Minimum chassis weights

[stever95]

Hi there! Recent alum on University of Delaware, ex- chassis team captain for Blue Hen Racing.

I've searched the forum on this topic for hours and not found a satisfying set of answers about team chassis weights.

There are lots of generalizations about how light a composite or space-frame chassis can be, but I'm really interested to see what your actual numbers are, especially including engine mounts or other similarly structural components. I'm personally most interested in 4 cyl teams, but all are welcome.

Anyway, I'll start. Feel free to criticize design flaws.

2013-14: Standard frame rules
Material: 4130
Weight: 75lb including engine mounts (not shown)
Unique: Chassis splits behind the MRH. Removable MRHB.
280

[Jay Lawrence]

Why have the removable section of your chassis? Any stiffness comparisons between this and a non-bolted version (with engine access)? I realise tub teams often do this out of necessity, but why would you introduce these compliance points on a space-frame?

From memory, the Wollongong chassis through the years have been between ~28kg and ~34kg. Each one since 2001 is for a 600/4 and is steel space-frame (typically 4130)

[Dunk Mckay]

Excluding all mounting tabs the frame I designed for my team last year (first time I'd done it) weighed in just under 30kg, excluding all mounting tabs. That's for a Yammy R6. A lot of wall thickness's were too high (no time to return material for the sake of a kilo or two) and the design was done primarily for total robustness as we'd been making folded honeycomb chassis until then (with little success) and I had no experience in chassis design, so there was tons of clearance on the driver templates and for engine removal, etc.

My suggestion to them for this years design, involved tightening the whole thing up (increasing stiffness), removing non essential members (reducing stiffness back to what it was, but increasing specific stiffness), getting the right wall thickness material, and, having now seen exactly how everything packages into the chassis, shortening the nose by almost 2 inches (this saves more than just chassis weight). All in this cut the total estimate weight down to almost 25kg. (55lb).

In my opinion ultimately sticking with standard frame rules I'm not sure there is much more weight to be had than this. As essentially the design was connecting up all the dots from suspension links to rule required points, and fits a human driver and engine. It's a layout I've seen used quite frequently (to the point that I was accused of copying), suggesting others have come to the same conclusion. An insanely tight package might save another kilo maybe two. Having a fully stressed engine to replace bracing to your suspension pick-up points could shave another bit of weight, but I'm not overly fond of that idea as it complicates things too much for a student built car (depends on the level of your students of course).

To get below 20kg with a frame I think you'd have to dramatically reconsider you suspension pick-up points. Either don't triangulate to them (not advised) or figure out a way to have less of them (lookup Lancaster link suspension).

[Jonny Rochester]

If you are using 4130 for most tubes, 27kg should be easy. 4130 is in theory twice as strong as mild steel, so you only need the bare minimum of regulation tubes of exact wall thicknesses (hoops 2.4mm, other tubes 1.6mm, and a fair few tubes can be 1.2mm or 0.049"). Then all the other tubes that are not regulated need not be 1", they can be 1/2" tube if you like. In your picture a fair few of you bracing tubes can be reduced in wall thickness or diameter, or even deleted.

Our chassis this year is heading towards 32kg, but it is mostly mild steel and for various reasons we have had to use thicker wall thicknesses than required in a few places (availability, cost, other mistakes...)

[stever95]

Why have the removable section of your chassis? Any stiffness comparisons between this and a non-bolted version (with engine access)? I realise tub teams often do this out of necessity, but why would you introduce these compliance points on a space-frame?

From memory, the Wollongong chassis through the years have been between ~28kg and ~34kg. Each one since 2001 is for a 600/4 and is steel space-frame (typically 4130)

1. The rear "half" is removable for a few reasons. It's most directly because our advisor has been pushing it for years, and we finally gave in. That's a whole mess of issues you don't want to get into. It's beneficial for a few reasons though. It made fabrication really, really easy - our welder could manipulate a light structure and get inside tight corners easily. It also makes assembly easy - bolt in the powertrain (barely have to lift the heavy and awkward 4 cyl), then bolt the rear to the front (MRHB, 2 engine mounts, and the 4 chassis bolts). There's also a small case to be made for ease of replacement - if the front or rear is totaled, you only need to remake half.

2. Unfortunately I don't have stiffness comparisons for you. But I can say that our simplified FEA model (no bolted connections) got us ~1800 ft.lb/degree, no engine. Physical testing may or may not have happened this year..

3. I would disagree with you on the point of compliance bit though. The front and rear are connected with >20 bolts in reality: 2 engine mount bolts (M10's), 4 chassis bolts (1/2"), and 16 MRHB sleeve bolts (1/4"). It's a remarkably strong connection, with little room for play.

28kg is respectable! That's what I was shooting for this year in design. Unfortunately it didn't end up that way.


...All in this cut the total estimate weight down to almost 25kg. (55lb).

...To get below 20kg with a frame I think you'd have to dramatically reconsider you suspension pick-up points. Either don't triangulate to them (not advised) or figure out a way to have less of them (lookup Lancaster link suspension).

I agree, 25 kg is about the lowest you can conventionally go. I don't know how the heck you would get any lower ... there are too many unavoidables. I'd love to learn more about the LL suspension. I recall seeing it on the cover of a magazine.


In your picture a fair few of you bracing tubes can be reduced in wall thickness or diameter, or even deleted.

Totally agree! I wish I had thought of decreasing both OD and thickness back in September. Worth nothing that a couple of tubes in CAD here didn't make it to the chassis after second, third, or fourth thought.

I'd love to hear from some alternative frame teams!!! I always wanted to engineer an AF.

[Luniz]

If you are using 4130 for most tubes, 27kg should be easy. 4130 is in theory twice as strong as mild steel...

While this statement is correct, the stiffness of 4130 (25CrMo4 for Europeans) is the same as mild steel. A chassis is usually designed with stiffness being the driving factor of development, and not the strength. So 4130 might not be as big of an advantage as you think, while having lots of disadvantages like poor weldability, need for heat treatment and just raw material cost.

[Menisk]

If you are using 4130 for most tubes, 27kg should be easy. 4130 is in theory twice as strong as mild steel, so you only need the bare minimum of regulation tubes of exact wall thicknesses (hoops 2.4mm, other tubes 1.6mm, and a fair few tubes can be 1.2mm or 0.049"). Then all the other tubes that are not regulated need not be 1", they can be 1/2" tube if you like. In your picture a fair few of you bracing tubes can be reduced in wall thickness or diameter, or even deleted.

Our chassis this year is heading towards 32kg, but it is mostly mild steel and for various reasons we have had to use thicker wall thicknesses than required in a few places (availability, cost, other mistakes...)

The only reason you'll find a heap of people using 4130 (at least in Australia) is availability of material for the roll hoops. We could only find 2.6 wall in mild. I'm the electrical guy so I don't know the specifics of it but our tech head didn't like welding mild to 4130. Something about Mardensite. Said it would probably last just fine for the period the car will be used, but it's better to weld moly to moly. Also 4130 and mild have the same stiffness. The challenge in chassis building (for a typical FSAE car) is getting your specific stiffness (stiffness per weight) nice and high. Each team will likely have a different stiffness target but the aim is to build a chassis that's stiff enough that you can tune the roll stiffnesses at either end to get the lateral load distribution where you want it. The chassis is a big torsional spring between the two ends so if it's not an order of a magnitude higher than the ends you won't be able to tune the lateral load distribution of your car.