Measuring trail

[Hagen14t]

Hey guys i was just wondering what the best way to measure trail. Ive never really seen it done so i was just wondering how to do it and what the best option is. ??

[Dirtrunner35]

Alignment rack

[outacontrol]

I don't know much about alignment racks, but I assume they use the position of the front wheels to align the rear axle. I'm probably wrong.

One thing I've noticed is, on a modiied is you can't rely on both front wheels being in the same location. Frame turning or manipulation can make the right front set farther forward than the left front.

There's one thing I've done on a modified that doesn't have refererance marks on the rear stub. The upper bar behind the seat, running left to right that the upper tail bar comes off of is a good place to start, because it's usually square with the frame. When I do this, I take the rear wheels off and support the rearend in a manner that leaves the car at proper ride height. Start on one side and hang a plum bob from the upper tail down to the rearend. Measure from the string on the upper tail bar to the bar behind the seat. Do the same thing on the opposite side, and you'll be able to see how much or how little trail you have. You should be able to use this method to put reference marks on your upper tail bars to make it easier next time.

[Dirtrunner35]

They will tell you how much set back the front and the rear has, wheel base on both sides and rear axle offset. A garage might not charge that much just to check it, about ten min worth.

[washeduptoo]

I pretty sure you can't measure that Allen from ball jts, front stub is turned. We built some blocks out of wood for 4 corners and put the car on them. We use a big square and go from floor on one edge and the other edge on axled tube. Then a tape measure to back on square against rear end from bar behind seat that we set ride hts with. Maybe this will help.

[Hagen14t]

Thanks for the replies guys. One quick question. If i have the bottom rods in the same holes on both sides on the frame, shouldnt the difference in the rods be the trail since they are they same?

[outacontrol]

It should be, assuming both brackets are set the exact same way.

[TCmod]

If the cars built with trail then you can go by that. If the cars built with 1/2" of trail and you have your right side bars set an 1/8" longer than you'll have 5/8" trail. Find how much is built into the car first then figure it out. Call your chassis builder.

And measure from a point on the chassis that is the same distance from the front tires like a 4 bar brackets mount tubing.

[dirttrackrocker]

my car is built to use a 2" shorter Bottom bar on the LR

[zeroracing]

Stock car driver hit it on the head, the mid plate should be square to the world. If not something is probably drastically wrong. Drop plumb bobs off the mid plate and measure to the reaerend.

[outacontrol]

I know you can measure mid plate to right side of rearend, but how do you do the left side? Also, the mid plate is narrow, you want a measurement as close to the hub as possible.

[clm15]

Take a string and a bolt tie the string to the bolt.Drop the bolt over the axle tube, put it out as far as you can to the tire,and in line with the lower balljoints. (Bolt cant hit the floor) DA!
Take the grease zerts out of the balljoints.Find a new or old set of grease zerts. drill the ball and spring out with a 1/8th in drillbit. run some string through the zert. Tie a bolt to the string,screw them in by hand,measure the distance from string to string.
Over the last few years,Builders have been turning the front clips to make the rf longer. Check with the builder for the proper distance. Goodluck have fun.

[oldtrackchamp4x]

I pull the door and qtr tin off the left side and clamp a piece of angle or strap to the midplate at an angle so that it is in line with the rear end tube closest to the outside of the tube. Have someone hold the tape or just hook it to the strap or angle. Add or subtract as needed depending on the strap or angle being clamped on the front or back of the midplate. That is how i do it and it works for me. I also do the right side off the mid plate. After i get rear end where it needs to be i use a spot on the frame on both sides where the 4 bar brackets are mounted and measure to the front of the rear end tube from then on, unless the car gets a hard hit. jmo.

[clm15]

Thats fine if you have a new car,or if you know if its not bent. .But if its a used car? It could have a bent a- frame or most anything. Stock a frames look ok sometimes but can be bent. I have seen cars that teams set the 4bar rods wrong ect. The string dont lie.

[oldtrackchamp4x]

That's why i measure from the midplate as there can be a bent a arm or slight frame damage that can't be seen. Also my rf stub has been turned. Measuring from the midplate is the only way to be sure. imo.

[outacontrol]

The mid plate is straight. drop two points straight down and draw a line as long as you want on the floor accross them, then measure back from that line.

If you really couldnt figure this out on your own you might need a bit more help than forums and stangers can provide, I would try and talk to whoever built your chassis or who you bought it from.


The problem with forums and asking questions is you get a lot of stupid answers or at least answers that are not really very helpfull to week thru. lots of guys arent as smart as they think.

I know how to set the trail in my own car. I've never understood why anyone would need to use the mid plate, it's not the only thing square in a modified. The upper bar behind the seat is, and it's much closer to the rearend. It was just a question, I didn't need a smart assed remark.

[johnny v]

I know how to set the trail in my own car. I've never understood why anyone would need to use the mid plate, it's not the only thing square in a modified. The upper bar behind the seat is, and it's much closer to the rearend. It was just a question, I didn't need a smart assed remark.

you have to remember what the main advertising points of this forum is....." The most opinionated forum out on the net" or something like that..... he is right on some points.... you never know who is giving the info and if it is worth a crap..... use it as research guidelines and starting points..... you still have to figure it out on your own....

[Dirtrunner35]

I'm glad I use a alignment rack lol

[mudeater18]

My suggestion is if your chassis is built by a current builder, give them a quick call. They'll know the most accurate measurement method and the builder won't steer you wrong because making you faster makes them more successful.

[zeroracing]

The main upright would be difficult to measure off of, it is typically angled back, so distance varies with height, your decking may not be level, and your frame rails may not be level with each other. So where do you measure from on this bar?

To get a more accurate mid plate measure, drop plumb bombs from each side, then runa string line or laser level along this line, now drop your plumbs off the rear end, or even hub end, you can now measure to be accurate out to the very edge of the hub.

The best is to use manufactures square marks, but you can double check with a mid plate measurement. This can help you determine if the car is bent, if it should be square then both should be good.