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Author: Subject: Midengined chassis
Livio

posted on 23/3/10 at 10:05 PM Reply With Quote
Midengined chassis

Hi All,

I have been reading the forum for a while but I wanted something substantial for first post. I am Hungarian so please forgive me for incidental language mistakes.
I have started to design a midengined car. I would like to show you the progress of the chassis. Now it is in the concept phase and I am curious about your opinion. Don't hesitate share with me your advices!





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twybrow

posted on 23/3/10 at 10:15 PM Reply With Quote
Hey Livio - welcome along. Look like you have been busy.

I don't like the look of some of those chassis tubes. You seem to have areas where you have little or no triangulation in your design (the area front half of the chassis). Plus, it seems odd to use bent tubes to triangulate your back end....

I hope this is of some use. Keep up the good work!

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hicost blade

posted on 23/3/10 at 10:17 PM Reply With Quote
Pro Engineer??
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andylancaster3000

posted on 23/3/10 at 10:26 PM Reply With Quote
Looks like a pretty interesting chassis concept

Have you looked at what components (engine, gearbox etc etc) you're going to use and your suspension geometry much yet? If not I'd say that'd be your next step. As these dictate your chassis design.

Once you have a suspension design and have determined where and how you will mount all of the key components the chassis design is really just an elaborate dot-to-dot exercise to join various components and suspension in the correct place.

With a spaceframe the ultimate aim is to have full node-to-node structure of triangles with straight tubes to as greater extent as possible. Obviously things like engines and occupants get in the way of this so they're never perfect!

Technically speaking you should also be looking at a desired chassis stiffness depending upon your suspension characteristics to design to (as light as possible ) but this might be more than you want to know for the moment!

Anyway, enough rambling. Hope this helps! Oh and welcome and good luck!

Andy



[Edited on 23/3/10 by andylancaster3000]

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Kwik

posted on 23/3/10 at 10:34 PM Reply With Quote
keeping in mind he might be using stressed skin method for some areas... which is esentially cross sectioning...

welcome by the way, and nice looking car at the moment, you obviously know you way around CAD

is that Solidworks or pro engineer? i use solidworks for my stuff, mainly because its what i can get my hands on...

also bear in mind if your making and welding the chassis together, you don want to over complicate it with too many curves, they might look nice, but are hard to make, though curves and bends in the roll hoop is normally the way to go

have you thought about ackermans? and steering/suspension geometry?

this might be usefull to you:
http://www.racingaspirations.com/?p=286

nothing else i can think of at the moment..

good luck

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hughpinder

posted on 24/3/10 at 08:44 AM Reply With Quote
I like it - similar to what I've been plannig for the last couple of years(but mines on a scrap of paper!)

I'd add the following triangulation:
A cross brace across the front rectangle and the 'floor'/'top' of that area.

I'd consider raising the 'floor' of the frint rectangle so it is the correct height to fir the lower suspension mounts directly to - for my design, that means welding the mounts to the top of a 25mm tube, which is itself welded to the top of the 25mm tube that runs across the bottom of the sheeted area.

The roll bar back braces I would continue down so they pick ip the upper suspension mount. Then lower the top rear rails to meet it there.

The intermediate horizontal rail in the centre section, I'd incline it so it goes from the point the roll bar meets the top of the chassis to the bottom rail at the front of the centre tub, and the front horizontal rail that joins to it I'd angle down at the back to also meet the lower chassis rail.

Obviously this is just what I'd prefer, and I'm not a mechanical engineer or anything like that, just areas I think would make the chassis look stronger

Nice design though
Hugh

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eddie99

posted on 24/3/10 at 09:49 AM Reply With Quote
I can't see anything? am i doing something wrong?





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tegwin

posted on 24/3/10 at 10:38 AM Reply With Quote
quote:
Originally posted by eddie99
I can't see anything? am i doing something wrong?


Yes.... open your eyes!





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iti_uk
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posted on 24/3/10 at 01:07 PM Reply With Quote
I don't like the "X" in the rear chassis - rather see a "/" there instead - less wasted mass.

There seems to be an obsolete side-rail (middle, between upper and lower rails). Either remove or triangulate to it not through it.

The sills seem very high - will this car have doors or a roof?

As mentioned, you need more triangulation in the front of the chassis.

The front roll-hopp seems very shallow and one-sided - is this deliberate?

Also as mentioned, you don't want curved triangulations (rear of chassis).

You would benefit from having a "transmission tunnel" (I know this is an MR car). This will aid bending stiffness and provide a location for gear linkages, coolant pipes, wiring looms, etc to run from front/mid to rear.

Unless you have just hidden them for the purposes of showing the spaceframe, side and floor shear-panels will help with your chassis stiffness without adding too much mass.

Another "as mentioned"; the suspension pick-up points need to be on chassis nodes. Design the suspension geometry first, then design the chassis to suit, not the other way round!

Overall, the chassis looks to be fairly ok in regards to longitudinal bending stiffness, but possibly poor in torsional stiffness with excess un-used mass.

Another thing to consider is that these kit-cars generally don't have chassis stiffnesses comparable to production cars (especially performance production cars), and don't need it due to their extremely light weight. If you do any stiffness analysis, don't be disappointed with ~2kNm/deg for torsional stiffness - anything more will require added mass in the form of more bracing, and the whole design will snowball. Keep it simple, define your wheels, suspension geometry, engine choice and driver seating position first, then "join the dots" to make the chassis.

Good luck and have fun designing it!!!

Chris

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Livio

posted on 25/3/10 at 07:59 AM Reply With Quote
Thank you guys for the useful information!

