trailing or leading arm rear suspension?

[twofoot]

I've seen both in use by successful cars. Is there any reason to choose one over the other? Is the mono shock preferable over a split pair of shocks?

Does anyone have drawings they could share for illustration purposes?

Thanks

Chris

[kwillmorth79]

In my limited experience, the biggest difference I have noted is how the car behaves under braking and acceleration - while cornering seems to be the same either way.

With the trailing arm (pivot at the front) braking torque is transferred to the middle of the car, pressing down at roughly the CG. This means that the torque from the arm has little effect on the rear of the car rising under braking, transferring additional load to the front. Under acceleration, the trailing arm lifts at the pivot, like an old styl ladder bar, pushing the tires to the track, slightly lifting on the rear, which reduces squat - although FV cars have little to worry about in this regard... Overall, a trailing arm car will pitch a little bit more front to rear between braking and acceleration than a leading arm car. The trailing arm geomtery also imposes some toe-in as the suspension moves up and down. This is interesting in that if you run over a curb with one rear wheel, it actally bump-steers slightly off of it (toes into the center line of the car, away from the curb), which can loosen the rear end as it exits the corner, although I am not aware of anyone feeling this was significant.

With the leading arm (pivot behind the trans), brake torque on the arm attempts to lift the rear of the car, but since this is so far behind the CG, it actually results in the arm trying to lift the rear wheels off the ground, which results in less total rear lift, reducing some load transfer. The feeling in the car is that the whole car feels like it is squatting down on all four tires under braking. This is also why you see cars with leading arm setups set to low to the ground, as the amount of front compression is less. Under acceleration, the leading arm presses down at the rear, which actually attempts to increase squat. If the cars had 150hp, this might actually be an issue, but at 60, not so much. A little more spring and adjustment to the rear zero-roll rocker geomtry minimizes this, plus a little additional rebound damping at the front to slow the reaction down, results in a car that between hard on the brakes to hard on the gas, is very stable in ride hight front to rear. The leading arm creates toe-out s the suspension moves up and down. This means running over a curb at the apex can steer the rear into the curb, slightly... again, the effect of bump steer using either leading or trailing arm is very small, so is not a serious factor.

Neither impart any more or less actual total traction, so both are competitive. They each have pros and cons under bumpy conditions under braking and acceleration that seem to end up in a null comparison overall.

As far as mono vs. dual shocks... the mono shock is the only way to attain rue zero-roll resistance, and is preferred today. It also gets the shocks out of the wind. There have been many other rear setups used at various times, from dual lay down shocks, to longitudinal mono shock, but the current genearl lateral shock and rocker setup, with droop limiter rod, is simple to set up, durable, and aerodynamic - so has become the most common.

[twofoot]

Thanks Kevin. That helped!

Chris

[FV80]

Chris,
In addition to Kevin's thorough discussion of the pro's and con's ...
IMHO - *my* PRIMARY reason for using a single shock rather than 2 is *COST*. With the price of good shocks, this is quite significant. If you can make a car work with just one, and it's legal, WHY would you use 2?
As for front/real 'trailing arms' - the front type is in your way when you remove an engine or try to adjust valves. With the arms in the rear, that area is clear. Simpler maintenance...
YMMV,
Steve

[rstackjd]

I don't have anything to add, I just want to subscribe to this thread and don't know how to do it aside from posting a reply.

[pillowmeto]

The leading arm system will allow you to build a less bulky frame rear of the engine since the suspension does not need to be located there. This will allow you to move more weight up front and have a lighter frame. The weight of the arms will also be farther forward.

On a lesser scale of importance, the leading arm allows you to have a lower moment of inertia about the CG of the car allowing for easier (less work required) rotation of the chassis about that point.

The rear trailing arm allows you to build a zero bump steer rear, where the front arm will not (unless your butt is very very thin or fitted high in the car).

