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PostPosted: September 14, 2014, 2:11 pm 
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LateralScience wrote:
I guess this is more of a simple standard book chassis build, just with a transverse FWD engine pushing it.
I think you should call it "Buford"... :mrgreen:

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PostPosted: September 14, 2014, 3:34 pm 
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LateralScience wrote:
Note the harness bar is at the same height as the rear uppermost member (Book chassis member "O" I think), to meet K3 and K4 members. The bottom of the triangulation members (meeting the main roll hoop and harness bar) ties into the rear lower control arm inboard point of the radius rod.


Isn't it considered dangerous to have the harness mounted below the level of the slots in the seat? Something about compressing your spine in case of an "event"?

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PostPosted: September 14, 2014, 3:58 pm 
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Acerguy wrote:
LateralScience wrote:
Note the harness bar is at the same height as the rear uppermost member (Book chassis member "O" I think), to meet K3 and K4 members. The bottom of the triangulation members (meeting the main roll hoop and harness bar) ties into the rear lower control arm inboard point of the radius rod.


Isn't it considered dangerous to have the harness mounted below the level of the slots in the seat? Something about compressing your spine in case of an "event"?


There is an acceptable range of downward angle that can be used. Most race seat and harness companies publish info about safe practices with harnesses.

See http://www.schrothracing.com/sdocs/2009_Competition_Instructions.pdf


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PostPosted: September 14, 2014, 5:10 pm 
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The cross bar is too low to be the anchor with that seat.

Also, the holes in the seat are not to support the belt, only to get the belt to the drivers shoulders.

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PostPosted: September 14, 2014, 5:32 pm 
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Acerguy wrote:
LateralScience wrote:
Note the harness bar is at the same height as the rear uppermost member (Book chassis member "O" I think), to meet K3 and K4 members. The bottom of the triangulation members (meeting the main roll hoop and harness bar) ties into the rear lower control arm inboard point of the radius rod.


Isn't it considered dangerous to have the harness mounted below the level of the slots in the seat? Something about compressing your spine in case of an "event"?


The harness bar must be at or below the level of the seat holes so that the belts are horizontal or downward-sloped, with a limit on how the slope's maximum angle.

From esp42089's link:
Quote:
Shoulder belts must run from the shoulders horizontally or down, at no
more than a 20° angle.


From the 24 hours of Lemons rules:
Quote:
Shoulder-harness bars are necessary for proper shoulder-harness mounting in nearly all applications (the harness-to-bar attachment point must be between zero and 15 degrees lower than the harness's seat-entry point).


If the harness bar is above the seat holes, in the event of a rollover, occupants will be able to move too much and may end up striking the roll cage, roof, or ground.

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Last edited by Laminar on September 15, 2014, 8:25 am, edited 1 time in total.

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PostPosted: September 14, 2014, 10:35 pm 
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Gents,

With regards to the placement of the harness bar, in FSAE the shoulder harness must be no greater than 20 degrees from the horizontal. I recall getting out of the 2010 car after a full day of driving doing suspension tuning with bruised shoulders and a sore neck from the harness with the belts at the horizontal . (I guess I'm a softie.)

I do agree that spinal compression may be an issue with 9" wide hoosiers and 4 piston wilwoods on a 1300 lb car whilst standing on the brakes.

It's not too big of a deal to relocate the harness bar upwards. I'd rather not have back problems going into old age.

From SCCA GCR:
"The shoulder harness shall be mounted behind the driver and
supported above a line drawn downward from the shoulder point
at an angle of 20 degrees with the horizontal. "

In terms of progress today, I cut the uprights, and I will be placing a 1/2" grade 8 bolt in double shear through a 0.85"x1.3" cross section. (With the 1.3" cross section in compression.)
Attachment:
cut uprights.jpg
cut uprights.jpg [ 992.24 KiB | Viewed 2739 times ]

Attachment:
upright cross section.jpg
upright cross section.jpg [ 615.45 KiB | Viewed 2739 times ]


it took me longer than expected to make these cuts today, as I continually stopped to check, and recheck the evenness of the rectangular cross section. I blended the cut into the I-beam shape of the upright making a smooth transition to the bearing.
Attachment:
smoothed transition.jpg
smoothed transition.jpg [ 976.45 KiB | Viewed 2739 times ]


Therefore stress risers from geometric discontinuities will be kept to a minimum.


