Patrick's build thread. (IT RUNS!)
04-25-2012, 12:24 PM
#571
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Steve, I'm sure you're right in what you're saying and thanks for putting that all down. Here are some observations from the engineer to add to the mix. I knew something was up....
Hi Patrick
I’ve been analysing some of the data in more detail and what I have discovered is the following.
We have been using 45mm of rear damper travel at the track. This probably equates to around 70mm of wheel travel.
Unfortunately the car needs to have more droop travel through some of the higher G force turns. The inside rear wheel is lifting off the ground, this is the jacking effect that Paul was talking about, at the time we thought it was being caused by too much rebound damping and we removed some.
Factory setup have the droop controlled by the bolts retaining the outer torsion bar bush, your application probably has the rear droop controlled by a spacer above the piston within the damper so some damper or rear suspension setup mods will be required before the next outing.
If you look at the data particularly suspension position [position each corner in a new graph on the page] you will see that the lr suspension pots flat lines during some corners at around -22.
At this time the rear wheel is starting to lift of the ground and the rear will start to become unstable.
Your video also confirms this as you have to correct your line on entry , you were travelling 16 klms quicker than the previous lap and the gpsmap shows you wider on entry before the apex which all added to the event.
The rear end appears to be rolling more that the front but a more accurate measurement of motion ratios would be required before you could call this a fact.
Everywhere that the lr suspension starts to bounce off the full droop point is where you were complaining about a lack of rear grip so we need to rectify this before next outing.
If there are no spacers within the damper then we may need to get extended rose joints, Im a bit surprised that this has occured given that you bought the dampers to suit this application.
Mark
Rsport Race Engineering
Hi Patrick
I’ve been analysing some of the data in more detail and what I have discovered is the following.
We have been using 45mm of rear damper travel at the track. This probably equates to around 70mm of wheel travel.
Unfortunately the car needs to have more droop travel through some of the higher G force turns. The inside rear wheel is lifting off the ground, this is the jacking effect that Paul was talking about, at the time we thought it was being caused by too much rebound damping and we removed some.
Factory setup have the droop controlled by the bolts retaining the outer torsion bar bush, your application probably has the rear droop controlled by a spacer above the piston within the damper so some damper or rear suspension setup mods will be required before the next outing.
If you look at the data particularly suspension position [position each corner in a new graph on the page] you will see that the lr suspension pots flat lines during some corners at around -22.
At this time the rear wheel is starting to lift of the ground and the rear will start to become unstable.
Your video also confirms this as you have to correct your line on entry , you were travelling 16 klms quicker than the previous lap and the gpsmap shows you wider on entry before the apex which all added to the event.
The rear end appears to be rolling more that the front but a more accurate measurement of motion ratios would be required before you could call this a fact.
Everywhere that the lr suspension starts to bounce off the full droop point is where you were complaining about a lack of rear grip so we need to rectify this before next outing.
If there are no spacers within the damper then we may need to get extended rose joints, Im a bit surprised that this has occured given that you bought the dampers to suit this application.
Mark
Rsport Race Engineering
That really shows how nice it is to have all that information. Now, to interpret.
What I get from that is more questions. You mentioned that you removed the front sway. Clearly more weight is allowed to transfer toward the rear. Are you actually bottoming out the rear shock? ( I think probably not since He is not saying that your are flat lining on compression on the other side of the droop and he would have pointed that out) Is your sway bar in the rear now too large and lifting the inside wheel when it compresses (brings it higher into car with it?) That is my best guess.
So maybe removing the front sway bar may not be the answer? Put it back and make other changes to the front? Or keep that set up and do;Softer sway in the rear? Do the rear shocks need more travel or do they need to be stiffer? Of course the key to doing any one of these successfully is making sure the rest plays nice with whichever change is made.
Still. Great info. Thank you for posting these.
Last edited by 95ONE; 04-25-2012 at 01:07 PM.
04-25-2012, 03:24 PM
#572
Rennlist Member
That really shows how nice it is to have all that information. Now, to interpret.
What I get from that is more questions. You mentioned that you removed the front sway. Clearly more weight is allowed to transfer toward the rear. Are you actually bottoming out the rear shock? ( I think probably not since He is not saying that your are flat lining on compression on the other side of the droop and he would have pointed that out) Is your sway bar in the rear now too large and lifting the inside wheel when it compresses (brings it higher into car with it?) That is my best guess.