[brian]

Since trailing arms can be made longer, or be able to simulate a longer connection by using an intersection pickup point ie Caldwell, Citation and VDF, suspension travel is less likely to generate secondary responses. Secondary responses are bump steer, roll steer and the dive, antidive actions. Leading arms or ones that are behind the axle are much more critical on frame pickup point to minimize secondary reactions. Advantages of the rearward arms is that it makes packaging, maintenance and aero easier. A properly designed rear arm has little disadvantages to the traditional trailing arm.

[problemchild]

If one wants to tune those "secondary reactions" as a performance tool, it is probably easier with the rear arms.

When was the last time a FV with forward arms won the Runoffs. Why is that .... I wonder.

[jpetillo]

Kevin, I think you hit the nail on the head that the main difference between the lead & trailing arms is in the effect under braking. But I believe the difference in feel is not from how the car pitches due to force loads on different points of the frame, it's from trailing arms being under tension under braking while leading arms are under compression. The leading arms under compression will flex, and can cause the tires to bounce as the arms wiggle.

The leading vs. trailing arms in themselves have no net effect on front to rear load transfer or pitching. Whether during braking you pull up on the back (leading arm) or down in the middle (trailing arm) of the car, the only difference in car dynamics is from the frame & arms flexing. For example, you can neither lift the rear tire or push down on it differently based on where you send the load. I'm leaving out bump steer and moment couples of where the different frame & arm weights are, etc. That said, no two suspensions feel the same, but it's those secondary issues that are the reason.

Pillowmeto, I think you got your trailing vs. leading arm terminology backwards (trailing arms are in front and leading are in the rear), but otherwise I think you nailed it!

I agree, I thought the main reason for the move to leading arms (arms in the rear) was that they allow you to build a zero bump steer rear suspension because their pickup points are closer together and "aligned" with the rotation points of the axles. You can't easily do that with the trialing arms because they want to locate their pivots where the driver sits.

Then the leading arm's larger frame is a bad thing, and the improved aero & reduced cost a good thing. I'd say that the bending of the arms under braking the worst thing

I believe this is right - please let me know if I strayed from reality!

John

[clubford00]

I asked this question to Al Varacins who builds the speedsport (second place car at the runoffs the last 2 years) and although he didnt get into specifics he said that the rear arm car is actually a weaker design than the front arms. Now i know this might start a whole new batch of posts , but thats what he said. Mike, does he read these posts? if so have him comment.

[jpetillo]

I asked this question to Al Varacins who builds the speedsport (second place car at the runoffs the last 2 years) and although he didnt get into specifics he said that the rear arm car is actually a weaker design than the front arms. Now i know this might start a whole new batch of posts , but thats what he said. Mike, does he read these posts? if so have him comment.

That was my understanding as well. I understood it to be that you can't make the rear arms strong enough (the bars themselves and the pivot joints) without adding too much unsprung weight.
John

[fvracer]

I gave Kevin's response some thought and I don't think that it is correct for the following reason: all the trailing/leading arms do under braking is transfer the rotational motion from the rear axle into the frame, centered around the rear axle centerline. The net result doesn't matter if the car is using leading or trailing arms. This is somewhat counter-intuitive, but when you thing of the force as only resisting the rotation of the wheels and how it acts upon the car it becomes clear. There are several other advantages/disadvantages between the two types but I don't think that performance under braking is one of them.

Doug

[pillowmeto]

My terminology was backwards, for some reason I got the impression from earlier in the thread that you guys were referring to them differently than standard convention, so I went with it.

[robert]

Here’s some stuff to ponder. Non of this should be accepted as fact.

The main fact to be found in this thread is that leading arms, as Greg states, continue to win the run offs, and they have won the bulk of races in FST (his car).

For equal brake torque, a shorter control arm applies a greater load at the frame. Think of using a wrench in reverse. To obtain equal torque applied to a nut, the longer the wrench, the less effort required. That means to me that the rod end on a generally shorter leading arm is more heavily stressed than the rod end of a longer trailing arm.

I don't care how one chooses to think about how brake torque is reacted, there is little doubt in my mind that a torque arm such as our leading or trailing arms generate anti-suspension forces. Brake torque becomes a vertical load on the frame, and prevents freedom of motion in the suspension when brake torque is present. We can't by rule, prevent that with the single pivot point we are permitted, but less anti-suspension would seem present as arm length increases. Trailing arm layouts permit longer arms.

