First Year Time: Chassis Design

[OspreysGoSWOOP]

We're a first year team, planning to compete in 2012. We feel that our chassis is entering the final tweaking stages so I'd like to post it up for critique. We use the following steps to build our design.

1. Research vecicles dynamics and suspension (Miliken, Gilespie, etc.) We read a ton of books.
2. Define wheelbase/trackwdith
3. Define chassis constraints.
4. Define front/rear RCH
5. Optimize using OptimumK suspension software.
6. Rocker/Damper Geometry.
7. Make compromises between performance and packaging.

Specs:
52in TW
64in WB
Tires: 20x7.5 Hoosier
0.7 deg camber gain per 1 deg roll
Front RCH: 1.25
Rear RCH: 1.5
Ground clearance: 1.75in.
Steering Ratio: 3:1
100% Ackerman

Overall I'm happy with the rear. I think the front is a bit bloated but the suspension yields excellent simulations so performance > looks.
NOTE: Damper & rocker geometry is not in the pictures.

Cheers!

http://i.imgur.com/ks32R.jpg
http://i.imgur.com/1BH2P.jpg
http://i.imgur.com/NPfFT.jpg
http://i.imgur.com/rNBUe.jpg

[OspreysGoSWOOP]

We're a first year team, planning to compete in 2012. We feel that our chassis is entering the final tweaking stages so I'd like to post it up for critique. We use the following steps to build our design.

1. Research vecicles dynamics and suspension (Miliken, Gilespie, etc.) We read a ton of books.
2. Define wheelbase/trackwdith
3. Define chassis constraints.
4. Define front/rear RCH
5. Optimize using OptimumK suspension software.
6. Rocker/Damper Geometry.
7. Make compromises between performance and packaging.

Specs:
52in TW
64in WB
Tires: 20x7.5 Hoosier
0.7 deg camber gain per 1 deg roll
Front RCH: 1.25
Rear RCH: 1.5
Ground clearance: 1.75in.
Steering Ratio: 3:1
100% Ackerman

Overall I'm happy with the rear. I think the front is a bit bloated but the suspension yields excellent simulations so performance > looks.
NOTE: Damper & rocker geometry is not in the pictures.

Cheers!

http://i.imgur.com/ks32R.jpg
http://i.imgur.com/1BH2P.jpg
http://i.imgur.com/NPfFT.jpg
http://i.imgur.com/rNBUe.jpg

[Hub]

Your rear roll hoop support is not ok with the rules.
h t t p : / /www.fsaeonline.com/Downloads/2010_FSAE_Rules_Clarifications_and_Examples.pdf

[blister]

- Make the toe base bigger on the rear axle.
- Does the engine fit behind the driver?
- Is the seat back angle tested with a mock-up?
- Why do you think the roll centre needs to be defined so early?

Cheers

[Dsenechal]

Looks like a good start.

Maybe model in a percy instead

[Big Bird]

Nice work, full credit to you for being brave enough to post your work up here for all to criticize.

Some quick points:
* The driver can't move. It seems you have modelled a static driver, and then fitted a frame as tightly as you can around him. In the heat of battle, there are going to be arms and elbows flaying about everywhere. Driver comfort and movement seems to be highly underrated in FSAE, (and I can speak first hand on this front!) Make the cockpit bigger, don't sweat the minor weight difference.
* Not much room for a pedal tray?
* Rockers and dampers are significant load inputs into a chassis. I'd be hesitant to say the chassis is right to go until I was sure rockers and dampers were mounted at node points, and not putting any frame tubes into bending.
* Triangulation!! I get scared when I see squares - think about your load inputs and how they might "lozenge" your square bays.

Your chassis is just a big bracket. Make sure you define where all the major components will go, maybe try to line up attachment points where you can to reduce your number of node points - then join the dots.

Good luck!

[DMuusers]

Do all the pick-up points fit inside the rims (especially in the rear)? And like blister said, increase the toe base on your rear upright. It looks like you've got anti dive geometry. Be sure you've modelled it right, and reevaluate if you actually need it. I'm not saying you shouldn't have it, I'm saying you will need to know why you have it.

Looking at your CoG. It seems it's going to be pretty far to the back. That means you'll have to compensate pretty bad in your spring stiffness or your anti roll bars.

Also, think about producability and packaging. Will the powertrain and drivetrain fit in there? Look at possible attachment points for the spring damper system and anti-roll bar. Rack mounting points, pedal box mounting points, impact structure, etc. etc.

