Chassis Development - PeachParts Mercedes-Benz Forum

Roncallo · #1 12-31-2010, 09:25 PM

OK I now have finished putting the M120 into the 560SL and it runs very well. I put about 13,000 miles on it and all was going very well up until I cracked the rear subframe. That is now well underway to being fixed. So this summer I would like to turn my attention to chassis tuning. I don't mean hard race car chassis tuning. I just want to make sure things are well balanced, after all I did add 200 Lb under the hood of the car. So I'm interested in knowing your thoughts on tuning as far as getting the car well balanced.

I'm not a race car driver by any means but right now the Modified V12 car feels more well balances and stable at low speeds and that is probably due to all new suspension components and shocks and summer tires. But at high speeds I feel more confident above 90MPH in my stock 560SL, but a cant put my finger on what doesn't feel right in the V12 car at these speeds. All I can say is the hair on my back starts to stand up earlier in the V12 car. Then again it could all be my imagination playing with my head, not wanting to wreck the V12 107.

So what might be your thoughts how to identify a problem and how to solve. I will be interested in trying things out.

matsalleh76 · #2 12-31-2010, 11:25 PM

What was the weight differential V8 -Vs- V12?

--Adding weight to the front end will increase understeer and bugger up the front spring rate/body roll "comfort zone", resulting in the slightly disconcerting handling you describe.

We went through this with our 2.3-16 when we increased the front end weight with a supercharger/water/air intercooler system. There were other factors involved but the ultimate solution was heavier front & rear ARB's that brought the car back into balance.

Here is a link to a write-up we did on the effects of ARB changes on the car as it went through various iterations leading to the final blown configuration. It may give you some ideas.

Regards,
bobf

Roncallo · #3 01-01-2011, 11:35 AM

Quote:

Originally Posted by matsalleh76

What was the weight differential V8 -Vs- V12?

--Adding weight to the front end will increase understeer and bugger up the front spring rate/body roll "comfort zone", resulting in the slightly disconcerting handling you describe.

We went through this with our 2.3-16 when we increased the front end weight with a supercharger/water/air intercooler system. There were other factors involved but the ultimate solution was heavier front & rear ARB's that brought the car back into balance.

Here is a link to a write-up we did on the effects of ARB changes on the car as it went through various iterations leading to the final blown configuration. It may give you some ideas.

Regards,
bobf

The weight difference between the M120 with 722.6 transmission over the M117 with 722.3 trans is about 200 LB. I was able to bring the ride height back to stock using heavier springs up front for an iron block 450SLC. The car drove real nice threw the twist and turns of Deals Gap "Tail of the Dragon". Its the high speed straight line stability that seams amiss.

matsalleh76 · #4 01-01-2011, 09:29 PM

Increasing the front spring rate will also increase understeer.

We have never had any high speed straight line stability problems: the suspension bushes, steering components, etc are fresh and the car has always been smooth, steady, & true @155KPH. Perhaps something in the suspension/steering needs attention.

What caused the rear subframe to fracture and where did it fracture? 124's will sometimes develop a crack on the bottom of the rear diff carrier section between the two diff mounting bosses.

Regards,
bobf
(ex-Westbrook CT.)

Roncallo · #5 01-01-2011, 11:57 PM

Quote:

Originally Posted by matsalleh76

Increasing the front spring rate will also increase understeer.

We have never had any high speed straight line stability problems: the suspension bushes, steering components, etc are fresh and the car has always been smooth, steady, & true @155KPH. Perhaps something in the suspension/steering needs attention.

What caused the rear subframe to fracture and where did it fracture? 124's will sometimes develop a crack on the bottom of the rear diff carrier section between the two diff mounting bosses.

Regards,
bobf
(ex-Westbrook CT.)

The 560SL and 450SLC springs have the same spring rate. The 450SLC spring is just longer. See picture.

This cars suspension is not as refined as the 124. It's a 1971 handed down from a 115.

Alot of this could be me. I'm really not comfortable with either the stock 560SL or the modified 560SL above 100 MPH and that number has been coming down as my age goes up. But it does seem to come on earlier with the V12 like 90 MPH.

