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 Post subject: Exo Mid-Engine Design
PostPosted: June 19, 2014, 12:43 am 
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Joined: June 8, 2014, 1:43 pm
Posts: 57
Location: Houston, Tx
Ive been on this forum for a while, reading plenty of posts trying to get ideas about my build.

A little info on myself:
Graduated in December with an Engineering degree(not actually using it though) and the company i am working for has SolidWorks, so I have been modeling a frame in various designs for a few weeks(the only modeling i did was in Pro-E and i had to figure most of it out myself).

I have seen how much input and help this forum has been for different builds(Proximacentauri, JonW,mjalaly,cheapracer's how to build an Exo Middy), so I thought i would start my own...

A little back story on the frame:
my uncle has a machine shop(no CNC machines) and used to build nascar frames so he has plenty of tubing that i can have. It is 1.75in diameter, 3 Wall Thicknesses available: 0.125, 0.083, &0.065 ERW tubing. He also had DOM tubing available(not sure on sizes). For the front and rear boxes, horizontal tubing and roll hoops are 0.125WT, most of the remaining tubing is 0.065. The engine compartment is oversized right now to fit any 4 or 6 cyl depending on what donor car i can find at a reasonable price at insurance auctions.

I am planning to speak with one of the designers at work tomorrow to show me how to find CoG in SolidWorks and an Engineer to help me work through a drop test analysis for impact/impulse loading.

I have attached some of the images of the frame and static analysis that i have done so far(if they don't work, i attached a URL also). The frame was initially more curved, but my uncle recommended switching to less curved tubes.

One of the things I still need to learn a lot on is suspension geometry design. Does anyone have any recommendations on info sources?

Thanks,

-Austin

Image
https://drive.google.com/file/d/0B93HqB ... sp=sharing

Image
https://drive.google.com/file/d/0B93HqB ... sp=sharing

Image
https://drive.google.com/file/d/0B93HqB ... sp=sharing

Image
https://drive.google.com/file/d/0B93HqB ... sp=sharing


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PostPosted: June 19, 2014, 9:35 am 
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Joined: January 3, 2014, 10:47 am
Posts: 75
Location: Raleigh, NC
Where do you live? I'm originally from the home of Nascar and have a similar experience as you regarding engineering school and program familiarity. I'm also working on an exo middy design, I haven't gotten much feedback on it but it's progressing.

Did you make your frame using solid bodies and creating sweeps/extrudes or did you use the weldments feature? I started with sweeps but once I found the weldments modeling has become much easier and faster.

so far your design premise looks sound but definitely incomplete.

suspension geometry is tricky because it's not about designing around any set parameters but figuring out what you want the suspension to do....and most people don't know what they want a suspension to do because they've never had a say in the matter. There are many books on the subject but the best you can do it figure out how to balance the good with the bad.

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PostPosted: June 19, 2014, 10:54 am 
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Joined: June 8, 2014, 1:43 pm
Posts: 57
Location: Houston, Tx
I live in Houston, Tx(updated my profile to include it)

The first revision of the frame was made through sweeps, but after watching numerous FSAE SolidWorks tutorials, i created tubing profiles and have been using the weldments feature for a few weeks. I am having issues running the FEA sometimes though. I completely re-did the frame last night(the pictures in my initial post) and on the analysis you can see that tubing is missing, the joints are also not meshing correctly. I had hoped to speak with a stress engineer today for help meshing it, but i wont have time until monday.

As for what i want to do with it as a basis for suspension, I want 90% road 10% track. I have read some internet resources and i have a basic understanding of Camber, Caster, KPI etc., but not nearly enough understanding to put a design together...


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PostPosted: June 19, 2014, 2:53 pm 
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Joined: January 3, 2014, 10:47 am
Posts: 75
Location: Raleigh, NC
Entec1 wrote:
As for what i want to do with it as a basis for suspension, I want 90% road 10% track. I have read some internet resources and i have a basic understanding of Camber, Caster, KPI etc., but not nearly enough understanding to put a design together...


We are very much alike in this respect (although I think realistically I'll end up closer to 100% road and track as time allows).

