Do it all three link suspension for a challenge car?

Been doing a lot of reading, and watching some videos (including some of Asphalt_Gundam's suspension videos).  I am still learning, but wanted to toss out an idea to see if someone knows if it is utterly horrible, or if it deserves a bit more review.

I'm in the early stages of planning out the rear suspension in my challenge project, and there is definitely conflict when optimizing the design between drag and autocross.  Items of note are that I will probably not have a ton of adjusting holes for everything, and I'm not in a position to weld to cast iron, so the upper link will get mounted to the right of the differential, on the axle tube.  The cheap kit I ordered has two holes per mounting point, and I am trying to keep additional expenses as low as possible.   Basically, I am trying to decide if a little extra flat sheet and some tubing will be the best way to gain a few tenths in the drags, versus spending the money elsewhere.

For autocross, a longer upper link appears to be the ticket, to help control pinion angle, and keep from transferring all the weight to the rear wheels when accelerating out of a corner, and for drag (especially when not using slicks, and with less horsepower), a shorter upper link helps generate some separation with higher anti squat, and makes the instant center move and transfer load to the rear tires more quickly.

If the above is correct, would it make sense to set up a bracket on the upper frame as far forward as practical for autocross use?  Then, making sure that bracket is very beefy to handle excess load, bolt an extension bracket to it that will allow for a much shorter upper link during the drag portion of the event?

I'd guesstimate it'll cost about $25-50 in steel to do this, so it becomes of matter of deciding if the money is best spent here or somewhere else(such as coming up with slightly better shocks/springs, etc).  Oh yeah, for reference, the car is likely to weigh around 2900-3000 lbs with a driver, and make in the 300-400 HP range.  Lateral control will be a fairly long panhard bar that may or may not be adjustable.  Tires will likely be some sort of 200TW, or A7s.  If I'm really lucky, maybe a set of used drag radials.  If I somehow score a set of drag radials for the same cost as the metal to build this, and am at the budget limit, obviously, it makes sense to just keep the tires, and abandon this idea.

The only change I make between autocross and drag is tire pressure and shock settings. The same arm position setup I like for autocross will cut a 1.8 60ft on 200tw tires.

I'm not a drag racer but talking with people who are they agree that it's hooking up and just needs some dialing in on the shocks to improve...apart from slicks being the obvious.

If stress into the mounts is a concern you could always build a 4th link for drag and remove it for autocross. I've considered this myself because of all that launch force through one upper arm. Autocross will flex around the frame area without reinforcement as it is

Asphalt_Gundam said:

The only change I make between autocross and drag is tire pressure and shock settings. The same arm position setup I like for autocross will cut a 1.8 60ft on 200tw tires.

Similar result from our Challenge 3link. AutoX FTD and a 1.6sec 60ft without touching the links. 

Sounds like it is unnecessary work, then.  The main reason I was worried about load was extending the bracket length, but it does make sense that I could go ahead and run a fourth link (it comes in the kit) just to the left side of the differential.  Even if it won't be an optimal position, it should take a bit of load off the other link on a hard launch.   The spare link could also be combined with the panhard bar hardware in the kit to try to build a watts link, instead, but I'm not sure that is worth the effort.

Edit:  One concern I do have is my budget may not allow for adjustable shocks, so I'll have to live with whatever I have.  Guess I could add a mounting tab at a different angle to change the effective rate.

Unless something about autocross tires on drag strips has changed, nothing will matter if you're stuck running A7s back there.  I'd go hunting for drag radials and not worry too much about the rear suspension tweaks between autocross and drags.

In reply to ¯\_(ツ)_/¯ :

I've acquired some used 200TW tires cheaply, so my hope for a worst case if I also run A7s is that I can include two of them and a couple wheels in the budget, but yes, ideally, I'd snag some drag radials.

I also think it's worth noting that suspension geometry theory, especially for things as specific as drag racing, gets really, REALLY far into the last tiny little percentage points and it's easy to get carried away chasing those little bits.  Sometimes the fun is in chasing those little bits, but sometimes good enough is really good enough.

Most live axle rally cars run completely parallel, level (at ride height), 4 link rear suspension.  Not "optimal" just predictable and simple and they'll still launch and accelerate quite well in most scenarios.

Asphalt_Gundam said:

The only change I make between autocross and drag is tire pressure and shock settings. The same arm position setup I like for autocross will cut a 1.8 60ft on 200tw tires.