Flexing arms, if it is an issue, is an issue of arm design. In my mind, it should not be a factor in choosing to use leading or trailing arms. I doubt any arms in use are marginal in their ability to handle the demands on them in compression or tension. Leading arms often carry spring (push rod) attachments, and it is not unusual to see them sag from spring loads.

As far as bump and roll steer goes, a leading arm can eliminate bump steer or roll steer, not both. At least if you believe in geometric roll centers and their related roll axis. Sounds like another good reason to reduce roll angles.

A trailing arm won't eliminate roll or bump steer, but I’m sure a long trailing arm can keep both within reason. Trailing arms possibly offer some driver protection for side impacts, and might help protect valve covers and exhaust tubes.

Stiffer cars reduce wheel travel and the associated bump or roll steer.

I have a hard time believing that the dirty air in the wake of the front wheels, beam, tie rods, brake hoses, turkey legs, etc permits much aero gain with leading arms.

So . . . despite my in stinks its hard to argue against success . . . leading arms have the success.

[Speedsport]

[quote="clubford00"]I asked this question to Al Varacins who builds the speedsport (second place car at the runoffs the last 2 years) and although he didnt get into specifics he said that the rear arm car is actually a weaker design than the front arms. Now i know this might start a whole new batch of posts , but thats what he said. Mike, does he read these posts? if so have him comment.


Yes, I do browse this board from time to time. In reference to my dad's comment, it's difficult to build enough structure behind the motor to place the pick up points for the leading arms. The chassis usually has to be narrow behind the motor, making for a long section without a good way to keep it from wanting to twist or bend up in even the slightest hits. The extra frame material behind the motor also adds a lot of extra weight.

On the flip side, the leading arm cars provide the big advantage of keeping the air flow a little cleaner to the cylinder heads.

[jb_11]

[quote="problemchild"]If one wants to tune those "secondary reactions" as a performance tool, it is probably easier with the rear arms.

When was the last time a FV with forward arms won the Runoffs. Why is that .... I wonder. [/quote]

Maybe because Vortechs have good aero, good cooling, and oh yeah, tend to have good drivers

[problemchild]

Justin,
We are talking almost 25 years. Citation in the eighties.
Mysterians, Caracals, homebuilts, .....
An Adams won .... no cooling, no aero, a driver, and rear arms

[hardingfv32-1]

"making for a long section without a good way to keep it from wanting to twist or bend up in even the slightest hits."
Assuming zero roll rear suspension, why does it matter if the frame twists?

How do we know the Adams has bad aero, the Vortech good aero and good cooling?

Brian

[FV80]

"making for a long section without a good way to keep it from wanting to twist or bend up in even the slightest hits."
Assuming zero roll rear suspension, why does it matter if the frame twists?

How do we know the Adams has bad aero, the Vortech good aero and good cooling?

Brian

Brian,
Mike was referring to the tendency of rear arm cars to get BENT badly in the rear in IMPACT situations. Front arm cars don't HAVE anything at the tail of the car structurally to worry about (much).
As for your second question ... we're looking at the record and guessing I think .
Steve

[rstackjd]

Ok - I know this is a REALLY stupid question - but since I'm new to this - can anyone tell me if this is a trailing or leading arm rear suspension?

[ external image ]

Thanks!

So much to learn - so little time

[jpetillo]

Ok - I know this is a REALLY stupid question - but since I'm new to this - can anyone tell me if this is a trailing or leading arm rear suspension?

Thanks!

So much to learn - so little time

It's a leading arm. It's not a stupid question, since a lot of people - not necessarily on this site - mix it up. I think it's bad terminology. The front's a trailing arm, for example.

[rstackjd]

Thanks! One piece of information down, about 20,000 to go!

Seriously - thank you

[FV80]

Actually, both configurations are TRAILING ARMS. The FV GCR does not mention the allowance of 'leading' arms ... therefore no cars use them
It just a matter of PERSPECTIVE..
Steve