-Yes, it is made in ProE.
-Its not going to have any roof or doors.
-I am going to implement some of your advices on the chassis, thanks again.

Some additional info on the concept:
-The "/" from the front side is missing because of the limited space for the feet but I am aware of the problem and something will be made with it.
-I am not going to buy many things for the car as long as the design is not finished. The donor car is planned to be a Honda 1.6 esi or a B16 if I can get one for a reasonable price. They are quite light I think (approx 115-120kg altogether and wet).
-The connecting points of the suspension are planned to be modifiable. I want to make an other fixing point at 5-6 cm lower position to be possible to rise the whole body for street usage without ruining the suspension geometry.
Suspension geometry: the approx positions are defined but the final points require the final mass and inertia to position the appropriate roll centers.
-Not yet absolutely sure but I want to put the gas tank between the seats maybe hanging out backward a bit. This way I could place the exhaust manifold right before the engine and compensate something on the fact that the originally FWD engine is getting quite far behind together with CoG. 60:40 is my the desired mass proportion.

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posted on 25/3/10 at 08:33 AM Reply With Quote
Cool. Should be an interesting car.

As far as the "adjustable suspension geometry" goes, having an "upper" and "lower" mounting option will give you more of a compromise than just jacking the front of the car up a bit for the road. Lets say you drive to the track on the "high" setting, get into the pit area, move the wishbones down to the "lower" settings... then align the whole setup again?? Are you really going to want to do an alignment (at least) twice every track-day? Why not just design the suspension to have a little more travel (droop) and jack the car up 40mm for the trip home? Keeping the chassis narrow at the suspension pickups, allowing longer wishbones, will help you achieve this.

As far as fuel tank - how about a long and thin fuel tank down the "transmission tunnel" (spine - just remembered the proper term :p). This keeps it right in the middle of the car and packages it quite nicely. Another done-thing is to put a fuel tank in either sill with a cross-pipe connecting. However, there doesn't look to be a substantially wide sill available for this option.

Chris

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Gakes

posted on 26/3/10 at 06:23 PM Reply With Quote
I agree with iti_uk. Keep it simple. I can now see how much time it takes to setup a rose-jointed suspension-I sometimes wish I had made a simpler wishbone design with bushes instead

In the end it boils down to how finicky you are.

....and good luck with the build,its always good to see others' approach





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cheapracer
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posted on 27/3/10 at 03:46 PM Reply With Quote
quote:
Originally posted by Livio

-The connecting points of the suspension are planned to be modifiable. I want to make an other fixing point at 5-6 cm lower position to be possible to rise the whole body for street usage without ruining the suspension geometry.
.


Hmm I think I'll drop my wishbones 50mm.

Wow, why have I got 30 degrees of toe out at the front?

Wow, why have the CV Tripods just fallen out of the gearbox onto the ground?

You should just lower it 50mm by letting all the air out of the tyres, won't go very fast but at least you will be able to drive it.

Some clever people just run different wheels sizes, smaller for track and obviously larger for road use.





It's coming....

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Livio

posted on 30/3/10 at 05:33 PM Reply With Quote
Cheapreacer,

This is my oppinion:
-It should be possible to move upper the outer connection of the tie-rod, or lower the whole rack I know. But it can be worked out. I have already thought about that.

-I think if originally the car has approx 120mm bump than in my case with 60mm travel it will allow another 60mm change.

-Changing the wheel size is a good way but it does not allow as much lowering or the max speed and acceleration will change drastically

Anyway the reason of choosing this difficulty is that in our country it costs to much to road legalize a kit car so basically it is being designed for track use. But if I could manage road using somehow than I dont have to redesign the whole car just "simply" raise it up. Above all this would allow some fine tuning with the connection points by using additional holes on the bracket. Thats all about this crazy lowering story.

Anyway what diameter tube do you suggest for the chassis if I have already removed the middle rail on the side. I suppose it should be different for the diagonals.

Thanks, Peace!

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posted on 3/4/10 at 12:43 PM Reply With Quote
quote:
Originally posted by Livio

-Changing the wheel size is a good way but it does not allow as much lowering or the max speed and acceleration will change drastically




Thats just not true, I suggest you investigate further about tyre sizes.

You set up for racing and have taller gearing for street/highway cruising - I can't see an issue myself.





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iti_uk
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posted on 7/4/10 at 09:04 PM Reply With Quote
In fact, I'd go further and say that taller wheels for road use is a very good idea - you'll have a nice high "final drive" ratio for the track, and a decent low "final drive" for the road, giving possibly better mileage and a more comfortable cruise RPM. If you are worried about the subsequent loss in max speed/acceleration, then you shouldn't have a road license. Seriously, what do you expect to be doing on the way home from the track?!?

The adjustable suspension to that degree is just not realistic - if you think acceleration los from taller tyres is bad, you should experience what a bad alignment does for the handling...

Chris

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Livio

posted on 8/4/10 at 10:49 AM Reply With Quote
I have to admit that changing the wheel size can be a good way for changing ground clearance. Although the car would look funny with 10cm bigger/smaller wheels.

Adjustment method is not yet clearified. I am still thinking about it a lot. Maybe it will be canceled and only a single position will be used.

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iti_uk
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posted on 8/4/10 at 01:32 PM Reply With Quote
I agree that changine sizes may look a little odd - it would be hard to "get used to" the look of the car in either configuration.

Good luck with the build, and let us see any photos of progress.



Chris

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Livio

posted on 21/4/10 at 07:27 PM Reply With Quote
I have redesigned the chassis mainly according the advices. I like this one much better.


Large size: here


Large size: here

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