So maybe removing the front sway bar may not be the answer? Put it back and make other changes to the front? Or keep that set up and do;Softer sway in the rear? Do the rear shocks need more travel or do they need to be stiffer? Of course the key to doing any one of these successfully is making sure the rest plays nice with whichever change is made.
Still. Great info. Thank you for posting these.
What I get from that is more questions. You mentioned that you removed the front sway. Clearly more weight is allowed to transfer toward the rear. Are you actually bottoming out the rear shock? ( I think probably not since He is not saying that your are flat lining on compression on the other side of the droop and he would have pointed that out) Is your sway bar in the rear now too large and lifting the inside wheel when it compresses (brings it higher into car with it?) That is my best guess.
So maybe removing the front sway bar may not be the answer? Put it back and make other changes to the front? Or keep that set up and do;Softer sway in the rear? Do the rear shocks need more travel or do they need to be stiffer? Of course the key to doing any one of these successfully is making sure the rest plays nice with whichever change is made.
Still. Great info. Thank you for posting these.
I'm also curious what the roll moment is in the rear. I wonder if the RC is too low, causing too large of a roll moment. This *shouldn't* cause all of the ill handling effects (unless it's ridiculous), but it may point to a larger issue. What is your ride height? Typically the rear static RC is pretty high off the ground on 951's so I'm not convinced this is the culprit.
Doubtful you're bottoming out on bump -- that would indicate something really wrong was going on.
I'm not convinced the inside-rear wheel lifting is the whole story (though I'm not convinced it's not). I haven't done the calculations in a long time, but a significantly lower percentage of your weight is over that wheel (I want to say it'd be in the neighborhood of 20%). This means your outside wheel is providing 80% of the friction. With the suddenness you lost traction, I'm wondering if something else was there to catalyze the spin.
Like I said though, I'm not convinced it's not the whole story. 20% of your friction is a significant amount. If I can get some free time in the next few days I'll try to do some calculations to see estimates of what your friction curve might look like on that inside wheel. Hell, when completely unloaded it could just be the weight of the inside wheel's unsprung mass that's contributing to your friction, which comes to <10% of the total friction.
Food for thought.
04-25-2012, 05:20 PM
#573
Nordschleife Master
I just have a very hard time accepting the inner rear wheel lifting at basically steady state in a corner with that little steering angle. Something is fishy.. I would take a look at the suspension, brakes, wheel bearings, bushings etc to see if it binds or somehow limits travel.
04-25-2012, 05:47 PM
#575
Rennlist Member
Thread Starter
Yes, rear sway not front Bruce.
I also wonder about the rear inside lifting. Perhaps he means on the track that day as opposed to just at that corner before the crash. However even if that was the case it may mean that the inside wheel was not actually lifting off the ground but still could have been getting pretty light. Also, as has been noted, I slip initially and correct before the main 'let go'. There are also some bumps through there which could have easily unweighted part of the car. Not everything has been discovered yet I feel. I want to resolve this before taking to the track again. Collin, I don't have the ride height but it was not slammed low at all. I suspect it wasn't all that much lower than normal ride height but don't have the actual measurements.
I also wonder about the rear inside lifting. Perhaps he means on the track that day as opposed to just at that corner before the crash. However even if that was the case it may mean that the inside wheel was not actually lifting off the ground but still could have been getting pretty light. Also, as has been noted, I slip initially and correct before the main 'let go'. There are also some bumps through there which could have easily unweighted part of the car. Not everything has been discovered yet I feel. I want to resolve this before taking to the track again. Collin, I don't have the ride height but it was not slammed low at all. I suspect it wasn't all that much lower than normal ride height but don't have the actual measurements.
04-25-2012, 06:38 PM
#576
Rennlist Member
Not convinced about the rear rolling too much relative to the front either. I run my car as low as spring seat travel allows on the rear of my car and even wih the rear RC at its lowest point as a result it would never lift a rear wheel. It does lift front wheels occasionally.
Unless disconnecting the rear bar had a very dramatic effect, but it shouldn't have with the spring rates you were running. If the rear end was too high (too much rake) that also might contribute to the rear end wanting to move around more from simply having too high a C of G. Then again it could just be skipping on bumps.
