Trunk bar diameter question

[66Valiant528]

Im going to replace my trunk bars off of the main hoop. They are presently 1.75 diameter bars with .120 wall thickness. As I am going to lengthen them to mount just above the new leaf sliders I am also installing at this time, I was hoping to use 1.62 (1 5/8) bars with the .120 wall thickness to save a little weight. The main hoop is 1.75. Can I do 1.62 trunk bars? Rulebook just specifies .118 wall diameter for mild steel. I probably missed it but figured Id ask. Presently 8.5 certified. I can do either diameter and Im sure 1.75 would be fine, but Id like to use the smaller diameter if it's OK. Anybody know?
Happy New Year to all!

 

[MOPARMAGA]

Im going to replace my trunk bars off of the main hoop. They are presently 1.75 diameter bars with .120 wall thickness. As I am going to lengthen them to mount just above the new leaf sliders I am also installing at this time, I was hoping to use 1.62 (1 5/8) bars with the .120 wall thickness to save a little weight. The main hoop is 1.75. Can I do 1.62 trunk bars? Rulebook just specifies .118 wall diameter for mild steel. I probably missed it but figured Id ask. Presently 8.5 certified. I can do either diameter and Im sure 1.75 would be fine, but Id like to use the smaller diameter if it's OK. Anybody know?
Happy New Year to all!

Be careful with .120 wall thickness tubing.
Both rocker bars I installed in my dart, did not pass the ultrasonic test.
Mill spec is what they said. It's like give or take .007 either way. Funny thing was 2×2× .063 is good enough for a rocker bar lol.
Unfortunately, I don't know the answer to the question. I assume since you're already 8.50 whatever bars you add are okay.

 

[66Valiant528]

BTW this is in my 66 Valiant .

 

[Illahe]

Im going to replace my trunk bars off of the main hoop. They are presently 1.75 diameter bars with .120 wall thickness. As I am going to lengthen them to mount just above the new leaf sliders I am also installing at this time, I was hoping to use 1.62 (1 5/8) bars with the .120 wall thickness to save a little weight. The main hoop is 1.75. Can I do 1.62 trunk bars? Rulebook just specifies .118 wall diameter for mild steel. I probably missed it but figured Id ask. Presently 8.5 certified. I can do either diameter and Im sure 1.75 would be fine, but Id like to use the smaller diameter if it's OK. Anybody know?
Happy New Year to all!

Hit up GTX John or 9sec RR they should have some clarity on the specs.

 

[mopowers]

From what I'm seeing in the latest rule book I've got, if you're using only two (2) down bars toward the rear off the main hoop, they only have to be 1-5/8" 0.083"CM or 0.118"MS. I'm pretty sure the main hoop size is irrelevant, as long as it meets the minimum spec as well - which it obviously does. So, you should be good to go as planned.

On a side note, when I built the cage in my '66 Dart, my local NHRA rep was pretty helpful with these types of questions over email. Super responsive too. There may be one in your area that you could double/triple check with and at least have something in writing should it become a issue down the road.

 

[TheRacerDave]

Mopowers treed me! According to the 2026 NHRA rulebook, 1-5/8 (1.62)" diameter x .118" wall for mild steel in a door car. FWIW my car certs to 8.50 and is a rectangle frame rail chassis and all round tubes for the roll cage, including the FC cage, are 1.62" diameter.

You can download the 2026 NHRA rulebook for free at NHRARacer.com - NHRA RULES
The relevant pages are 418 - 427.

 

[ch1ll]

How much weight are you going to save between the two as opposed to the difference in strength?

As for thickness, I don't think its just a question of strength, but also how long it takes to grind through as your skidding across the asphalt before you stop.

 

[66Valiant528]

Any weight savings is a plus. But Im still confused. The roll bar spec says down bars must be same diameter as main hoop. But full bodied car seems to indicate if bar A is 1.62 then B can be 1.62. Main hoop is 1.75 everything else is 1.62....

 

[mopowers]

The roll bar spec says down bars must be same diameter as main hoop.

Where is that it stated?

 

[TheRacerDave]

The rule about rear braces being the same as the main hoop is in section 4.10 for roll bars. NHRA distinguishes between roll bars and roll cages, so if you have a full cage section 4.11 applies.
That's my interpretation, but I'd hate to steer you wrong and a quick call to your chassis inspector or technical support guy might be a good idea. Their contact info can be found on nhraracer.com in the Tech section.

 

[mopowers]

If that's what the OP is referring to, I totally agree. That section is specific to roll bars not cages. May want to reach out to an NHRA rep to get something documented though before burning in the new bars.

 

[66Valiant528]

Yes thats the section on roll bars. That's what's confusing. I presently have 1.75 trunk bars and was hoping to use 1.62. To be safe I will use 1.75. The weight won't be significant but ounces add up.

 

[66Valiant528]

I think at one point the car which was raced down south may have had a roll bar only and when upgraded to a cage they used 1.75 down bars. And I say that because a tech inspector was kind enough to look at my set up and say that he had seen this before where someone tried to convert a roll bar into a cage.

 

[66Valiant528]

I will send rdodge@nhra.com an email.

 

[mopowers]

I think at one point the car which was raced down south may have had a roll bar only and when upgraded to a cage they used 1.75 down bars. And I say that because a tech inspector was kind enough to look at my set up and say that he had seen this before where someone tried to convert a roll bar into a cage.

I don't think it's that abnormal of a setup. Several companies sell 1-5/8" kits to convert 1-3/4" roll bars to full cages. I'm pretty sure you'll be fine with 1-5/8" down bars. Let us know what NHRA tech says.

 

[66Valiant528]

Email sent

 

[66Valiant528]

So NHRA only needs a max diameter of 1.5/8 tubing. Paid 12.60 a foot for .120 wall MS. Delivery was $215 so I took the 175 mile round trip with the Ram and got 17.4 might at 3.50 a gallon for diesel. 35 bucks!
Cut out my spare tire well yesterday. Went rail to rail. Frame rails were excellent. Georgia car. Now a question about sliders. I'll start a new post.

 

[MOPARMAGA]

So NHRA only needs a max diameter of 1.5/8 tubing. Paid 12.60 a foot for .120 wall MS. Delivery was $215 so I took the 175 mile round trip with the Ram and got 17.4 might at 3.50 a gallon for diesel. 35 bucks!
Cut out my spare tire well yesterday. Went rail to rail. Frame rails were excellent. Georgia car. Now a question about sliders. I'll start a new post.

Is that .120 guaranteed .120 thick as in sonic tested before you bought it ?
I mentioned before, the .120 I bought and welded in for rocker bars did not pass the .118
I would have it checked before you weld it in, because the tech guy will catch it.

 

[66Valiant528]

Yes the steel vendor actually said that they would measure the wall thickness. My salesman told me they get large shipments and some walls are thicker. He had his people measure the wall thickness to give me stock that had thicker wall thicknesses. Marmon/Keystone was the vendor. I told him minimum wall thickness was .118

 

[MOPARMAGA]

Yes the steel vendor actually said that they would measure the wall thickness. My salesman told me they get large shipments and some walls are thicker. He had his people measure the wall thickness to give me stock that had thicker wall thicknesses. Marmon/Keystone was the vendor.

Awesome