Metric rear trailing arm mts??

[racer21]

Wondering what kinda angle people try to acheve on the rear uppers,i have to put new mts on top and was wondering if I should put more angle in the bars or less? Anyone know?
Pros and cons of each pleaseeeeeeee

[Dirt_Buster]

We had the left upper running up hill to chassis. Makes it like a 4 bar mod and limit with shock or chain. RR we've tried every where we liked it fairly flat/level.

[turtle1hp]

We had the left upper running up hill to chassis. Makes it like a 4 bar mod and limit with shock or chain. RR we've tried every where we liked it fairly flat/level.

When you run these upper angles, what are your lower angles set at with these angle? The reason I ask, We are limited to stock mounting points on the both the frame and rearend housing for the lowers but the rules are "kinda vague" for the uppers. I was thinking about cutting the top off of the housing off, welding a plate as low as I could without interfering with the third member, to get the mounts as low as I could on the housing.

[Dirt_Buster]

You have to raise the entire rear of car up. Taller springs.

[turtle1hp]

You have to raise the entire rear of car up. Taller springs.

OK. I have done that already to get more angle on the lowers. The highest point, the hump, of the rear frame rail is higher than the top of the tire sitting at ride height. I was just wondering if you were restricted to stock mounting points as well??

[Dirt_Buster]

If running a 9" we made our own top mounts. We also changed coil bucket locations. But our track had. 2.5" run from rear end after our car went out and left everyone every race. So now we jacked the car into the sky and even faster.

[turtle1hp]

Hmmmm.... so I am on the right track! Everyone around this area seems to think that raising the mount on the rearend will simulate a pull bar. I can see that theory working on a drag car but not sure how consistent that would be with all the suspension movement on a dirt car.

[Dirt_Buster]

We did that before but I personally liked the LR driving under the car like the mod and late model and using angle,roll steer, and a lot of dynamic weight transfer to load the right side. Limit how much the LF come off the ground with the LR chain or have multiple shock mounting points if limiting that way.

[stock car driver]

Lowering the top mounts lowers the roll center and will get the rear rolling side to side more similar to the front.

[turtle1hp]

Lowering the top mounts lowers the roll center and will get the rear rolling side to side more similar to the front.

Are you talking about lowering the mounts on top of the rearend or the ones on the chassis, or both? One of the reasons I am asking, is that I have seen the drawings for how to locate your rear roll center and it only shows it with everything sitting level. What happens if/when you change the angle of both of the left side arms, or when the body rolls? Where does it locate to then??? I haven't figured that one out yet....

[stock car driver]

Are you talking about lowering the mounts on top of the rearend or the ones on the chassis, or both? One of the reasons I am asking, is that I have seen the drawings for how to locate your rear roll center and it only shows it with everything sitting level. What happens if/when you change the angle of both of the left side arms, or when the body rolls? Where does it locate to then??? I haven't figured that one out yet....

If you know how to locate the roll center then you should already know moving the mounting holes on the frame down would raise it, not lower it.

[turtle1hp]

If you know how to locate the roll center then you should already know moving the mounting holes on the frame down would raise it, not lower it.

I see your point... I found a roll center calculator on The Dirt Forum Tool Box. It should answer the other question I had about where the roll center moves as the body rolls and the links move.

[stock car driver]

where it moves to doesn't matter does it? are you going to stop your car from rolling to have your roll center not move?

Id suggest making yourself a jig if your going to be messing with rear end mounts. moving the holes on the rr lower for example 1/8 of a inch forward or back will make it so you cant get either of the upper bolts in... in other words lots of bind. some people don't mind that bind and figure that's what the bushings are for I guess, Ive seen guys need jacks and porta powers to get their rearend back in or remove a trailing arm for tech...

[7uptruckracer]

The way the arms are angled SCD is 100% correct, Moving the mounts closer to the rear end lowers roll center, this isn't a DLM so don't try to apply it to a metric 4 link.

[turtle1hp]

The way the arms are angled SCD is 100% correct, Moving the mounts closer to the rear end lowers roll center, this isn't a DLM so don't try to apply it to a metric 4 link.

Ok. So I have been trying to figure out exactly what mean by it's not a Dirt Late Model? Are you saying that I should not be trying to get mechanical grip from the bar angles, and that I should only focus on the lowering of the roll center as a tuning tool?

[oilman]

This could turn into a long thread...I will say that I know of several winning metric cars that have very tall upper mounts on the rear end. But this is with fairly soft tires, nothing like G-60's...I would think a lower roll center would be more important with IMCA tires.

[7uptruckracer]

Your biggest hinderance is the high rear roll center on a metric 4 link so getting that down will be your biggest gain you can't really "load the car" on the bars you don't have indexing birdcages. I keep the metric rear pretty simple. Work within your rules to lower the roll center, get the rear end free of binds. Know how your car rear steers but lower the roll center will have you moving your bars closer to the rear end. Get your front end in the track the metric has some bad camber curves and bump steer issues so if your rules allow it address those to. Side bite in the rear and camber curves and bump steer in the front

[stock car driver]

This could turn into a long thread...I will say that I know of several winning metric cars that have very tall upper mounts on the rear end. But this is with fairly soft tires, nothing like G-60's...I would think a lower roll center would be more important with IMCA tires.

When I took my IMCA stock car and put the D hoosiers on it that sucker was a 3 wheeling sob, I got up on my rs tires twice in one heat race on entry to a corner at a big special that the local ump guys thought was a dry track. My car 3 wheeled until I entered the next corner, by the end of the two day event I had 225# bolted by my lf valve cover and was still 3 wheel 2 car lengths out of the corner.

[fast_crew]

Not to hijack, but does anyone know what angle the lower mounts are at in relation to the axle tube. Thanks

[Racer96m]

The lowest rear RC you can get is with all arms parallel to the ground (no angle) . Putting the stock "W" style suspension into a bind is your biggest enemy. Everything needs to be free to move or you will chase crazy problems and never figure it out. When we ran the SS my top mounts were tall, I had 17 deg or angle in the uppers. That of course raised the rear RC, so you need to design for that, like all the weight moved up high, fuel cell up high etc, etc. High rear RC will cost you side bite also so you have to compensate for that with front end width, wheel offsets etc. etc. You can set up a car to win 1000 ways, you just need to understand whats going on and adjust everything else to work with that not fight it.

Dave