[ThePaddock]

Hi,

I'm debating a Watts link setup both for a client's car and possibly my GTV race car. I'm currently running a Panhard setup in my car and I do like it, but see the theoretical (and practical?) advantages of the Watts link.

My question is, aside from looking at pictures and just "guessing", is there a recommended geometry at ride height for the left and right bar attachments on a Watts link? This would help me visualize the amount of work required to fabricate and weld-in the brackets.

Thanks.

 

[bianchi2]

search ' watts linkage ' in the above ' search ' area...lots of imfomation there

 

[Alfar7]

Verses the Panhard set-up a watts link on a 105/115 chassis has little to offer, other than complication.
Critical is the point of the center pivot. It should be at the same level as the tops of the axle tubes to raise the roll center. Not much room for it. Look at the differential position at race ride height!
Avoid a low roll center as has been the usual "build".

Critical is the angle of the upper link ,from the side of the differential. It should have a downward angle to control squat under acceleration, on both Panhard and Watts

 

[conedriver]

Hi Richard,

I guess this is appropriate to the original question, and your knowledge is greatly appreciated. I thought, per Fred Puhn's book, that the original 105/115 chassis roll center was located where the T-arm connects near the top of the differential, and the benefit of a Panhard bar or Watts link was to lower the rear roll center and make the roll axis less angled downward to the front. My understanding is that the 105/115 chassis is basically understeering due to the angled roll axis and changing to less angle is beneficial. Your thoughts?, thanks.

.....Avoid a low roll center as has been the usual "build".

Critical is the angle of the upper link ,from the side of the differential. It should have a downward angle to control squat under acceleration, on both Panhard and Watts

 

[Alfar7]

Sorry, but you are only correct that with a trunion bar as used by the factory the roll center is at the top at the diff mount. But that is tremendously higher than the front roll center even with dropped spindles. With stock spindles and lowering with shorter springs the front roll center is well below ground.

The trunion`s purpose was axle location side to side and roll center apparently was not even considered.

The trunion should never be used as a top link when side to side movement is controlled by a Panhard or Watts link system. With the axle in full loaded ride height the free to move upper adjustable link should have the chassis mount for the front rod end about 3 inches lower than the rod end on the side of the differential.
And that mount has to be securely braced inside the chassis as it has to survive full acceleration pull and braking push as it`s what keeps the rear differential from moving and controls the diff`s pinion angle. Most are built with multiple holes so the angle of the upper link can be adjusted.
Without the downward angle the car will both squat heavily on acceleration, and weight transferred to the front is uncontrolled.
Remember under acceleration the full force ot the torque is attempting to roll the top of the axle rearward, and under braking trying to roll it forward.

 

[PSk]

If you are going to the effort of building a watts link, build it like modern race cars with the center pivot attached to the chassis and the link ends attached to the axle (see picture from Peg Adjustable Roll Centre Watts Linkage | Race Products). This permits dead simple roll center height changes by just moving that center pivot ... think multiple vertical holes so you can track test and find the roll center height you prefer

Pete

 

[conedriver]

Thanks Richard. Good discussion. I know that the trunion bar and it's original dual function must be replaced by a fore-and-aft only link if a Panhard bar (as on my '76 Autocross Spider) or watts link is installed, and also understand the front roll center may be under ground level. But, surely lowering the rear roll center in relation to the front results in less angle of the roll axis and doesn't roll axis angle affect handling?. Regardless, while it's interesting to discuss, maybe this is splitting hairs and not pertinent to the posters original question. Or we can continue in the vein of the "Brits vs. Yanks spring rates" thread a few years back where it became obvious after a long discussion that there is no specification that suited everyone .

Sorry, but you are only correct that with a trunion bar as used by the factory the roll center is at the top at the diff mount. But that is tremendously higher than the front roll center even with dropped spindles. With stock spindles and lowering with shorter springs the front roll center is well below ground.

The trunion`s purpose was axle location side to side and roll center apparently was not even considered.

The trunion should never be used as a top link when side to side movement is controlled by a Panhard or Watts link system. With the axle in full loaded ride height the free to move upper adjustable link should have the chassis mount for the front rod end about 3 inches lower than the rod end on the side of the differential.
And that mount has to be securely braced inside the chassis as it has to survive full acceleration pull and braking push as it`s what keeps the rear differential from moving and controls the diff`s pinion angle. Most are built with multiple holes so the angle of the upper link can be adjusted.
Without the downward angle the car will both squat heavily on acceleration, and weight transferred to the front is uncontrolled.
Remember under acceleration the full force of the torque is attempting to roll the top of the axle rearward, and under braking trying to roll it forward.

