Author Topic: Pagoda Instability over 100MPH  (Read 4541 times)

n/a

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Pagoda Instability over 100MPH
« on: July 06, 2006, 22:11:25 »
Dear Members;
After the post about E-Type or Pagoda, I get the feeling most believe the W113 has excellent handling. I agree on corners at speeds below 50MPH...but I have had a few scares with mine recently. My car's suspension and steering are all good and tight,  but this past week after playing tag with a Porche at speeds at and over a hundred (don't ask...), on a couple of grades, I almost lost it as she exhibited severe oversteer.
Question; can something whacky be happening when the car rises over a hill and the wheels camber inboard thus causing a camber thrust, especially with the independent rear? Tire pressure was high at 40PSI and perhaps the bite on the road was less with a tire slightly convex?? Tires are not high speed rated as I wanted whitewalls...they are BF Goodrich P185/75R14's...
Any ideas? Anyone have their car pull suddenly on hills/curves?
Appreciate any input. Thanks.
Regards,
Joe
 :)

George Davis

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Re: Pagoda Instability over 100MPH
« Reply #1 on: July 06, 2006, 22:53:09 »
Joe,

My 2 cents.  The rear suspension is a modified swing axle design.  Swing axles have a bad reputation for severe oversteer, the most famous examples being VW Beetle, Corvair, and the 300 SL Gullwing.  Under certain circumstances, the outside wheel tucks under the car (severe positive camber), and then you're corning on only a small part of the tire, where the tread and sidewall meet.  Camber thrust may play a role, but I think it's mostly the big loss in traction that causes the oversteer.  The low-pivot swing axle in the Pagoda reduces the tendency to oversteer but probably doesn't entirely eliminate it.  I would guess that with your car going over a hill, the chassis rose up resulting in positive camber on the outside wheel, and you had essentially the same effect, corning on the edge of the tread.  I would guess that you found just about the worst possible situation there.

George Davis
'69 280 SL Euro manual

Vince Canepa

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Re: Pagoda Instability over 100MPH
« Reply #2 on: July 07, 2006, 06:32:36 »
George's comments are pretty much on the mark, except that I don't consider these cars to have "severe" oversteer if you compare them with Porsches.  The rear weight bias of the 356 and 911 Porsches give them what I would consider severe oversteer.  The W113 will understeer to a point, then transition to a mild oversteer if you exceed the limits.  The trick is to anticipate how the car will react.  For instance, if you are cresting a rise it is better to stay on the gas rather than backing off.  You must learn to control the weight transfer in order to keep the rear suspension compressed.  The goal is to minimize positive camber effects.  I like to set them up with a touch of negative camber at rest to help.  If you have ever spent any time in a Porsche, many of the same lessons apply, i.e. slow in - fast out and never brake or back off the gas mid-corner.

Vince Canepa
1967 250SL
113.043-10-001543
568H Signal Red
116 Caviar MB-Tex

peterm

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Re: Pagoda Instability over 100MPH
« Reply #3 on: July 07, 2006, 08:13:21 »
First of all P rated tires are speed rated to run less than 100,   Second your driving a 35 year old car with 35 year old technology with no airbags/non pretensioning seat belts, ?age of your suspension components etc many reasons to use this car as a cruiser.  I have no problem with speeds like that at the track, its the other cars on the road that make this more of an issue.  There was a case a few years back here on Long Island of a Lambo and a vette mixing it up and the lambo head on'd volvo killing both drivers.  The vette driver went to jail for manslaughter as this was considered a speed contest directly responsible for both deaths.

n/a

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Re: Pagoda Instability over 100MPH
« Reply #4 on: July 07, 2006, 16:36:26 »
Ok,
I agree it was a dumb thing to do, but if there is a defense itwas only at this speed for a short distance. While the speed limit is 65, cars routinely do 80 and the Pagoda does this just fine. I'm trying to understand, and I think the responses confirm this, the swing axle although much better than the 300 Gullwing, can still do unpredicable things. And yes, the instinct is to let off the gas...I wonder if this was the unprdicability that affected many early Gullwing drivers. Anyone out there who has taken a Pagoda on the Autobahn over 100MPH?
R/
Joe

Vince Canepa

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Re: Pagoda Instability over 100MPH
« Reply #5 on: July 07, 2006, 19:54:40 »
Not on the Autobahn, but on Hwy. 1 between Santa Cruz and San Francisco in California.  I used to run my 250 routinely between 80 and 120.  I find the cars have excellent straight line stability and as long as you drive them as I stated earlier, I think they corner quite nicely.  If the car is unstable, and to me that would mean uncertain about where it is headed, I would take a real serious look at the hanger mount next to the differential, the cross strut mounts and the trailing arm mounts.

