebound. A softer spring requires higher compression and lower rebound.

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post #56 of 70 (permalink) Old 09-15-2014, 06:20 AM
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Originally Posted by holmz View Post
^This^ is not quite correct.
Under static conditions there is, say 1000 pounds on the rear.
Under braking it might be 600 pounds.
The car will be lower with 1000 pounds than under braking with only 600 pounds.
The springs force on the rear wheels will linearly change exactly the same as the rear height changes.
The dampers either increase or decrease the springs force.
When the high rebound is set correctly (for braking) the weight/force of the rear moves in more of a step function versus time, than in a ramp. So even thought the rear is initially low, as soon as the brakes are jammed on the weight it off the rears even though it is low.
With no dampers the weight is on until the car rocks forward in pitch.

I forgot the OP's question, but his interpretation was correct.
I'm not exactly sure what you are saying by "With no dampers the weight is on until the car rocks forward in pitch." ??? I'm also not sure how what you have typed is essentially any different that what I typed.

Anyway, it's been my experience that adding rear rebound keeps the car more stable under trail braking... in all the cars I've ever driven in anger. David, I'm confused how it seemed to have no effect or make it worse. Am I correct in remembering that you are running 500/800 on street tires? Yowza. That probably makes it very difficult to tell what your damper settings are doing at all.
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post #57 of 70 (permalink) Old 09-15-2014, 07:44 AM Thread Starter
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Yeah, I'm running 550/800 on street tires for now -- the rates will make more sense once I get the R comps on the car.

Yeah, I'm not sure. I've done a sweep of -10 to +10 clicks of rear rebound from where Fred suggested the shocks start, and couldn't tell a difference in rear stability through transitions. The stiff springs may help mask any effect the shocks are having at this point without having more tire, so it may all be pissing in the wind right now. I'm pretty worried that when I throw the R comps on (255->275 rear and 225->275 front) that it will dramatically shift the balance of the car toward loose given the massive front tire increase, which is only going to make my problem worse.

More on topic of the thread, I actually tried lowering front rebound 6 clicks yesterday when I was getting some pretty nasty ice mode in a hard braking zone (>60 to 20mph) right after a transition, and that actually did seem to help quite a bit, though I may have also forced myself to get into the brakes slower instead of stabbing it.

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post #58 of 70 (permalink) Old 09-15-2014, 07:54 AM
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So far Vulcan Grey and Glagola1 have made correct statements about damping, though they are talking about different effects due to rebound. Vulcan Grey is talking more about the fronts loosing traction due to too much high-speed rebound. Glagola1 is talking about instability due to rapid load decrease of the rear tires due to too little low-speed rear rebound.

To Vulcan Grey's point: If a bump is encountered while braking, the suspension will compress. If there is too much high-speed rebound, the load on the tire will actually decrease as the tires tries to drop back down after the bump because the damper is restricting how quickly the suspension & tire can extend. If the load decreases too much, the braking torque will overcome the friction between the tire and the ground causing the tire to lock up. This could trigger an ice mode event.

To Glagola1's point: Rear rebound is vital to controlling weight transfer during braking. Think of rebound as a tool that slows the weight transfer during a transition. During braking, weight is transferred from the rear to the front. Increasing rear rebound will slow the rate of weight transfer, keeping the rear tires loaded more for a longer period of time. The front grip increases at a slower rate and the rear grip decreases at a slower rate. Decreasing the rear rebound will allow weight transfer to occur more quickly, giving additional grip to the front tires in a shorter period of time. Similarly, more grip is being taken from the rears in a shorter period of time. This causes the feeling of instability if rear rebound is too low. Once the system reaches a steady state condition, the dampers are no longer in control of anything because the pitching motion is complete and the dampers are not compressing or extending.

In general, rebound damping steals grip from the opposite end of the car during a transient event. Compression damping adds grip to the end of the car that it is added to. Here's a summary of those statements:

Generalizations during a transition (pitch or roll, or some combination there of):
More front rebound = less rear grip
More rear rebound = less front grip
More front compression = more front grip
More rear compression = more rear grip

Of course, it is possible to go too far with compression and/or rebound damping. Too much compression will cause the tires to load too quickly, causing the car to skid. Too much rebound will cause the tire load to decrease too much, causing loss of traction.

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post #59 of 70 (permalink) Old 09-15-2014, 08:40 AM
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David, those spring rates didn't work for me until I added the rear wing. A ton of rear toe-in helped but it was still too loose before the wing. This was on 225/245 A6s. I also did not enjoy how stiff the car was. It made it kinda skittish and difficult to drive in the hamfisted style I employ.

I've also found that too much front rebound can have a negative effect on braking. I only ran into that with some revalved Koni Yellows. I have not found that in any of my sets of Motons or the Ohlins I'm running now.

Jake, I agree with everything you posted regarding what I said but I'm not sure if Vulcan Grey was talking about the realm of TOO MUCH rebound valving. I mean, yeah, if it's too much resistance the wheels can jack up but damn. I hope we're not in that range of valving.
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post #60 of 70 (permalink) Old 09-15-2014, 12:58 PM
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IMO, I would start with minimal to zero rebound damping front and rear, as the Penske manual says. Then get compression damping where you like it increasing both equally to a point and then changing front or rear a tiny bit to balance the car. Then I would start increasing the rebound a little at a time (equally) until the car feels skittish, as they say, and then I would back rebound damping back down. Then slightly adjust front or rear balance to suit your feel. Again, IMO rebound damping should be the least required to help the car stay composed over a variety of situations.

I have double adjustables by JRZ in my Esprit, and I had adjustables on my daily driver. On the daily driver I kept getting ice mode in the rain if I tried to brake for an intersection that had some ripple to the pavement. The front brakes would at first lock up from minimal braking, and then the ABS would freak out and the car would slide with all 4 wheels locked up. Reducing shock damping fixed this.

In the Lotus with the double adjustables I read the book by JRZ's founder, it explained his philosophy , and a big part of that was to use least amount of rebound damping possible, especially on rough or rippled pavement.

I've also played a lot with my mountain bike adjustable suspension. Too much rebound damping will allow the front fork to compress, and not re-extend, all your weight will go forward and you will flip!

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