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The Liquor Makes Me Think
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Discussion Starter #1
This may be a dumb question but I'm preparing to back half my truck. How much of the oem frame can you replace with round tube before becoming tube chassis? Seems like most classes only care if the oem frame a few inches infront of the spindle to a few inches behind the firewall.
Does that mean I can replace oem frame rail from just behind the firewall and replace with round tube and still be considered "back halfed" for street classes? How will this affect sfi cage certs?
 

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I would look to the associations that you want to run with, so there is no chance of a misunderstanding of "the rules".
 

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Back half used to be from behind front seat to rear. But if it is a unibody, frame connectors to front subframe are common.
 

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Mine starts at the rear of the front suspension mounting. Seems like as the factory front style suspension is intact there’s no issue. My truck has an s-10 front clip but uses stock style bolt on a-arms and I’ve never had an issue anywhere.
 

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The Liquor Makes Me Think
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Discussion Starter #7
I think I may start the tube at the drive shaft loop. That should still give a good amount of oem frame and weight savings.


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Inwas looking at 25.2 book. Why not build a front cage with all sorts of bars needed and leave existing rails to back and tie in that way its stock rails and all that tube work is for "safety"
 

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The Liquor Makes Me Think
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Discussion Starter #9
Inwas looking at 25.2 book. Why not build a front cage with all sorts of bars needed and leave existing rails to back and tie in that way its stock rails and all that tube work is for "safety"
For weight. Factory rails are heavy compared to the tube.
Im thinking of going 25.3 or 25.5. I cant find good info on the differences in design and all.
 

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Their not drastically different. 25.3 certs to 6.5, 25.5 certs to 7.5 at 3600 lbs. I would do the 25.3 for sure if using CM. 25.5 can use CM or MS.
 

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From what I've heard 25.5 has requirements for rear of the car. I know 25.2 is purely a box around the driver and rest is nhra approved. Unsure of 25.3
 

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I have the 25.5 and 25.3 books, have to look, but I believe the 25.3 is all round tube, and 25.5 allows some rectangular and square tube. The rear frame requirements are pretty basic,
 
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