LocostUSA.com

Learning how to build Lotus Seven replicas...together!
It is currently October 19, 2017, 6:46 am

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 415 posts ]  Go to page Previous  1 ... 13, 14, 15, 16, 17, 18, 19 ... 28  Next
Author Message
PostPosted: January 3, 2012, 3:09 pm 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7367
Location: Massachusetts
Lonnie, the answers your looking for are in this thread. Mostly right above your post. I think you are somewhat over complicating things.

In response to your question, how much is enough - I think the answer is when you can tune your car's handling. Seven13bt, says right above his car responds well to tuning. If it twisted too much, the anti-roll bars and spring rates would not change the handling when they were adjusted. The car needs to be stiff in comparison to it's springs and anti-roll bars. Andrew directly comments on this too.

You can also see from Seven13bt's post that it is not out of reach for anyone to measure their stiffness ( of the frame ). So long as you can measure accurately, you do not even need high loads.

Leftover stock from your construction and cement blocks would work too. You need a good scale, a dial gauge and tape measure for the measuring.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
Reply with quote  
PostPosted: January 3, 2012, 3:23 pm 
Offline
User avatar

Joined: January 1, 2012, 12:44 pm
Posts: 220
Location: Boise, Idaho
It has been suggested in this thread that a factor of ten in chassis stiffness over wheel rate should be adequate.
In my case, the front wheel rate is 207 lb/in and the rear is 289 lb/in. The rates for a good street setup would be much lower.


Top
 Profile  
Reply with quote  
PostPosted: January 3, 2012, 3:30 pm 
Offline
Always Moore!
User avatar

Joined: November 9, 2007, 3:40 pm
Posts: 3881
Location: Fort Worth, TX
Driven5 wrote:
I haven't really followed who is using GRAPE and who is using LISA, but is anybody using both programs yet such that the outputs of the two programs can be compared directly against each other when using the same structural designs and loading conditions?


The snow and cold made it a good day for LISA. :)

Here you go (node 35 in Grape is node 43 in LISA and node 36 in Grape is node 44 in LISA):
Grape: 5,033 ft-lbs/deg (node 35 displaced 0.126", node 36 displaced -0.126")
LISA: 4,328 ft-lbs/deg (node 43 displaced 0.147", node 44 displaced -0.146")

Attachment:
LISA test.JPG
LISA test.JPG [ 156.32 KiB | Viewed 1628 times ]


The numbers not being identical do not surprise me but the ~10% difference does. Now I really wonder how ANSYS stacks up.

_________________
-Andrew
Build Log
Youtube


Top
 Profile  
Reply with quote  
PostPosted: January 3, 2012, 3:37 pm 
Offline
Always Moore!
User avatar

Joined: November 9, 2007, 3:40 pm
Posts: 3881
Location: Fort Worth, TX
This is the easiest way I've heard it explained Lonnie:
Attachment:
stick model.JPG
stick model.JPG [ 17.51 KiB | Viewed 1628 times ]


The chassis is the blue tube and the front and rear suspensions are the red tubes. The entire sprung mass of the car is suspended above the chassis by the infinitely stiff black rod. As the car corners the black rod places a torque on the blue tube (the chassis). The front and rear suspensions resist this roll.

If the sprung mass is at the center of the chassis (50/50 weight distribution) and the front half of the chassis and the suspension have the same stiffness as the rear half of the chassis and the suspension (and the tires are the same) then the car should be neutral in a perfect world.

Since the suspension at each end and that half of the chassis are springs in series, you need the chassis to be stiff enough so it does not have a large effect on the system (where the 10x comes from). You could design a chassis with a really stiff front half but a really soft rear half and it will be just like adding a stiff front ARB and removing the rear ARB - you do not want this.

Optimum's website has a 5 part tutorial that shows how to calculate your roll stiffness from springs and bars - it is really where the required chassis stiffness starts.