Vince Canepa
1967 250SL
113.043-10-001543
568H Signal Red
116 Caviar MB-Tex

Longtooth

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Re: Pagoda Instability over 100MPH
« Reply #6 on: July 07, 2006, 20:25:43 »
For what it's worth, my dad owned a '67 250SL in Europe from about '70 to '80 while he lived in Italy.  He also owned a '62 or '63 190SL.  Both of these he drove thru-out Europe at high autobahn level speeds.  He told me several times that the 230SL had a severe oversteer, shifting suddenly from the normal bit of understeer.... and that this occurred occurred suddenly, no warning 'feel' when cornering at high speeds... i.e. the rear would kick-out and spin around before you knew it.  The story he related to me was that this severe, without warning high speed oversteer was the reason MB changed rear suspensions on the 250SL and later versions.... as opposed to the 230SL's.  He said, & I've experienced in my own '67 250SL that this sudden kick-out was dramatically reduced in the 250SL and later versions, but all it did was dramatically increase the speed at which the sudden kick-out occurred ... with a short warning period before it happens.

My dad's since passed away, but he was an SL enthusiast in Europe from the time we moved there in '59 'til his passing a couple years ago. ... for example, he had MB Stuttgart do any major overhauls and engine rebuilding even though he lived in Naples most of the time he owned the 250SL.  He loved the Italien mountain drivers going over the Alps back and forth to Germany several times a year.... they raced over the Alps.... picking up 2 or 3 other's they would pass by, but that wanted to join the race / push it to the limits on those roads with some white knuckle adrenalin soaking "relaxed" competition... the only thing they had to watch for on the Italien side were the carbinari... but it was hell bent for leather once they hit Switzerland and Germany.... Brenner Pass as I recall.

By the way, braking with the 113 drives the center of gravity toward the front wheels big time, so severe positive camber in the rear wheels (inside wheel on a curve) will result no matter what with a swing axle... it's only a matter of how severe.  The outside wheel cambers due to the forces (centrifugal force at the axle, countering resistance where rubber meets road).  This forces the center of gravity to the outside... hence taking the vertical force off the inside wheel/tire, letting it lift.  The noticable nose-dive on a 113 when braking is actually closer to 1/2 a nose diving and 1/2 the rear lifting!  If you therefore brake at all, even decelerate at high speed on a curve, you get a double whammy.... and if you do this while cresting a hill, not only does the entire car loose traction, as soon as the rear wheels start to grab with the front wheels 'hanging' with low load on them, a much greater than half the weight of the car goes to the front wheels... while the rear wheels start to lift.... leaving nothing but big time positive camber to develop on the outer rear wheel... in short, you're lucky you didn't roll it.... so it's probably a good thing you were over-inflated since if under-inflated the side-wall could have buckled easier and you'd been riding on the outer edge of the rim... and if it weren't for the broad stance on the 113 you probably would have.    

If you're going to race porches on hilly curves at 100mph +, get wheels that allow you to the lowest low profile tires and widest tread width you can find... which will mean some modification at the rear fenders when bottoming out... then don't brake into the curve or at the crest, but before you enter the curve and before you crest.... then accelerate like hell out of the curve after the car bottoms on the other side of the crest.  Parenthetically, I use every inch of both lanes on a two lane road.... taking it wide on entry, cutting the corner at the apex to within an inch of dirt, then taking it wide out of the curve.... watching for oncoming traffic --- pedestrian, horses, bicycles, bikes... and oh, cars and trucks too... but those are usually easier to see before entering the curve wide.... the others aren't.  But you know all this.

Longtooth
67 250SL US #113-043-10-002163
95 SL500

Chad

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Re: Pagoda Instability over 100MPH
« Reply #7 on: July 07, 2006, 21:46:07 »
Of course, the modern cars have more safety features for the racing than our "antiques." This is one illustration.
http://www.wreckedexotics.com/special/enzo/

I wonder how many of the 300SL were lost because of the handling concerns/oversteer.

1967 230SL (Manual, rustless driver)
« Last Edit: July 07, 2006, 22:11:51 by Chad »

Benz Dr.

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  • Benz Dr.
Re: Pagoda Instability over 100MPH
« Reply #8 on: July 08, 2006, 14:31:44 »
You have to remember that the Gullwing had a  joint on both sides unlike later cars where they only have it on the right side. Aside from disc brakes, the 230 and later axels are pretty much the same mounting into the car the same way. Only major difference would be the hydro compensator that's often replaced with the cheaper spring. Is that what you mean by better or different?

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
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1966 230SL 5 speed, LSD, header pipes, 300SE distributor, ported, polished and balanced, AKA  ''The Red Rocket ''
Dan Caron's SL Barn

1970  3.5 Coupe
1961  190SL
1985   300CD  Turbo Coupe
1981  300SD
2013  GMC  Sierra
1965  230SL
1967 250SL
1970 280SL
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n/a

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Re: Pagoda Instability over 100MPH
« Reply #9 on: July 08, 2006, 21:51:19 »
Wow,
Thanks for all the responses...
First of all, I'm not going to do this again..."Lomgtooth"...you are absolutely correct. I forgot how much these cars nosedive when braking and that would camber up the rear wheels big time! I wonder if altering the proportioning valve to brake more on the rear discs would help.
"Dr.Benz" mentions a "hydrocompensator"...I've seen this in a few pics but thought it was just a shrouded spring. I presume it is a pressurized spring with hydraulic damping? If so, would chaning over to one help with the ride and handling?
Some humbling scary pics at Chad's hyperlink...
http://www.wreckedexotics.com/special/enzo/
Again, thanks for the input...
Regards,
Joe
 :)

Longtooth

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Re: Pagoda Instability over 100MPH
« Reply #10 on: July 09, 2006, 02:06:37 »
Benz Dr....
I'm not differentiating between rear suspension differences in 230's and later, rather the full cornering capability.  There are several differences in the 230SL's and later version 113's... especially in the front end of the 1st version 230SL's (which ended with what chassis number I don't know for sure, but my big blue book refers to Aug '66 as the distinguishing date.