The rear sub frame cracked from engine torque. Once I put in a limited slip phantom grip rear and stick summer tires it stared to hook up and cracked the Diff mounting pad. I have substantially beefed that up and don't expect any more problems there. I attached pictures of the old and new mount. Old one was made with a 1/8" tube and 0.095" plate tig welded from the top only. New one is made with 5/16" tube and 1/8" plate tig welded from both sides.

docsaab · #6 01-06-2011, 10:27 PM

OK, lets start with suspension settings. How about camber/castor/toe in/thrust angle, front & rear. Thrust should be at Zero, front castor at max positive(+3?)(1/2deg+ more on the right side), slight negative camber (-1deg), just a touch of toe (1/16"-1/8"), front and rear, then test drive. If you still have that floaty feeling I'm guessing you're feeling, add more positive castor, retest. Remember, it's not a race car, it's a rallye car, as roads are not like a race track, especially in CT. Whatta ya think?

JayRash · #7 01-07-2011, 12:02 PM

if the car feels like its lifting its nose up at speed, try to run an under cover, try a tad more -ve camber and make sure the front of the tires is pointing inside.

Castor is a gr8 thing if ur looking for front end stability and weight in the steering wheel. with more castor the more the wheel is turned the more -ve camber the outer tire has, while the inside gets +ve effectivly planting the tires in a high speed corner. the downside of alot of castor is a slow turn in.

oh and make sure the trailing arms in the rear have new bushings since bad ones cause the rear tires to toe out thus cause the car to wonder around at high speeds. (even the wind force AT HIGH SPEEDS on the tires will cause them to toe out.)

spacer the rears with the same ammount. it helps.

Roncallo · #8 01-07-2011, 01:44 PM

Thanks for the reply's. At this time I still have the rear sub frame and axle out of the car so its a good time to consider replacing trailing arm bushings and new rear sub frame mounts. Although they do look OK, I have all new stuff up front so I might as well do the rear end overhaul.

I will be looking into spacers as well on the rear at least until I can get custom wheels that don't require them.

JayRash · #9 01-09-2011, 02:34 AM

Ron, I have never setup my cars according to stock. i always play around with the setups, i for one hate understeer, and i make sure my cars have as planted as they can front ends, i also set them up to be tail happy on trail braking. but thats me.

JayRash · #10 01-09-2011, 10:32 AM

thanx ggr for the read
as for stiff springs and shock stiffness; i am interested in examples
as far as i know stiff springs work better with relatively softer compression and stiffer rebound, right?

GGR · #11 01-09-2011, 11:12 AM

Quote:

Originally Posted by JayRash

as far as i know stiff springs work better with relatively softer compression and stiffer rebound, right?

Yes, this is what I read. But I also read that it depends on the spring rating and they should be matched. I don't remember where, otherwise I would be posting the links. My experience is with a W111 Chassis. I wanted it to handle better and I was considering adjustable konis. But then I was not really able to find the konis characteristics, the spring characteristics and the relation to match them. I was also reading that people had better experieces with stock bilsteins than with konis on the W111 chassis. I then looked at the table of correspondence in the Workshop manual and discovered that for the W111 Chassis bilsteins with one green bar were recommended for normal use and with two green bars for heavy duty. I chose the second option. I could also see that even stiffer bilstein with four bars were available but fitted as standard on the W113 chassis which has softer springs (similar chassis but shorter wheelbase and lighter car).
These heavier duty four green bars shocks fitted as standard on the W113 are even not recommended for use in countries with bad roads for the W111 chassis.

This is what makes me say that stonger springs may work best with softer shocks and vice versa, but I was not able to come with a quantified formula.

JayRash · #12 04-13-2011, 05:08 AM

glad to hear. and i think its like i suspected all a long. u spaced the fronts but did nothing to the rear track, and with the new et on the sl rims pushing the rims out u solved it

Roncallo · #13 04-16-2011, 03:48 PM

Quote:

Originally Posted by JayRash

glad to hear. and i think its like i suspected all a long. u spaced the fronts but did nothing to the rear track, and with the new et on the sl rims pushing the rims out u solved it

Car seams very well balanced now. And with the new stearing box its like a new car. Thank you everyone

Roncallo · #14 08-01-2011, 12:09 PM

I actually think I really learned what is going on here this week when I put Drag radials on the car.

With the drag radials the high speed stability problem became more pronounced. I have come to the conclusion that this is due to the Phantom Grip rear end acting like a spool while you are going straight and your initial steering inputs from straight are fighting the rear tires. This is causing an amplified amount of steering hysteresis when the steering box is centered. No feeling of instability around a high speed curve.

Any other Phantom grip users notice this?