_________________
-Andrew
Build Log
Youtube


Top
 Profile  
Reply with quote  
PostPosted: January 3, 2012, 3:47 pm 
Offline
Always Moore!
User avatar

Joined: November 9, 2007, 3:40 pm
Posts: 3881
Location: Fort Worth, TX
a.moore wrote:
The numbers not being identical do not surprise me but the ~10% difference does. Now I really wonder how ANSYS stacks up.


Disregard - I screwed up. I should have scratched my head for another minute or two before hitting send. Apparently those little 1/2" tubes next to the transmission actually DO something. :D

The numbers match now and LISA actually says my chassis is a little stiffer (0.125" at node 43 and -0.124" at node 44) which equates to 5,093 ft-lbs/deg. So essentially the two programs solved within a percent or so of each other.


Attachments:
LISA test.JPG
LISA test.JPG [ 168.33 KiB | Viewed 1626 times ]

_________________
-Andrew
Build Log
Youtube
Top
 Profile  
Reply with quote  
PostPosted: January 3, 2012, 11:19 pm 
Offline
User avatar

Joined: October 24, 2008, 2:13 pm
Posts: 3162
Location: Carlsbad, California, USA
horizenjob wrote:
Lonnie, the answers your looking for are in this thread. Mostly right above your post. I think you are somewhat over complicating things.
. . . <snip> . . . .

Thanks for being diplomatic, Marcus. I didn't make it clear why I referenced the dynamic case. I know the static measurements are pretty do-able.

There was an interesting documentary on the development process for the chassis of the current generation Mustang. It had a number of different elements like actual road tests, race track testing and a big simulator where hydraulic activators were connected to the hubs in place of wheels and tires. That last test was the most telling because they could simulate so many different circumstances you might encounter on real world roads of various types. That's when the whole, complete chassis had to work as a system not just be a model in a computer or a chunk of metal in a torsion test rig. The initial results were pretty ugly with a lot of vibration in certain circumstances and less that stellar responses to certain simulated roads.

However, the test environment also had excellent analytical tools and the engineers were able to work through it pretty quickly, mostly making small changes like shocks and valving, springs, motor mounts, additional welds in critical places, etc. The basic design was just fine and plenty strong in static testing, but "the Devil was in the details" and some of the chassis responses were not knowable in the static case; only when dynamic loads were applied to the real-world unit - the completed car. It just made me aware that something that works well on paper or in theory or in a computer model can be lacking in practice.

Cheers,

_________________
Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


Top
 Profile  
Reply with quote  
PostPosted: January 4, 2012, 12:11 pm 
Offline
User avatar

Joined: October 24, 2008, 2:13 pm
Posts: 3162
Location: Carlsbad, California, USA
Lonnie-S wrote:
. . . <snip> . . . .
But, what's a good enough target for a street Locost? 3,000 lb-ft/degree? 4,000? 5,000? More? It's really hard to know. I do suspect there is a number above which no practical benefit is delivered for any extra stiffness added. With a race car you would expect to need a higher number than for street applications. Anyone know a real chassis engineer we can ask?

Cheers,


I don't usually answer my own questions, but I wondered how much I could glean from the reference books I have in my own library. It turns out there was more there than I realized. Here is an answer for both torsional stiffness and bending from the book “Vehicle Body Layout and Analysis” by John Fenton, Mechanical Engineering Publications, London, UK, 1980, page 7.

“In a typical family saloon 4500 lbf ft/deg (6100 Nm/deg) should be a minimum with 5000-5500 (6500-7500 Nm/deg) preferred.”

Further on Fenton writes:

“Mid span bending deflections for a car should not exceed 0.05 in (1.27 mm) and door aperture deformation should not exceed 0.05 in (1.27 mm) for a 1500 lb (680 kg) mid span load.”