Also, I polished the cast iron flashing, and removed a few of the unnecessary bosses for ABS sensor, etc. I took great care ensuring I take as LITTLE MATERIAL away from the upright in sections which will experience heavy loads. I'm pretty certain this cross section will be more than sufficient to handle the loads induced by the AFCO tubes. Also, consider that the LCA pickup point is 0.68"x0.25", and the tie rod (soon to be toe link) pickup point is 0.77"x0.25" at the thinnest cross section.

Also, I did some suspension planning today and looked into using GSXR1000 shocks/springs and ran the math myself, and unfortunately Kurt is correct with regards to his analysis. In order to have a decent wheel rate (100 lb/in to 200lb/in), I will need an installation ratio of 0.3. (WR= SR*IR^2)

I love the fact that I can get GSXR1000 shocks for 20 to 30 bucks on ebay, but with 1.5" of travel, I will need a spring of 1000lb/in, (which will require shock revalving), to have 5" of total travel. (1" droop, 3" compression, 1" bumpstop). Also, with a spring rate this high, at full bump, the loads applied to the inboard damper support would have to support 1500lbf. (Which would necessitate additional bracing and weight support the load.)

I REALLY want to use GSXR1000 shocks, mainly due to the lightweight, and VERY low cost, but that math simply does not work out.

Say for instance, I have a direct actuation shock with the outboard point mounted on the control arm, and inboard point on the chassis. With the inboard shock point 7" above the LCA pickup point, and the outboard shock pickup point 8" away from the inboard LCA pickup, this gives an Installation Ratio of 0.86. This arrangement results in a 41.1 degree shock angle (measured from the horizontal). If we move the A-Arm up 1" in bump, and using the Law of Cosines, I can solve for the reduced shock length. Taking the before and after lengths gives me shock displacement, with which I can use to solve for the installation ratio.

I suppose this may be a blessing in disguise, as I really need to keep this build CHEAP, which means I'll probably use the integra springs/dampers. (But with 450lb/in spring, this results in wheel rate of 351 lb/in--way too high for my liking.)

So my hands are starting to develop vibration fatigue from the angle grinder and miter saw. (I'm wearing Mechanix gloves underneath heavy leather welding gloves. And I may need to somehow add some vibration isolation between my hands and grinder. Any thoughts?

Also, as I don't want to inhale TONS of metal dust, does anyone have any recommendations of a good respirator which does not suffocate me in the process?

I guess it's a good thing that it's back the office tomorrow, and I can go to work and rest my back which is starting to kill me.

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PostPosted: September 14, 2014, 11:17 pm 
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Also my wife raised a valid point, in that a second Momo Start seat will not fit. It is 20.7" wide. The interior "cabin" space is 40" wide. I could just buy a kirkey economy aluminum seat which is 15.5" wide, and call it a day. The wife would fit in it as she's on the petite side, but in my experience the Kirkey economy seats were absolutely painful to sit in for 30 minutes at VIR.

I'd love to get some GRP seats (Tillet clones), but they are only UK distributed I believe. I was thinking some Lotus Elise/Exige seats in alcantara would be ideal, but not very cheap.

Any thoughts?

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PostPosted: September 15, 2014, 7:13 am 
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Re: shocks, I think it's ok to use an installation ratio of 0.6, figuring that after 2" bump at the wheel you'll be into the rubber on the shock. You should have 70mm total travel on the shock, minus 25mm for rubber. Every little bit helps! Wheel rate would then be 173#/in for a 480#/in shock.

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PostPosted: September 15, 2014, 7:55 am 
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Pete B, seeing as I've never driven a Locost7 on a street before, and only rode as a passenger on a racetrack, as well as never driven a Formula car on the street, from past owners experience would 70mm of shock travel and a 0.6 IR allow for sufficient wheel travel? This gives 4.5" of total travel. With 1" droop, this leaves 3.5" for compression.

Well, I do plan on having a 3" to 4" ride height. (Sacrificial skid plates for when the chassis bottoms out.)

