What is acceptable tolerance for a frame??

I defer to the less fortunate.

 

Thanks for answering that question.

I didn't know what a tram is, so I did some Google and eBay searching and it appears these cost at least $300 and usually twice that and more.

What is the margin or error on the typical inexpensive tram?

Would you think I'm nuts if I told you I took a 24 inch Harbor Freight caliper and mounted it on a 10 foot piece of steel conduit in order to create a tool to make consistent measurements across up to 12 feet. Wheelbase on my current project is just under 12 feet so my homemade tool is a little short for triangulating the axles. I'm working on that. So far total cost is less than $20.

My tool will not give perfect correct measurements and I know that. My goal in making it has been to get consistent measurements. Triangulating the frame doesn't require getting some exact number but rather matching numbers, so I think I'm good. I also think that means I can replace my homemade pointer with something that will slide over the conduit and be at least four feet long so it can extend my tool to 14+ feet. A couple of slits along the length of the sliding tube, with some type of clamp will hopefully hold it in place well enough to provide consistency.

From the pictures I saw it looks like a tram is similar to what I've built at least in principle.

Describing my homemade measuring tool may be a little off the topic of the original post, but my hope in doing so to illustrate the difficulties in getting something dead on. Though some of the faults in my design may be eliminated in the commercial tools, they have faults and limitations that mean some amount of error.

I'll confess I still don't believe dead-on often happens in the real world. I know Boeing has had problems with tolerances on airliners and I think until you know the margin of error in your tools and have mastered the techniques to minimize their impact you aren't likely getting dead-on results either.

I wish I could claim to have that all figured out. You might have matching measurements but your measuring tool still has some margin of error. Your dead-on is really within the total margin of error of each measurement you used in coming to that conclusion.

I'm not trained as a machinist or a fabricator or even a carpenter so I'm sure others can teach me a lot about this topic. I'd like to learn more about how you manage to measure and achieve what you call dead-on.

I like it as an objective but need some help believing anyone ever achieves it. From a powertrain standpoint my current project should be capable of well over 100 mph and if I ever decide to try it I'd like my truck to run true and predictable. A dead-on frame and suspension would sure make me more confident that's what will happen.

The only cars I've ever driven over 100 mph were made in 1971 or before so I know some of the less exacting standards of the past could do what I want. I'd like to finish my project rather than work to unnecessary perfection in the frame and suspension mounting.

If you read this far in this long post I thank you.

 

Your worrying about how straight the frame needs to be then your gonna put different sized tires on it?

 

Steve,

There are some that would like you to believe that your measurements have to be exact, they don't. Years ago, I made a crude tram, out of trunk torsion springs, that we used in several shops that I worked in. It worked great, back in those days most of the shops equipment was fairly bare.

Most people would be surprised if they started "x" measuring cars that have never been wrecked. I've had to make frame corrections on new cars and trucks that didn't sit level.

There was this one new Ford pickup that the owner brought it to the shop that I worked at and asked if we could fix it, he was pissed. He had taken it to several dealerships and they changed springs and played around shimming the body and several other things. We hung some frame gauges on it and it was down at the cowl; that was the way the truck was made.

I have been looking for some other frame books that could be use as a reference for frame tolerances, but I haven't found them. The Ford books that I have didn't list tolerances in them. The Chevrolet reference that I posted is 3/16"; which would make it within 3/32" on the "x" measurement. Most of the others that I have seen were a 1/4" difference on the "x" measurement; which would be within 1/8".

The most import thing, and this might sound funny, is to get the rear wheels facing or following the front, on the same track. The frame is a rectangle and if either of the frame rails are back, commonly known as a diamond condition, you have a parallelogram.

http://www.answers.com/topic/parallelogram

If you look at the diagram in the above URL, and picture your front and rear ends attached to the end lines; you can see that they aren't 90 degrees to the sides.

If you see a car going down the road and it appears to be dog tracking (or going somewhat sideways) this is what has happened. It is nice to have it perfect, but in reality this isn't always possible or realistic economically; hence the tolerances. Every piece on the frame, every hole, bracket or anything else that you might measure from or use as a reference has a locational tolerance; which one do you believe?

 

Thanks Ford.

I was getting a little frustrated at the responses that suggest dead on is what they attain. I don't believe they are managing that level of accuracy and their answers are either tonge-in-cheek or BS.

The truth is any measuring tool is only so accurate. If your digital caliper says 1.000 what is the fourth, fifth or twenty-fifth digit after the decimal. Everyone is working with tools that have tolerances whether they admit it or not.

I've bought every book on hot rod chassis engineering I can find, but most are geared to buying parts, not explaining how to measure and construct an accurate and straight chassis. The other books I've seen have been accident repair and not unlike what you've reported.

As I write this I wonder if I'm looking in the wrong places. It may be the race car guys who are doing the kind of work I need to be reading about. Not that I actually believe I need that accurate of work on my project.

It would be great if someone building real accurate square chassis would do a tech on how they are doing so, especially if they are using tools like most of us have in our home shops.

 

There are a ton of frame builds covered on the HAMB already. The methods they're using will work. Squareness comes from a deliberate fabricator who checks as they go and adjusts accordingly. If you're gonna require a specific thread on how to do that, I'm questioning if you should be building a frame. Maybe smaller fab projects are the place to start until those lessons are learned.

