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?

Roncallo · #7 01-07-2011, 08:47 AM

I had to send it to Firestone to get it aligned and they did get it to within specs of the stock 560SL, but I no longer have the exact numbers. That's one of the things that has me kind of stumped. Normally I would say more caster for high speed stability, but the 560SL requires 10° degrees caster which is off the scale of all the hand type alignment gauges I have. So I wouldn't think caster woulds be a problem, but then I don't understand why MB requires so much.

The rear alignment of the 560SL is not adjustable but it was also measured to be within spec.

The front end of the V12 560SL has all new parts and sticky summer performance tires and the car feels like it handles better than the stock 560SL at low speed, but I attribute that to the all new components and tires. At above ~ 80MPH I'm more comfortable in the stock 560SL.

Theirs always the possibility this is all in my head. As I push the car further and further I do pay attention, but I really do believe that are things that could be done to balance it out better.

Basically this is a stock 560SL chassis with 200Lb added to the front sprung weight and 450SLC springs up front to maintain stock ride height. It also uses 129 wheels which require 20mm spacers up front and no spacers in the rear. Its riding on lower profile 225/50-R16 BF Goodrich G Force tires.

I am concerned about the effect of the wheel spacers up front only, I do have room to add 20mm to the rear and may try it.

JayRash · #8 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 · #9 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.

docsaab · #10 01-08-2011, 02:14 AM

"Normally I would say more caster for high speed stability, but the 560SL requires 10° degrees caster which is off the scale of all the hand type alignment gauges I have. So I wouldn't think caster woulds be a problem, but then I don't understand why MB requires so much..."

If you take a look at this photo, you'll see why, look at the axis of the LCA, compare it to the UCA, as the arms swing up in compression, the arcs send the castor into negative. I don't understand, except they're trying to jack weight through king pin inclination to the rear wheels, only thing that makes sense to me, what say y'all?

"The rear alignment of the 560SL is not adjustable but it was also measured to be within spec..."

Maybe an independant or dealer with a Hunter aligner, with a printer, can give you better input, so you can see for yourself. Too bad you can't do a bump steer measurement, but that's for later.

"Theirs always the possibility this is all in my head. As I push the car further and further I do pay attention, but I really do believe that are things that could be done to balance it out better..."

You said you were a race car mechanic in a previous life, I'd tend to trust your gut, we former race wrenches pretty much could figure out what the car was doing before the driver could.

"I am concerned about the effect of the wheel spacers up front only, I do have room to add 20mm to the rear and may try it..."

I'd for sure try a spacer, before I went nuts somewhere else, balance balance balance. Give her a go, see what happens, ya never know, huh?

Roncallo · #11 01-08-2011, 10:42 PM

Quote:

Originally Posted by docsaab

"Normally I would say more caster for high speed stability, but the 560SL requires 10° degrees caster which is off the scale of all the hand type alignment gauges I have. So I wouldn't think caster woulds be a problem, but then I don't understand why MB requires so much..."

If you take a look at this photo, you'll see why, look at the axis of the LCA, compare it to the UCA, as the arms swing up in compression, the arcs send the castor into negative. I don't understand, except they're trying to jack weight through king pin inclination to the rear wheels, only thing that makes sense to me, what say y'all?

"The rear alignment of the 560SL is not adjustable but it was also measured to be within spec..."

Maybe an independant or dealer with a Hunter aligner, with a printer, can give you better input, so you can see for yourself. Too bad you can't do a bump steer measurement, but that's for later.

"Theirs always the possibility this is all in my head. As I push the car further and further I do pay attention, but I really do believe that are things that could be done to balance it out better..."

You said you were a race car mechanic in a previous life, I'd tend to trust your gut, we former race wrenches pretty much could figure out what the car was doing before the driver could.

"I am concerned about the effect of the wheel spacers up front only, I do have room to add 20mm to the rear and may try it..."

I'd for sure try a spacer, before I went nuts somewhere else, balance balance balance. Give her a go, see what happens, ya never know, huh?

Spacers are #1 on my list. Cheapest and easiest knob to turn. One thing I did notice when I first put this car together is the profound effect alignment has on this car. So hopefully I will get some slip plates this summer and make up some better fixtures to do a good alignment at my house. Then I can play with it. But I believe I will have to end up setting to something slightly different that stock alignment to compensate for the increased engine weight.

Another area of concern that I want to try is custom front springs. By installing the longer 450SLC springs I was able to recover the ride height to withing 1/4" but the SCL and 560SL both have the same spring rates. I would have preferred to compensate for the increased engine weight with stiffer springs rather than longer SLC springs. I have no intention of lowering the car.

JayRash · #12 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.

GGR · #13 01-09-2011, 08:18 AM

Quote:

Originally Posted by docsaab

If you take a look at this photo, you'll see why, look at the axis of the LCA, compare it to the UCA, as the arms swing up in compression, the arcs send the castor into negative. I don't understand, except they're trying to jack weight through king pin inclination to the rear wheels, only thing that makes sense to me, what say y'all?

Isn't it camber that would be affected by different arcs rather than caster?

John, you should describe better what your car is doing to help us diagnose. Is it pumping up and down at speed? Or does it have difficulties tracking staight on the highway? ie do you feel insecure passing a truck when you have a wall on the other side while being in a curve, because you feel you can't really control precisely the trajectory of the car?

I'm no specialist, but this reading helped me understand better how toe, camber and caster work, interelate and act on the car's handling: http://www.ozebiz.com.au/racetech/theory/align.html

I second what Jay said: first put all new bushings in the back as a loose rear end greatly affects handling. In my case worn bushings were having me very nervous in curves as I had the feeling the car would go oversteering without being able to control it. I doubt this would have happened in the reality but it was the feeling I was experiencing. If your car has a tendency to go left and right under heavy acceleration this may also be due to worn rear bushings as toe of the rear wheels changes and sends your back end left and right.

Tracking problems is more related to the front end, and as docsaab said, can be corrected by more caster and by making sure your toe is in, not out.

I've also learned that spring rating and shock stifness are closely interelated. Putting too stiff shocks will put that relation out of whack and end up with bad results. So, if you go for higher rated springs, make sure you mate them with the correct shocks, that may end up being softer. This may seem strange, but I have some examples of this if you need them.

Finally you say you added 200 lbs with the V12. You also told me this was 150lbs heavier engine and 50lb heavier transmission. I wouldn't worry too much about the transmission as it is quite in the center of the car. Your engine is 150 lbs heavier than the alloy V8, but only 60 lbs heavier than the iron V8 found in the 450SL/Cs. If you move your 40lbs battery in the trunck (if not done already) you just have 20lbs more in the front, that may be partly taken care of by your alloy radiator, power steering pump and A/C compressor bracket. And the weight of the battery in the trunk should really put the weight distribution of the car back to where it was before the swap, if not improve it. All that to say your 450SL/C front springs may be up to the job. I would look into chassis adjustment first before replacing them for stiffer ones.

JayRash · #14 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 · #15 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.