Isuzu FTS 750 4x4 - Self Build

[Vansafrica]

Hi Guys,

I am new to the forum but have been following and reading on here for a number of years. I have started a build on 2000 Isuzu FTS 750 4x4 and want to finalize my subframe design and need some advise on a technical matter.



My truck has a chassis length of 4100mm (tipper chassis) and the factory length is 5100mm (non tipper). My subframe design is 5000mm in length with an 800mm overhang at 25 degrees.



I have been chatting to some people and everyone insists on extending the chassis frame to 5000mm. I was convinced of this however, I kept wondering if the chassis would be "strong enough" where the splice was made, given that I will be attaching the Subframe (rail on rail) to the rear of the chassis frame on the piece that was extended. I am now thinking of attaching the Subframe to the chassis at 4100mm and having the Subframe overhang by 900mm. I have attached an image of another build that I found with this concept.



Given the rule that the leading/trailing edge stiffness of the subframe should be gradualy reduced to minimise the effect of point loading the chassis frame. The rear part of the Subframe will be firmly mounted over a 300mm section on the chassis frame so I am assuming little to no point loading will take place on the edge of the rear chassis frame or subframe.


The weight in the section of overhang (900mm + 800mm) will be around around 500kg (Bunk Beds and rear deck with spare wheel etc.) The Subframe will be made from 100x50 Parallel Flange.



I hope I explained it well enough with my limited knowledge of the matter. If you have any input or advise I will really appreciate it.

Thanks In Advance.

VA

 

[SkiFreak]

When you talk about extending your chassis, are you wanting to do this between the wheelbase or simply by lengthening it?

 

[Vansafrica]

When you talk about extending your chassis, are you wanting to do this between the wheelbase or simply by lengthening it?

Hi Skifreak. Simply lengthening it at the end.

 

[SkiFreak]

Simply lengthening it at the end.

That's what I thought, but thought I would clarify.

My concern would be more focused on how much weight/length you have behind the rear axle, as that will affect the drivability of the truck.
It looks like you want to include a bike carrier, or something similar, on the back too, which would also exacerbate that issue.
Too much weight behind the rear axle can result in seesawing, which can be very dangerous.

Extending the chassis between the wheelbase would negate most of those issues, but comes with its own disadvantages.
A primary one would be the increase in turning circle radius.

It has been mentioned on this forum numerous times (mostly by me) that in Australia we have what is referred to as the 60% rule.
This rule specifies that the maximum allowed overhang behind the rear axle cannot be more than 60% of the wheelbase, measured from the centreline of the rear axle.
Here that has been determined to be the "safe" maximum overhang, but does not factor in any weight limitations.

I do not know what the rules are in your country, but no matter what they are, you should definitely factor in the total weight you intend to have behind the rear axle.

 

[Vansafrica]

Thanks Ski Freak. I am at 60% behind rear axle. Thanks for your input.

 

[SkiFreak]

If it were my truck, I would likely make a stronger subframe, as don't believe you would get much, if any benefit in extending the chassis.