It would be very helpful to see some outside video or photos of the car cornering to get a better look at how each end was rolling.
Unless disconnecting the rear bar had a very dramatic effect, but it shouldn't have with the spring rates you were running. If the rear end was too high (too much rake) that also might contribute to the rear end wanting to move around more from simply having too high a C of G. Then again it could just be skipping on bumps.
It would be very helpful to see some outside video or photos of the car cornering to get a better look at how each end was rolling.
04-25-2012, 06:51 PM
#577
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04-26-2012, 09:05 AM
#578
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Not wishing to drag this out any longer than needs be but thought I should continue to present feedback from Mark, the engineer. I put to him some of the questions that I've considered and also some of the ones you guys have asked. Without going into great depth he has come back with this:
Hi Patrick
Please see attached screen capture. Unless the rear wheel managed to hold at exactly the same height for over a second on each of the three occasions as you turned in I would believe my comments to be correct. I think the wheel lifted on entry and you were a victim of the grip slide shimmy scenario on entry to the corner which caused your mid corner correction and subsequent running wide on exit. I'm not saying that the rear shock has definitely bottomed out but something has definitely stopped suspension droop at that point, the most likely scenario is that the shock has runout of travel but regardless the situation needs to be reviewed to see what has caused the suspension to stop its travel and cause wheel lifting at that point.
Please note if you look at the capture you can see a similar trait in the right rear shock as you leave the track at about the same level of droop.
The 'grip-slide shimmy' is when the rear end grips and then slides and requires steering correction to maintain line through the corner.
Hi Patrick
Please see attached screen capture. Unless the rear wheel managed to hold at exactly the same height for over a second on each of the three occasions as you turned in I would believe my comments to be correct. I think the wheel lifted on entry and you were a victim of the grip slide shimmy scenario on entry to the corner which caused your mid corner correction and subsequent running wide on exit. I'm not saying that the rear shock has definitely bottomed out but something has definitely stopped suspension droop at that point, the most likely scenario is that the shock has runout of travel but regardless the situation needs to be reviewed to see what has caused the suspension to stop its travel and cause wheel lifting at that point.
Please note if you look at the capture you can see a similar trait in the right rear shock as you leave the track at about the same level of droop.
The 'grip-slide shimmy' is when the rear end grips and then slides and requires steering correction to maintain line through the corner.
Last edited by 333pg333; 04-30-2012 at 05:19 AM.
04-26-2012, 11:17 AM
#580
Rennlist Member
Very interesting. So, this is saying your max droop is 20mm, or about -30mm of wheel travel? That is not an unrealistic amount of expected wheel travel. If we assume the shocks were installed correctly, this tells me they are much too short for their application. The shocks should be capable of handling more than 40mm of travel.
However given that you bough a kit from Moton and they typically know what they're doing, chances are the shocks are correct. Which means it is either a problem with the install or with a defect in the construction of the shocks themselves.
Given gate your engineer is very competent and the installation is bolt on, I doubt that's the problem. Time to dissect some shocks!
So, I feel obligated to ask the following question so please don't hate me. Is there anything about the shock pot thay would be inhibiting either wheel travel or getting a proper measurement? I've seen it before so i just wanted to ask....
Thanks for posting those!
However given that you bough a kit from Moton and they typically know what they're doing, chances are the shocks are correct. Which means it is either a problem with the install or with a defect in the construction of the shocks themselves.
Given gate your engineer is very competent and the installation is bolt on, I doubt that's the problem. Time to dissect some shocks!
So, I feel obligated to ask the following question so please don't hate me. Is there anything about the shock pot thay would be inhibiting either wheel travel or getting a proper measurement? I've seen it before so i just wanted to ask....
Thanks for posting those!
04-26-2012, 01:00 PM
#582
Rennlist Member
I don't think you should be lifting the inside rear, especially with that little body roll and no rear sway bar. Either the data is not correct or something isn't right in the rear suspension. What could limit the droop in the rear? A shock that is too short or an issue with the spring plate bushing or ?
04-26-2012, 03:04 PM
#584
Rennlist Member
04-26-2012, 05:13 PM
#585
Rennlist Member
Thread Starter
Well I guess all those questions need to be answered. Something is clearly not right. I don't have the solutions as we speak. Just have to wait now for people to have time to get back onto my car.