My initial constraints for this type of setup are
-4" bump, 2" droop (6" overall travel)
-above ground roll centers front and rear
-upward sloping roll axis inclination front to rear (higher rc rear than front). This is not a steadfast rule but most suspension seem to be set up like this

suspension goals were
-no positive camber on 3* of roll
- ~1* camber gain in 1" bump
- ~1* camber loss in 2* roll
- 5* static caster although this is less important than many other factors

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PostPosted: June 19, 2014, 10:30 pm 
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Joined: June 21, 2013, 4:11 pm
Posts: 99
Pardon my intrusion-long time lurker :lurking: and you have made my day- as I am not an engineer, and really didn't quite know the question! Anravin, I am curious, not questioning, re your reasoning behind the suspension stats you propose- I guess I'm asking where these particular front end targets are from, and would the type(irs or salisbury, be it parallel 4 link, mumford, or any other) have some effect on these choices? Cheap- if you are around, I'd love to hear anything you can add to this- anything at all!
Again, pardon me for butting in- I'll go back to :lurking: Chris


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PostPosted: June 20, 2014, 8:58 am 
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Joined: January 3, 2014, 10:47 am
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Location: Raleigh, NC
mysmallwheels wrote:
Pardon my intrusion-long time lurker :lurking: and you have made my day- as I am not an engineer, and really didn't quite know the question! Anravin, I am curious, not questioning, re your reasoning behind the suspension stats you propose- I guess I'm asking where these particular front end targets are from, and would the type(irs or salisbury, be it parallel 4 link, mumford, or any other) have some effect on these choices? Cheap- if you are around, I'd love to hear anything you can add to this- anything at all!
Again, pardon me for butting in- I'll go back to :lurking: Chris


double wishbone on all 4 corners. Running a solid axle would be a trick with a mid engine car although not impossible.

I came to those suspension numbers after a fairly nauseating amount of reading in books and tech articles and forum posts around the web....they just seem to be the best compromise of characteristics.

the goal of a suspension is to keep as much tire contact with the road as possible at all times. Body and suspension motion complicates this greatly but cannot reasonably be designed out completely in a road car for the driver's sake. At the very limits of grip is where a good suspension is truly differentiated from a bad one and this comes at the maximums of cornering, accelerating, and braking. I set up the camber so that at my maximum theoretical body roll (3*, which may be too much even) camber does not go positive and the tire is nearly vertical because this gives the maximum possible grip out of that tire during a turn. The rest can be figured out later through component selection and design.

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PostPosted: June 20, 2014, 11:25 am 
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Mid-Engined Maniac

Joined: April 23, 2006, 8:26 pm
Posts: 5585
Location: SoCal
With traditional A-arm suspension it's always going to be a compromise. Get maximum traction in the turns and it'll wear the inside edges under hard braking. During street driving, the excess camber will also result in accelerated tire wear. The only solution is having two alignment settings, one for the street and one for the track. The reality is that we aren't competing for the F1 championship, and the light weight covers so many sins that the cars do pretty well regardless.

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PostPosted: June 20, 2014, 10:29 pm 
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Joined: June 21, 2013, 4:11 pm
Posts: 99
Thanks, guys! I guess I was hoping for an easier way thru what looks rather daunting to me. At any rate I'm better informed now than "by guess and by God!"
Back to my :lurking: Chris


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PostPosted: May 10, 2015, 9:01 pm 
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Joined: June 8, 2014, 1:43 pm
Posts: 57
Location: Houston, Tx
Its been a while since I started this post, but I figured I would post what I ended up with...

~4in bump; 2in droop

Caster: (+) 3.5
Camber: (+) 1.5

In the process of building the car now.

Image

Ive got a build thread on Grassroots motor sports. Any thoughts on starting one here also to get more feedback?


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PostPosted: May 13, 2015, 7:18 am 
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Joined: January 3, 2014, 10:47 am
Posts: 75
Location: Raleigh, NC
I'd be interested in seeing a CAD file of the chassis and plenty of build pictures. Link to the grm thread?

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PostPosted: May 13, 2015, 7:45 am 
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Joined: August 12, 2012, 6:38 pm
Posts: 1807
Location: worcester county, Massachsetts
Entec1 wrote:
Any thoughts on starting one here also to get more feedback?


Duh. yes. do it. lots of pics too. If this is going to become a build log, it belongs over in the "Non-traditional build logs" forum

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