 

[Alfar7]

Lets address this to Alfa production cars;
The roll axis is only a straight line between the front roll center and the rear roll center. It`s effect on handling is a function of how different those roll centers are. It`s not a real factor as to handling, but an indicator of the disparity between those roll centers so they can be optimized based on "center of gravity" at those two ends of the car. And they are very different.
If your race car had equal height roll centers torsional flex wouldn`t be stressing the chassis as much as low front, high rear. But the location of the roll centers on all alfa production cars isn`t blessed with that.
Using dropped spindles, which allows raising the lower A arms outer position, (and raising the top arm mount on the spindle to farther increase the angle of the upper arm on race cars) will raise the front roll center. That is critical as the front of the car has a high "center of gravity" and far more mass there than the car is carrying at the rear.

At the rear the suspension design as the factory accountants controlled (Trunion) causes the axle`s tire loads to be shifted severely to the the outer tire unloading the inside tire. With the roll center positioned in the center area of the axle See pic of mine on the Duetto and the Coupe) the force and load on the rear axle using a Panhard system designed so that the it`s axle`s pivot mount point, and the chassis pivot mount point is basically a straight line, and it`s upper link is angled downward appropriately, then you will have less unloading of the inside tire. This because of the axle`s upper arm`s reducing weight transfer to the front under braking and increasing axle load under acceleration.
But there`s more to having a great handling race car. With significantly stiffer front springs and sway bar, more of the front weight is transferred to the inside rear during cornering.

My observations regarding what works with Alfa race car set up:
Front: Dropped and raised spindles. 1200-1300 inlb spring rate. Stiff shock settings, Droop limited to 1.5" at the wheel
Rear: Properly designed Panhard and upper link position. Softer shock setting, 150-160 inlb spring. NO Sway bar, softer tire pressures .

 

[PSk]

I found this site useful: Roll Center Understood

Pete

 

[guillotineFI]

High Roll Center: "Use in high grip conditions to avoid traction rolling"

 

[Alfar7]

I read the article.

Unfortunately it`s rear assessment is based on cars with independent rear suspensions.
That`s a different application completely With a straight axle located with horizontal lower rear links (And they should be horizontal).with a Panhard (or watts) suspension with a proper downward angled center upper link that loads the diff under acceleration and restrains forward weight bias.

A proper straight rear axle doesn`t squat under acceleration or have serious nose dive under braking.

I have several Publications (books) on race car build and tuning. One I pulled yesterday was one by Steve Smith published in 1974. It`s theories are antique and not current thought. But even it will steer you away from the Autodelta stuff tried in the good old days. Use such stuf to impress the uninformed...
The most applicable and correct build info can be found in the 5 book series by Carrol Smith. If you want to buy the most useful regarding chassis & suspension, steering etc buy his book TUNE TO WIN.
Thinking of "Sliding Block" suspension there`s his quote for him regarding Sliding Pillar Front suspension which seems totally applicable to Sliding Block: "If you own a Morgan, theres is nothing you can doto improve your sliding pillar front suspension except to install Koni Shocks, and replace the pivot bushings constantly. If you do not own a Morgan, there is no reason that you should be aware of the existence of this system".

 

[PSk]

Unfortunately it`s rear assessment is based on cars with independent rear suspensions.

Sorry, that is annoying. I should have read it properly

Pete

 

[ThePaddock]

Great discussion. Thanks.

 

[genericwood]

I have two Alfa race cars, one with a watts link and the other with a panhard. Richard is correct. The only practical difference is complexity.

 

[ThePaddock]

Hi,

So, the gentleman in the picture convinced us to go with the Watts link after a lengthy white-board discussion (some of you may recognize him). We ordered the AH kit and I finished dry-fitting everything yesterday. I like the way this is coming out. I also like the Panhard setup RJ built for me a while ago that's in my race car, so we'll have something to compare to.

Thanks for the input.

 

[Alfar7]

What material is the chassis brackets made from? Looks like a lot of excess metal & weight.

 

[ThePaddock]

What material is the chassis brackets made from? Looks like a lot of excess metal & weight.

It's 16 gauge steel. Nicely welded and doesn't seem to weigh much.

 

[genericwood]

I’m not familiar with the Alfaholics kit. How does it mount to the diff.

 

[PSk]

Pity the centre was not mounted to the car and the ends to the axle. This is how watts linkages are done nowadays, so you can easily adjust the roll centre

Pet

 

[ThePaddock]

I’m not familiar with the Alfaholics kit. How does it mount to the diff.

The t-bar is replaced with a u-shaped (cross section) heavy-duty steel bar that has a mounting point for the adjustable rod off the top of the diff. This simplified things since we didn't have to try to mount this rod to the roll bar and/or reinforce the floor. One complaint... I haven't torqued it down yet but there's about a 1/2 inch gap on both ends and I'm concerned about what's going to "give" when I tighten it.