I'm not a drag racer but talking with people who are they agree that it's hooking up and just needs some dialing in on the shocks to improve...apart from slicks being the obvious.

If stress into the mounts is a concern you could always build a 4th link for drag and remove it for autocross. I've considered this myself because of all that launch force through one upper arm. Autocross will flex around the frame area without reinforcement as it is

I think you have some experience with sprung third links, aka torque links, etc.  I'm currently running a solid upper link.  Can you provide any guidance for those that might want to go to a sprung link?

In reply to Gimp (Forum Supporter) :

I'm still just starting out on sprung links with a decoupled 3 link being a primary part of my new build.

The overview is the spring length, rate, total travel distance can all be tuned to basically act as traction control if done right. The tires get loaded but the spring takes some torque off the spike on throttle application in order to keep the tires hooked vs spinning loose.

The decoupled link comes in when the above is a downward to the front link that loads the axle/tire. On decell if it was the only link it would unload the axle. So a second link is added going upwards. This slips and does nothing on accel. But on decell it engages and by pushing uphill will load the rear axle. Also very tuneable depending on the setup.

Best of both accel and decell can then be achieved by having optimal link geometry for both.

The real world implement of this is that optimization of corner exit with the upper link will by default compromise the brake zone effectiveness of the rear axle 

I'm eager to learn with real world experience sometime this year as that's the goal for the new build being finished around late summer 

In reply to Asphalt_Gundam :

Hugely helpful!  Thank you!  A decoupled third link is in my future, but I was considering replacing my solid upper link with a sprung unit as a test.  Hoping to get some more bite off the line and on corner exit.

In the 300-400hp range, don't overlook the clutch when looking for dragstrip traction. Could very well be that suspension/tires optimized for autocross will also work on the dragstrip if the clutch's engagement rate is dialed in. Easy to do if you know how.

Grant

I may be seeing this wrong, but the goal for autocross and drag race in terms of acceleration is the same.  You want to make sure both tires have the maximum grip.  Part of that is to get the weight transfer working for you- and the compromise there is that you need to worry about dive in autocross whereas you don't in drag race- braking is really important.

And the other thing you battle is torque wedge- which is the engine torque moment on the axle.  My Alfa has this in a bad way- when I turn left, I get great grip, when I turn right, the right wheel unloads and slips.  And when we drag raced it, the same issue happened- the right tire unloaded and it spun. 

If the upper link on my Alfa was just located on the other side of the diff housing, that effect would be very much negated.  

A 4 link set up does do anything to torque wedge- since it acts on the axle equally on both sides.  Three link will deal with both the axle squat as well as torque wedge- since it is asymmetric.

For race cars, the torque wedge can partially be dealt with under cornering with the lateral link below the axle line- so the cornering force will make an axle moment that pulls the inside wheel down on every corner.  But the set up is better if you get the torque wedge dealt with- as then you can use the locations to tune the characteristics as opposed to just fixing a bad problem.

The comments from Ron Sutton in these threads are some good relevant reading, but also I'd argue that the OP method in the first one is a much easier method for changing antisquat at the track...

https://www.pro-touring.com/threads/102560-3-link-anti-squat-adjustment-upper-vs-lower-link

https://www.pro-touring.com/threads/104947-Suspension-Optimization-and-Tuning-Help/page2

In reply to Driven5 :

Ron Sutton steered me wrong for a lot of things regarding Detroit lockers.

My opinion on 3 links is drag and autocross have the same requirement from the rear suspension geometry -  maximum forward bite.

In reply to Pete. (l33t FS) :

You think forward bite is more important than roll steer for autox?

In reply to Driven5 :

Forward bite and roll steer aren't exclusive.

In reply to Driven5 :

I've found roll steer most effective on corner entry. Predictable and point-able thanks to great rotation off throttle. Too much and exit suffers. I use it as the biggest tool for getting a balance I like. Think big notches...then fine tune with shock settings.

Just for context... I'm also a gear type diff and no rear sway bar preference (keep both tires on the ground). Overall soft setup on the car, and I keep moving the front roll center up and reducing the sway bar stiffness as of late, to good effect.

Still learning as I go

In reply to Pete. (l33t FS) :

There was no implication that they are exclusive. Only that the requirement for autox is not exclusively "maximum" forward bite, and thus not actually the same as drag. 

Do you have any relevant criticism of Ron Sutton's live axle suspension geometry explanations?

In reply to Asphalt_Gundam :

Yes, the way a solid axle reacts to roll center height vs independent is interesting, and generally under-explored IMO.