Remember, don't be afraid to make changes to your chassis/suspension if something doesn't fit right!

Lastly, make sure everything is rule compliant.

[Chris Craig]

Good work guys, you look to have a pretty good start there.

A few things ive noticed

Rear chassis may need a bit more triangulation and with a bit of creativity you could increase the torsional stiffness of the cockpit section ,while improving your load paths

Also you track rod position and steering geometry looks dubious, i would think you will probably get a large amount of bump steer from that, what are the simulations showing for toe gain in bump?

[Pete Marsh]

Once again, nice post.

Watch the front roll hoop minimum bend radius, it looks too tight on the sides.
Don't forget Percy has to fit properly with the head rest padding in place.
The leg bay template looks to be a big problem with that rack placement, and the cockpit template will hit any gear shift that might be put there.

You might be a little ambitious in your component packaging. Many of the solutions you may have seen at events are the result of several iterations of a concept, and develop over many years. Don't commit to a package envelope unless you are sure you can do it. As a first year team this will mean a bigger and heavier car, but that shouldn't matter as you should simply be trying to score dynamic points at first(ie - start and finish). Even some top teams can't do that! And then the tightest packaging and lowest CoM doesn't count for much.

Is your steering ratio wheel to wheel? ie 90 deg at the steering wheel gives 30 deg at the tyre? If so that's way too fast. Steering weight and speed are hard to get right and make a huge difference to the car. This is a good thing to put effort into for a first car, and will make way more difference than power output or 25KG extra weight.

Pete

[exFSAE]

Not a bad start. Focus on getting mechanical design done, getting the car built, and getting test and tune time in. The designed product, and the final product, are generally two different things http://fsae.com/groupee_common/emoticons/icon_smile.gif

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">5. Optimize using OptimumK suspension software. </div></BLOCKQUOTE>

A little vague, don't you think? In your optimization, what's your objective? What criteria are you using for "optimal" design?

Why did you define wheelbase and track width so early?

Why the CG roll axis height of (roughly) 1.375"? What percentage of the total lateral load transfer is elastic, versus inelastic, and why? (Since that's really what you're tuning with RCH, IMO). Why the forward inclined roll axis?

Why that camber gain rate? Is that constant? Since the upper and lower a-arms look to be different lengths, I would assume not. How much does your VSAL change through roll travel? Ride travel?

What's the total estimated roll stiffness? That should be determined at the same time as the roll camber rates and static alignment, yes? How much of the roll stiffness is taken up by the front axle? How does that compare to your CG placement?

Why the 20x7.5 Hoosier tires (which btw, are 9.5" wide tread in reality)? Do you plan on testing a couple different tires back to back? If you find a faster bolt-on tire, what's the implication for your kinematics and spring rates? How much air inflation do you plan on running?

How did you determine how much Ackermann you need?

What's the minimum ground clearance by rule, and how does that compare to how much you have?

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I think the front is a bit bloated but the suspension yields excellent simulations so performance &gt; looks. </div></BLOCKQUOTE>

What's excellent about the simulations?

Just throwing some things out there that could potentially come up in the design tent - or at least that I'm curious about. Don't need to make a long drawn out thread now.

Also,
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Your chassis is just a big bracket. </div></BLOCKQUOTE>
I like this comment.

[MalcolmG]

I agree with Pete and Big Bird regarding steering effort and driver ergo. Assuming your first car is likely to be in the 250-300kg range, your drivers are going to be very unhappy in that position with that steering ratio.

Also, with the driver that far reclined you're going to have the engine and driver quite separated, which will contribute to a high yaw inertia. I'd think you'd be better off looking at a seatback angle around 40-60 degrees from horizontal, taking the hit on your CG height but benefitting from better driver ergo and a reduction in yaw inertia. You have a relatively wide track width so you should be fine with regard to rollover/passing tilt. If you're concerned about CG height then consider lowering your ride height closer to 30-35mm (although I don't know how much clearance is needed for whatever comp you're going to).

I'd suggest you need more details/load inputs into the chassis before you can get some really good feedback on your load paths, mounting of your pedal box, bellcranks/shocks, seatbelts, engine etc will need to be considered before you finalise the chassis.

[MalcolmG]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by exFSAE:
Why the 20x7.5 Hoosier tires (which btw, are 9.5" wide tread in reality)? </div></BLOCKQUOTE>
We used to run these on the back a few years ago and they're more like 8" tread width. I'd suggest you may struggle to get much temperature in them at the front. If your budget will stretch it may be worth testing narrower tyres too

[flavorPacket]

lots of great input here so I won't repeat. The major take away I have is that your chassis design does not place an appropriate emphasis on controlling the wheel toe angles. If you have access to tire data, I suggest you re-examine it to better understand what effect changes in toe will have on your car's performance. hint: It's much more important than roll center height.