My experience with the racing world is the chassis are far from perfect unless you're talking absolute top pro level series. You'd be better off listening to what the street guys here say, cause most racers will tell you to porta-power it straight after it's built crooked.

good luck with your project

 

I have read The Race Car Chassis HP1540: Design, Structures and Materials for Road, Drag and Circle Track Open and Closed-Wheel Chassis<o></o> (http://www.amazon.com/gp/product/1557885400/) and while somewhat short on actual how to methods, the design information is excellent. The best how to I have found is (http://www.amazon.com/Chassis-Engineering-HP1055-Herb-Adams/dp/1557880557) which is also worth reading for all content.

Another good source is Circle Track magazine (http://www.circletrack.com/index.html) as they do a lot on how to on the cheap fix busted old chassis after they hit the wall.

While I know this isn't traditional HAMB stuff, I am of the opinion that you should whatever you can from where ever you may find it.

 

I have and have read the Adams book. I've looked at the other and not picked it up because I judged it short on methods. Years ago I read Circle Track but not being a racer I quit picking it up. I may need to find some related boards and cruise the forums for techniques.

First I'm not building a frame, for my own project I'm swapping the front suspension from one frame to another. I'm also pretty comfortable with tolerances in the 1/16th to 1/8th range in most of the work I've done.

I'm trying to be tighter with the mounting of this IFS to the old frame. Dead-on would be nice. Just 1/32 would be great.

All these posts about dead-on or dead nuts are what have me pushing for more information. To me those terms mean a zero error tolerance. I'd like to see someone posts the details of how they achieve this dead-on or dead nuts they claim. I don't believe the measuring tools they are using can possibly be that accurate and the cutting tools available are not that accurate either. Real world accuracy is hard, but I'd be delighted to see and read how others are coming closer than I am.

However its clear I've hijacked the original posters thread along with beginning to beat a dead horse so I'm going to withdraw from this thread.

 

i am with a 1/16 diagonally and under that length wise of a inch on my frame that started off being welding on a rough dirt floor. now i have a nice cement floor and not a rough but i wouldn't trust it for being level. i bought 6 scissor jacks and used them, 2 protractors, level and 2-3 tape measures just to be on the save side. This is my first hot rod but not my first frame. i started small with a bar stool racer then i built a complete space frame buggy. and with no work table the buggy was my table. as for the chassis its been 4 years but i think this is my year to have it finished. Just take your time and think things through and if you are smiling and saying boy did i get a lot done today you might of gone to fast.

 

I assume in order to get a frame "dead balls on", the frame rails and their components (X member, cross members, etc.) would have to have been manufactured "dead balls on" to start with.

I am willing to assume that if a person took a factory left frame rail from a 32 Ford, and compared it to the right rail from the same car, that the length would not be close enough to the same to really ever be dead balls on apart from blind luck.

Having done a few cross measurements, I want to think that getting an "EXACT" measurement involves how a person holds the tape, which eye you close reading the tape, and how long we held the grinder on the end of the frame when we were cleaning it up.

Too damn many variables for "perfect" every time. Don't hate me, but I call bullshit.

Even if "perfect every time" does exist (like Sasquatch) I will never have that kind of patience, and I feel sorry for those who do.

 

I've found that it takes two people to get a good X measurement. The same person has to do the reading of the measurements and the other person has to hold the "dumb end".
This way each knows how he made the previous measurement or tape placement.
There is also a big difference if the X measurements are off 1/16th in say 100 inches
and they are off 1/16th in 200 inches.

 

I fabbed up a frame that used 1/4" wall 2" X 4" mild steel box tube (It was there). I was running a dragster coupler and no drive shaft, so, it had to be perfectly straight. I used a Mark Williams alignment tool that aligns the journals to the rear when setting up a dragster. Everything was right on the money...until I came out to the shop the next morning....and it was off an 1/8". I did this twice and realized that tempreture change can affect accuracy more than you think. Even with 2" X 4" and 1/4" wall it would move around. A cage would have bridged it and helped but the project got canned for being too heavy.

 

Shit guys, i'm already on wheels!! some of you guys are pretty late!! I got it within 1/8 on the rails and perfect on the crossmembers

 

if its within a 1/16th i call it good

 

Here's one you'll like!! I remember Frame Spec Books from the 50's calling for 1/4" of tolerance. On a Full Frame this was considered good. I.E. 57 Chevy.

I like to go for "0" to 1/8" max for all Measurements. Lenght, Width, Height and Diagonals.

The Old Tinbasher

 

It seems to me that tolerance on squareness or whatever measurement your talking about on a frame should be within the amount of adjustment you have for alignement etc. Cabs, beds and such are mounted to the frame through oversized holes for a reason. The same with front suspension - shims and other forms of adjustment.

 

I am building a frame for my T right now out of 2x3 120 wall tubing. I am out of square by 1/16 when I measure diagonol. I think I am just gonna run with it. I dont want to undo clamps to try to fix and make it worse or something.

 

1/16...1/8? When I was 18, I bought a 55 Chrysler from a body shop and drove it 100,000 miles myself with no noticable abnormal tire wear, pulling or other problems. (Did I mention that the wheel base on the rt. side was 1" shorter than the left side?)

 

I can almost guarantee you, all the buggy sprung model A's and 32's with any HP have a drooping RH frame horn from the torque....Go measure yours...

 

In building [4] model A ladder type tube frames, 1/16th was good, esp. in the X measure. Mostly get the frame level, not half a bubble off! The suspension will sometimes benefit with shimming (spring to perch) if you want it (the body) dead level in assembly.