Those requirements are higher than I expected. I thought 4,500 lb-ft/deg sounded pretty good for a street sports car. Additionally, I found some specific figures for both sedans and race cars in “Design of Racing Sport Cars” by Colin Campbell, Robert Bentley, Inc., Cambridge, MA. In the 1976 edition gives the following for torsional stiffness (as tested) for these vehicles:

On page 137:
Ford GT40, Mark I – 12,500 lb-ft/deg
Volkswagon 1500 unitized body – 7,000 lb-ft/deg
Rolls Royce Silver Shadow – 10,800 lb-ft/deg

On page 142:
For the Bobsy C Sports racer (USA, pop rivet aluminum, semi-monocoque) 12,000 lb-ft/deg

It looks like both FEA camps here, street car and race car, may need to increase their goals somewhat.

_________________
Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


Top
 Profile  
Reply with quote  
PostPosted: January 4, 2012, 1:20 pm 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7367
Location: Massachusetts
Quote:
It looks like both FEA camps here, street car and race car, may need to increase their goals somewhat.


I look at the numbers you mention and it makes me think we're doing it right...

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
Reply with quote  
PostPosted: January 4, 2012, 2:33 pm 
Offline
User avatar

Joined: July 22, 2007, 10:58 pm
Posts: 388
Location: Zagreb, Croatia
Lonnie-S wrote:
It looks like both FEA camps here, street car and race car, may need to increase their goals somewhat.



The key word here is vehicle mass... The lower the vehicle weight, less torsionall stifness is nedeed... so it is a bt pointless (but informative) to compare to a RR... :)

_________________
Building a single seater middy BEC with GSXR 1000 power :)
build log: http://locostusa.com/forums/viewtopic.php?f=36&t=5899

day job: http://smg.photobucket.com/albums/v723/turbolimac/portfolio/


Top
 Profile  
Reply with quote  
PostPosted: January 4, 2012, 3:09 pm 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7367
Location: Massachusetts
Not really trying to be difficult, but try looking at the problem from different directions. I think you know enough, looking at it from different angles will make your more comfortable with using what you already know about the problem.

Context and perspective are very important. Increasing the stiffness of my frame by a substantial amount would reduce the flexing of the frame at the coilover by say 1/16". How much faster would the car lap or how much safer would it be because of this change? Will it rattle or fatigue less because of this reduced flex?

My judgement at the moment is that if you are interested in the lap times, safety, rattles or fatigue we are at the point that it would make sense to look at those problems directly and see if they lead back to torsional rigidity as opposed to just blindly improving that single factor out of context with the rest of the car.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
Reply with quote  
PostPosted: January 4, 2012, 8:48 pm 
Offline
User avatar

Joined: January 2, 2009, 1:45 pm
Posts: 1140
Location: Halifax, Nova Scotia
The race cars are a different matter but don't forget that production cars also need stiffnes so the doors won't jamb ... at least we don't have doors.

_________________
Warren
Isuzu Pickup/SR20DE, +401 COLD frame
Build Log: viewtopic.php?f=35&t=11601


Top
 Profile  
Reply with quote  
PostPosted: January 4, 2012, 9:09 pm 
Offline
User avatar

Joined: October 24, 2008, 2:13 pm
Posts: 3162
Location: Carlsbad, California, USA
horizenjob wrote:
Not really trying to be difficult, but try looking at the problem from different directions. I think you know enough, looking at it from different angles will make your more comfortable with using what you already know about the problem.

Context and perspective are very important. Increasing the stiffness of my frame by a substantial amount would reduce the flexing of the frame at the coilover by say 1/16". How much faster would the car lap or how much safer would it be because of this change? Will it rattle or fatigue less because of this reduced flex?

My judgement at the moment is that if you are interested in the lap times, safety, rattles or fatigue we are at the point that it would make sense to look at those problems directly and see if they lead back to torsional rigidity as opposed to just blindly improving that single factor out of context with the rest of the car.


I'm not meaning to sound like critic either. I thought 4500 lb-ft/deg was pretty impressive. Andrew's 5K+ figure was too and my little visualization (500 lb Sumo on a 10 foot lever) I thought drove the point home. Also, as I indicated earlier, I thought there would be a point at which additional torsional stiffness would not yield any further, practical benefit. I just didn't know where that point would be or how you'd find it either pragmatically or theoretically.