[RobbyObby]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by OspreysGoSWOOP:

5. Optimize using OptimumK suspension software
</div></BLOCKQUOTE>

A word of advice, don't even think about uttering the "O" word at competition. In fact, delete it from your vocabulary right now! http://fsae.com/groupee_common/emoticons/icon_wink.gif The judges will tear you apart. Just sayin.

Ill try not to repeat what others have already mentioned but from a quick glance:
1. The front bulkhead support structure does not meet the rulebook.
2. Internal template will be an issue.
3. Although upper echelon race cars position the driver practically laying down, in a FSAE/FS car, you will gain much more by shortening the cockpit/frontal structure and situating the driver more upright. Better visibility, easier egress, etc.
4. I am going to assume you are running a 600cc engine given your other design choices, and packaging the engine/final drive assembly may get a bit hairy.
5. Also, with regards to chassis stiffness, the cockpit is always going to be the weakest section of the frame, due to the massive opening needed for the driver. Trust me, run a torsion test in CosmosWorks, add upper cockpit braces and run the test again. You will see a world of difference.

You guys look like you are well underway though.



P.S.- Lol Todd!

[PatClarke]

Which 'O' word is that RobbyObby ?? :-)

I would never take umbrage at a mention of Claude's company.

Oh, you mean the 'Optimize' word =] =]

Osprey, if we knew what the 'Optimum' was, all cars would be 'Optimized' and all would be the same!

Good effort though and I agree with most posters here.

Pat

[RollingCamel]

Todd's finger got suppressed!

As you advance you will need to add more components and check for space. The foot are quite close to the bulkhead so I wonder if there is space for MCs.

Every single bit of your components must be added and leave no chance to "Shit! It doesn't fit!". I see that you have taken brackets into consideration which is good. Did you create your template 'volume'?

It may seem that there is enough space in Solidworks, but in real world it is much smaller.

For the steering rack length did you get from a ready made one or you are going to order a custom one?

Todd's bottom can't be that low because of the seat thickness and mounting.

I won't push too much on the vehicle dynamics but based on what you chose the Ackerman percentage and other settings?

[Adambomb]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by RollingCamel:
Todd's finger got suppressed!

As you advance you will need to add more components and check for space. The foot are quite close to the bulkhead so I wonder if there is space for MCs.

Every single bit of your components must be added and leave no chance to "Shit! It doesn't fit!". I see that you have taken brackets into consideration which is good. Did you create your template 'volume'?

It may seem that there is enough space in Solidworks, but in real world it is much smaller.

For the steering rack length did you get from a ready made one or you are going to order a custom one?

Todd's bottom can't be that low because of the seat thickness and mounting.

I won't push too much on the vehicle dynamics but based on what you chose the Ackerman percentage and other settings? </div></BLOCKQUOTE>

To reiterate what everyone else has mentioned about driver packaging, the "Todd" model was originally developed at ISU, but I'll just say that we haven't used that model in years. and BTW, he used to have much worse things modeled than flipping the bird. Oh yeah, the googly eyes were the first thing I did in CAD for the team. It took me like 4 hours, LOL. Could probably do it in like 30 seconds now.

[RollingCamel]

Hehe, I posted some 3D images in a magazine with his finger waving. Checking against our bodies Todd was quite close, although a real wooden mock-up of the chassis must be done to try it ourselves.

[OspreysGoSWOOP]

WoW! Thank you all for the positive feedback. And I'm honored that the infamous Pat Clarke responded to my lowly post. THANKS!

Regarding the main roll hoop structure. Does this new design solve the problem? It's similar to Figure 5 on the link provided.
http://i.imgur.com/ojHim.jpg

The toe base will be adjusted. Thank you for the pointers.
We defined roll centers to get an idea of camber gain per degree body roll.
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Is your steering ratio wheel to wheel? ie 90 deg at the steering wheel gives 30 deg at the tyre? If so that's way too fast. </div></BLOCKQUOTE>
Why is that? This was calculated based on max hairpin turn radius of 4.5m from Dynamic Events.


Thank you to ISU for providing Todd. I took some liberties in improving his head and helmet. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif
Re: Todd vs. Percy. They are nearly identical. I have Percy in our model too.
http://i.imgur.com/Xl2QW.jpg