Tiago,
Have a look at the photo on this post, from another thread asking for frame advice. There are many things on that car that make me feel rather ill, but let's start with these three.
1. Look at how close the steering-wheel is to the driver's stomach. Imagine a slightly fatter driver! These cars should be COMFORTABLE to drive. The driver should be able to QUICKLY rotate the wheel 90 degrees left, then 180 right, then 180 left, ... again and again, almost non-stop for the ~15 minutes of the Enduro.
This is why building an ergonomic mock-up is so important. Many newbie Teams (and also many not-so-new Teams) make the mistake of ignoring ergonomics ("Hey, it's all about power, and turbos...!!!"). Bad ergonomics is NOT something you can see in CAD. You must be able to sit in the car, push the foot pedals, turn the steering-wheel, change gears, and really simulate yourself driving fast. (A racecar video-game in front of you can help here.) More on mock-up frames below.
~o0o~
2. The overall "mass-distribution" numbers of the car are very important. These are perhaps even more important than power, or frame torsional-stiffness, etc. The main numbers you want to get right are;
2.1 Total mass. For a lot of reasons LOWER is better (obvious, especially when you have a smallish engine).
2.2 CG-height. Again LOWER is better (obvious).
2.3 Yaw-Moment-of-Inertia. Again LOWER is better (this one is very often ignored).
The car in the photo has at least a foot (0.3 m) of fresh-air between the driver's feet and the Front-Bulkhead! That is an extra 9 x ~0.3 m tubes (plus floor-sheeting and bodywork) adding to total mass.
Furthermore, the Rules mandate quite heavy tubes for the FB, and the area inside these tubes must be filled with a heavy plate. Then the IA must be mounted in front of the FB. All this adds greatly to Yaw-MoI!
There is no Rule saying that the Brake-Master-Cylinders must be in front of the pedals. Think about this, and then maybe find a way to put the Brake-MCs under the driver's knees. This will shorten the car by 0.3 m, reduce its total mass, and greatly reduce its Yaw-MoI.
Similar thinking can be used at the back of the car. Try to limit the rear frame to the mandated Main-Roll-Hoop-Bracing, and just enough to hold the engine and diff in place. Rear-suspension can attach to the MRH and the engine-diff supports.
~o0o~
3. Look at the position of the front Spring-Dampers in the photo. Read 2.2 above, and ask if these SDs (and their pushrods&rockers!) could be in a WORSE position!!!??? Also consider the SD's effect on the driver's sight-lines (ie. driver should see the cones).
My suggestion is low-mounted Direct-Acting-SDs, with these mounted to the frame at a pre-existing node (eg. the top wishbone's rear frame mount, possibly on the FRH). Result is better visibility, less total mass, lower CG, lower Yaw-MoI, less cost, less friction, etc...
~~~~~o0o~~~~~
ERGO MOCK-UP FRAME - There have been many suggestions for how to do these posted over the years. Eg. plywood, PVC piping, etc. Here is another "quick-and-easy" way.
Buy several lengths of 12 mm (or 1/2") diameter mild-steel bar. These are dirt cheap, typically used in reinforced concrete, etc. Buy a small "stick welder", again dirt cheap. Do multiple quick sketches of "frame concepts". Pick the one that looks best. Cut up the MS bar with a hacksaw, or BIG bolt-cutters, or angle-grinder (easiest but noisiest), and stick-weld together to make the frame. Bends are easy to do, NO fish-mouthing is needed, and big gaps are easily filled.
Bolt in the engine, and weld on whatever suspension configuration you have in mind, with the wheels at the ends. Do some quick torsional-stiffness tests on the frame. These are very easy to do, and you only need +/-10% accuracy. Fit a seat, steering-wheel, and pedals (just bits of the 12mm bar). Jump in and start "driving".
All this should only take a day. Ok, for students, maybe a week. At the very least your Team members acquire some fabrication skills.
Now the REALLY IMPORTANT BIT. After all Team members have done some "driving", you all sit around the car, perhaps with some refreshing beverages, look long and hard at the car, and think of ways to MAKE IT BETTER. This means making it more comfortable, making the mass-distribution more like above notes, perhaps adding more torsional-stiffness, but mainly making it SIMPLER to build.
Tomorrow get the angle-grinder out and re-work the frame. Then repeat above paragraph. NO CAD is needed, and you should have a really good frame after a few weeks.
Z
(PS. Here is a link to an old post regarding frames and torsional-stiffness -> 44 gallon drum chassis. The rest of that thread may also be of interest to you.)
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