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Learning how to build Lotus Seven replicas...together!
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PostPosted: December 14, 2017, 5:14 pm 
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Location: Novato, CA
My rule of thumb on static roll center is 2-6". The UCA-to-LCA length ratio is not a factor on a Locost, because, as we discussed, the UCA length is determined for you by the frame and spindles. Of course you can vary the length of the LCA (per Graham's suggestion), which will also change your UCA length, but LCA length is going to determine your front track, which you might want to be close to your rear track.

Frame and spindles. Once you have those, your options are limited, which is a good thing if you don't want to do the math.


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PostPosted: December 14, 2017, 5:42 pm 
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For another perspective, look at the "Latest Updates . . . " section on this webpage ==> http://sevenesque.com/

There are 3 accompanying YouTube videos for his process and the first one is here ==> https://www.youtube.com/watch?v=6crOSs6LCTQ

The idea is that it takes very little math, but you also have some adjustability in the end-design for tuning purposes. I have not used the method myself, so you'll have to evaluate it for yourself.

Cheers,

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

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


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PostPosted: December 15, 2017, 9:42 am 
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The "fun to chase" might be a slight understatment :ack: . But given a choice, I would want to minimize roll center movement.
I would be very careful about assuming that most auto engineers know anything about suspension design. Most are only concerned with cost and packaging. There is only a small group in each company and they are not assigned to bread and butter vehicles.
Dave W


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PostPosted: December 15, 2017, 11:44 am 
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Location: Sunny-Okanagan, Canada, eh?!
This was the statement I was referring to (probably stolen of the SCCA D/E-Mod list):

Quote:
Roll center
----------------------------------------------------------------------
"I would like to respond to the many discussion points on the
subject of roll center & RC migration. Sorry to be so long winded but
this is a complex subject.

I have spent many years working within Ford Racing to study NASCAR,
Indy Car & sports car suspension geometry & the associated effects.
This has included design, modeling & testing both on the track & in
laboratories. Within Ford Motor Co. we have laboratories that can
actually measure the forced based effects of roll center height & RC
migration to a very accurate degree.

Here are a few interesting points:
1. Suspension models in general are not forced based but are
relatively simple geometric models.

2. Geometric models are very good for defining the basic character of
rigid suspensions (heims etc. no rubber bushings). These include
camber gains both jounce & roll. Roll center placement & migration,
roll rates, steering geometry etc.

3. Geometric lateral roll center migration, in general, increases as
the geometric roll center gets closer to the ground.

4. Geometric roll centers are an approximation of the real world of
forced based roll centers. (what happens on the race track & how the
vehicle transfers weight from tire to tire)

5. Forced based roll centers can be modeled in ADAMS or other such
complex models.

6. Force based effects can be measured in the laboratory very
effectively.

7. Force based roll centers can & do migrate & this can have a very
significant effect on weight transfer, jacking forces & the resulting
COG height and tire contact patch loads.

8. Vehicles with relatively high roll centers (2" - 3" or more above
ground statically).
In forced based testing those vehicles that exhibit significant
geometric RC migration, have higher tire contact patch weight
transfer than do vehicles with the same roll center height & little
or no lateral roll center migration

9. Vehicles with low roll centers (very near the ground).
In forced based testing these vehicles do not have siginificant
weight tranfer associated with roll center height & or roll center
migration.

10. The forced based roll center is always higher than the geometry
indicates. (for all vehicles we have tested)

11. Many NASCAR suspension geometries have roll centers that migrate
(geometrically) this results in greater force based weight transfer &
a resulting reduction of grip at the limit.

in summary:
1. Very few of us have the ability to conduct forced based modeling
or testing.

2. Use geometric models to define your basic geometry requirements.

3. Do your best to reduce geometric lateral migration of the roll
center without compromizing the important parameters.

4. If your roll center is very close to the ground lateral migration
of the RC is MUCH LESS important than with higher roll centers. To
say this the other way; cars with higher roll centers should have
MUCH less lateral roll center migration.

Again sorry to be so long winded & I hope that I have not confused
the issue."

Jay Novak
Lifted by Del Long

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PostPosted: December 15, 2017, 1:14 pm 
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My hang up with the traditional method is that it's only accurate with equal weight on both tires. Said another way, how can roll center location be determined when both tire geometries have equal say, while in a turn where one tire has twice the load - and traction - on it?

In the end though, I'm not sure how much it matters. The light weight of our cars is the primary reason for their performance.

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Last edited by KB58 on December 15, 2017, 1:23 pm, edited 1 time in total.

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PostPosted: December 15, 2017, 1:17 pm 
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The best useful design guidelines I have seen are that the force line should be under 4 degrees for IRS cars and absolutely under 8 degrees for live axle cars. The camber correction should be held to between %25 and %50. It's easy to want to do more camber correction but you quickly land up with other bad compromises.