The biggest differences are in rear camber spec's (greater positive camber in the 230SL's for standard springs, 1.75 degr vs 1.5 degrees)... and front end spring compression (PN's) with the early version 230SL's having a lower spring compression than the later ones.  The other significant difference between all 230SL's and later version SL's is the front torsion bar torsional stiffness... which is 46% stiffer in the 230SL's than the later SL's.  If you couple this with the lower loads on the wheels, the lower front spring stiffness of the early 230SL's, and the greater rear wheel camber, it translates to a greater front end and rear lean at speeds on the 230SL's than later versions.  With a lighter rear wheel load and greater camber, the 230SL's would spin-out at a lower speed than the later versions.... and with the softer front end springs the front would lean more on cornering so be more prone to lift the inside wheels with more loss of traction.

I can't attest to the derivation of my father's sources (and since he's recently passed I can't ask him) but since we lived in Europe at the time the 230's were first introduced, and then when the 250SL came out... with the suspension changes.... and his having been an avid enthusiast of the SL's while living in Europe from '59 thru '80, I can only surmise his source(s) were thru the European version equivalents of Car & Driver & the like as well as other MB SL enthusiasts with whom he associated while in Europe.  He told me once when I was visiting him in Naples in summer of '68 that the 230SL was "notorius" for high speed cornering spin-outs without any warning.... that the car would remain at an understeer condition until it suddenly went to a severe oversteer which caused the spin-outs.  He had, by that time, also purchased a '67 250SL which he said wasn't prone to the sudden spin-out syndrome.... by his own testing and comparison's with a 230SL on an air base's tarmac.  I had and have no reason to suspect my father's statements and experiences were in error or exaggerated, and indeed, upon careful review of the suspension spec's between the 230sl and later versions his statements are supported by the differences in specified suspension systems in direction of tendancy's he spoke of.  Since I'm an engineer I resort to an engineering analytic approach to my judgements or explanations.  

I can also surmise that my father's statements are the result of high speed driving conditions in Europe... and when I lived there 'til summer of '64 I drove all over europe at speeds near and in excess of 100 mph... 90 was just cruising along ... no white knuckles.  On the Bundesbahn's (highway's.... not freeways or Autobahn), most of the stretches outside cities and towns along the way were no speed limit stretches... so being a teenager, I drove as fast as I could go without crashing.... and I never did lose it, though there were multitudes of close, close calls.  I once drove back from the Italien Riviera to Central Germany in a b544 Volvo ('62) loaded to the hilt with my buddy's and my camping equipment without any brakes!!!! none... except the emergency brake and what alot of down shifting could muster.   That was across the alps (up Brenner pass wasn't so bad.... going down was a white-knuckle adrenaline pumping event.... as was crossing Germany on the autobahn's in summer vacation near bumper to bumper traffic going 80 in the slow lanes.

My dad wasn't a teenager, but he drove hell-bent for leather everywhere as well... just with a lot more finess and experience than I had.  Since he and my mom lived there from '59 thru '80, he pretty much did a lot of high speed agressive driving in his 190SL and 250SL for a large fraction of his adult life.  By the way, he never had an accident or fender bender despite what might appear to be 'risking it all' type driving.. on ice/snow, over Alps, up hill, down dale.  From my experiences driving for several years in Europe and later when I lived there in my early 30's for several years, and from my father's... one of the driving cultures is like a bunch of kids street racing.... kind of like "I dare you to keep up with me.... or dare you to pass me if you can"... which occurs frequently among drivers of high end MB's and other sports car drivers.  Things may have changed since the 60's when I was driving there as a teen-ager and early 20s.... and later in the early 80's when I lived there, but on several and frequent long duration business trips to Europe until 2000 I hadn't found it to have changed.... although on my latest trips the freeway's were far more crowded with trucks and cars than I'd ever experienced before.... result of the EU freeing up the border's I think.... and lack of having extended the number of lanes on the freeways... so the fast lanes were going no faster than the slow lanes... or not much faster anyway.

The net of this response is that I think my father had a better handle on the different response characteristics of the various '60's SL's versions in high speed driving conditions on twisting high speed roads than most outside of professional racers... not to mention the extensive coverage of auto MB's and BMW's (among others) performance in the European auto weeklys & monthlys of the times.  My dad thought nothing of simply hopping in the car with a bag of clothes and driving straight thru from Naples to Munich or Frankfurt on some occasions... no big deal and he loved that drive as fast as the SL could go.

Longtooth
67 250SL US #113-043-10-002163
95 SL500