There are people out there who almost certainly do know either from many years of experience or knowledge of engineering mechanics and physics. That's why I thought Fenton's recommendations were worth putting up here. It at least gives a target to shoot for.

There is more in Fenton's book and Campbell's too about the consequences of too little torsional stiffness and possible bad suspension behavior. There are minimums you do want to meet. However, since there are lots of happy people blasting around planet Earth in Locosts having less than 4500 lb-ft/deg of stiffness, I think we'll be alright. I'm likely to be one of them in a few months. I don't expect my own chassis will be as stiff as the ones you and Andrew have designed.

Cheers,

_________________
Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


Top
 Profile  
Reply with quote  
PostPosted: January 4, 2012, 9:40 pm 
Offline
User avatar

Joined: October 24, 2008, 2:13 pm
Posts: 3162
Location: Carlsbad, California, USA
kikiturbo wrote:
The key word here is vehicle mass... The lower the vehicle weight, less torsionall stifness is nedeed... so it is a bt pointless (but informative) to compare to a RR... :)


I'm not sure I'm with you on that - at least so far. It sounds like mixing inertia and acceleration forces of the vehicle body (sprung mass) with the forces that might act on same through the unsprung mass (suspension). I believe I could think up some counter examples to it having ultralight weight, low torsional stiffness and almost certain failure.

Do you recall where you saw the statement about the relationship between mass and torsional stiffness?

To me, it was very impressive that a huge vehicle with a nearly 10 foot (119.5 in ~= 3035 mm) wheel base, the Rolls Royce, was very nearly as stiff as a race car. The GT40 Mark I main chassis is made from 22 and 24 gauge sheet steel, designed in the 1960s and has 12,500 lb-ft/deg of torsional stiffness. That is just amazing to me. I'd love to be capable of doing design work on that level myself.

Cheers,

_________________
Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


Top
 Profile  
Reply with quote  
PostPosted: January 4, 2012, 10:19 pm 
Offline
The voice of reason
User avatar

Joined: January 10, 2008, 4:47 pm
Posts: 7367
Location: Massachusetts
The torsion is caused by forces thru the springs and shock absorbers. The springs and shock absorbers are scaled to the vehicle and it's purpose. So a heavy vehicle with strong springs and shock absorbers will generate large forces and require a stiff body. So will a lighter sports car which requires strong springs and shocks for other reasons.

Andrew is giving good reasons and examples in this thread. The stock Locost chassis example he gave above bends 1.4" under the loads we are talking about here. That's more then 10 times as much as his chassis. What happens on this chassis when you try to use an anti-roll bar to tune the handling?

Say you adjust the rear bar stiffer to reduce understeer. This should increase the dynamic weight transfer during cornering to the rear wheel. When you do this though the chassis is not stiff enough to cooperate. The chassis will just bend more and that dynamic weight will not show up at the rear wheel. Changing spring rates will not help much either because the big spring in question is the chassis itself.

Then you also now have a bunch of secondary issues plaguing you. You've upped your spring rates and anti-roll bar rates. Now the really big spring doesn't have a shock absorber. Your suspension is not so independent anymore with those really stiff bars. The list goes on...

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
Reply with quote  
PostPosted: January 4, 2012, 10:25 pm 
Offline
User avatar

Joined: September 26, 2009, 8:25 pm
Posts: 886
Location: Park Hills, KY
Horizon, I'm really liking your design over the typical locost chassis... If you have dimensions available, it may be the way I go...

I may have missed the dimensions listed in the thread "messy shop"... and I'm rarely viewing the forum on an actual computer, usually a kindle fire or my cell and am unable to open the google 3d warehouse files. I'll have to make some time this week to take a look...

How well do you think the lotus 907 would work in that frame? would I need to alter the mounted angle like seven13bt did in his? or is there more room for the tilt...? I can take some measurements when I get home...


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 415 posts ]  Go to page Previous  1 ... 13, 14, 15, 16, 17, 18, 19 ... 28  Next

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: Yahoo [Bot] and 5 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
POWERED_BY