Above I stated that the roll resistance was provided by both geometric resistance and elastic resistance. There is one major difference. The elastic resistance is produced by the act of rolling the car. At the initiation of a turn there is no elastic roll resistance, it's easy to see that the anti-roll bars are doing nothing. So there is no suspension tuning available at this point. At steady state cornering the anti-roll bars and springs can adjust the weight transfer.

For SkinnyG in an autocross where things are happening quickly this could be a big deal. It might be confusing if the geometric and elastic roll stiffness knobs are at different settings. It might also be what you want, but getting there without knowing that difference might be hard.

The geometric stiffness is there instantaneously as soon as a lateral acceleration is started. If the force lines or RC are moving around that stiffness will be changing, but it is always instantaneous.

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PostPosted: December 15, 2017, 10:06 pm 
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Location: Sunny-Okanagan, Canada, eh?!
KB58 wrote:
My hang up with the traditional method is that it's only accurate with equal weight on both tires.


But does get you in the ball park, for our "one-off backyard-built low-cost car."

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PostPosted: December 15, 2017, 10:55 pm 
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The thing is though you get into these discussions about stuff like lateral movement of the roll center and they don't really mean anything. Just because those force lines meet somewhere doesn't say anything. The car doesn't roll around that point and it makes no accounting of the weight on the tires or the magnitude of the forces involved... It's just a simple diagram someone invented years ago that seems like you know something, but in the end it leads you down a hole. I refer to roll centers because it's sometimes a useful shorthand and it can be helpful. There is a correlation there but it's not worth looking at too deeply.

SkinnyG, something I saw a few years ago was the way Modernbeat's car was set up to allow adjustment of the upper wishbone for roll center. The brackets where oriented to hold the rod end horizontal and their mounting bolt vertically. The mounting bracket had some extra space in it that was taken up by spacers, so he could swap them around to quickly change the height.

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PostPosted: December 16, 2017, 12:02 am 
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Yep.

If I've learned anything, it's this: make everything adjustable, because theory doesn't always intersect reality.

My focus on roll centers came from way before I built my Locost, and was autocrossing a B13 Sentra. I copied a friend's successful setup, right down to the letter - every part was the same, and mine ended up handling atrociously; TONS of oversteer. That's what got me learning suspension, and discovered the rear roll center was VERY high, and the front roll center was below ground. Once I sorted that out, the car was ~very~ competitive (until they put non-VTEC Civics in my class, haha!).

I do know that your basic string computer works reasonably well.

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PostPosted: December 16, 2017, 11:02 am 
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I've heard the mention of "string computers " here many times. Can someone expound on this a bit, or direct me to some place that might be able to clarify it to my foggy old brain?

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PostPosted: December 16, 2017, 12:08 pm 
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You make a 2D mechanical model of your proposed suspension, then literally use string in place of the lines to find where they intersect. Look at the picture on the first page of this thread. You just use string in place of drawing the lines.

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PostPosted: December 16, 2017, 2:07 pm 
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1/4 scale cardboard pieces and thumbtack joints are large enough to get accurate measurements from and small enough to manipulate on a desk or small table.


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PostPosted: December 16, 2017, 11:03 pm 
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OK! Gotcha. Thanx,guys!!!

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PostPosted: December 20, 2017, 4:33 pm 
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It may not be ultra-precise, but cheap mock-ups can be very helpful, in my opinion. Below are some photos of a foamcore mock-up I did with respect to my rear suspension design. I'm a very visual person, and I wanted to develop a sense of how much difference change is my 4-link distances would make in practice. Materials-wise, it probably cost me $10 U.S. to make this.

I was trying to understand issues like bump steer and the danger of collision between the links and my axle housing for various amounts of bump and droop. A similar model of the front suspension would require modeling both sides, as you want to determine the geometric location of the roll center and you really would want to vary the location of the two suspension sides together, but independent of each other.

If you have access to some basic drafting tools like a standard sized T-Square and table top with square edges (or a big sheet of plywood), you can do it. I did things full size as I didn't want scaling errors introduced. I found it very helpful. Staniforth's string model would be a good model to use for setting things up with a front suspension.
Attachment:
File comment: The last of 4 versions I "tested" using this model.
Version-4-Bracket-Setup.JPG
Version-4-Bracket-Setup.JPG [ 134.27 KiB | Viewed 324 times ]

Attachment:
File comment: Tracing out the movement of the axle center over bump & droop
May-23-2915-#2.JPG
May-23-2915-#2.JPG [ 137.88 KiB | Viewed 324 times ]

Attachment:
File comment: Measuring off movements of various locations from traces left by the model.
Ruler-at-3-Inch-Bump.JPG
Ruler-at-3-Inch-Bump.JPG [ 119.19 KiB | Viewed 324 times ]


Cheers,

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

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


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