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PostPosted: September 15, 2014, 8:21 am 
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Well, you know that's rubber in there, not concrete. 2" bump before hitting it seems pretty reasonable to me. Needless to say the rubber should be in good shape because it will get hit occasionally.

There's a guy who used bike shocks on his middy, of which he has sold some kits. Look up Graber Cars. He swore up and down that he had a nice ride (and it looked good in videos, too), and plenty of travel.

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PostPosted: September 15, 2014, 7:41 pm 
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Perhaps in the SCCA EP class Honda CRX that I had the chance to ride in on the street had JDM concrete inside their shocks--because it sure did feel like concrete when the front bottomed out. (Front was very soft, no front anti-roll bar, and rear was insanely stiff to get it to rotate).

Nevertheless I ran the math myself for GSXR1000 shocks using a 13" overall length, 650lb/in spring rate, and 1.5" travel. Unfortunately, I'm arriving at 2.5" for total rear wheel travel at an IR of 0.6, and wheel rate of 236 lb/in. For the front, it's a little better, at 3.2" of total travel with an IR of 0.47 with a wheel rate of 141 lb/in. Rear droop of 0.19" and front droop of 0.47".

The governing expressions are on the right, and the input and output values are on the left. The upper triangle is at rest, and the bottom triangle is under bump displacement d. Let me know if someone is interested in verifying my math, as there may be something that I'm missing.

Front:
Attachment:
Front IR.jpg
Front IR.jpg [ 130.51 KiB | Viewed 2701 times ]


Rear:
Attachment:
Rear IR.jpg
Rear IR.jpg [ 136.97 KiB | Viewed 2701 times ]


Note that this is direct actuation by the A-arm, as I plan on keeping this build caveman simple, thus no bellcranks. I've been approved for a 14 day copy of Optimum Kinematics software, which is what I used to design the FSAE suspension. I may resort to inputting coordinates of pickup points, but from the looks of it, it doesn't look like I'll be able to easily get the wheel rate I want without a tradeoff in wheel travel. I would REALLY want this to work, but hitting a big 4" deep pothole at 75mph would likely break the chassis.

With regards to the spring rate of the rubber bump stop, it appears to be very non linear:
http://www.1addicts.com/forums/attachme ... 1394988940
Attachment:
6 Bump Test.jpg
6 Bump Test.jpg [ 84.88 KiB | Viewed 2701 times ]


Miata bump stops:
http://www.virkki.com/jyri/miata/bumps.html
Attachment:
CHART_FCM_46_54_vs_63mm_NA.jpg
CHART_FCM_46_54_vs_63mm_NA.jpg [ 70.39 KiB | Viewed 2701 times ]


I recall our Ohlins TTX-25 dampers on the 2011 FSAE car were very hard. It's generally never a good idea to ride the bump stops, because once the bump stop is fully compressed the spring rate essentially goes to infinity, and the car is likely to jump and lose normal load on the tire. I'm sure it's possible to design a car to ride on the bump stops with a falling installation ratio for increasing wheel travel, but I'm definitely not smart enough to do that.

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PostPosted: September 15, 2014, 11:14 pm 
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You can get some rising rate from a bump stop, but at some point it's supposed to prevent metal to metal contact which might be shock internals or suspension components that might be damaged. So they should become extremely high rate the last little distance.

You can use the motorcycle shocks in pairs and argue they have better heat dissipation that way.

It's hard to know how much bump travel to have, it depends on street / track use and how mach clearance you need when the suspension is compressed - how rough the road surface is. Two or three inches is typical on these cars.

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PostPosted: September 16, 2014, 12:38 am 
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horizenjob, I was definitely playing with the idea of running two GSXR1000 springs/shocks in parallel for one corner, and have a very low IR. I think that would most certainly work. There's just that small added problem of having to package two springs/shocks with the same installation ratio. But hey, the Ford RS200 did it, right?

Pete B, here is the rear suspension from Graber Cars:
Attachment:
graber rear susp.jpg
graber rear susp.jpg [ 59.44 KiB | Viewed 2683 times ]

If I'm not mistaken, it looks like a 4 bar linkage on a solid rear axle, with R6 or R1 spring/damper with R1 or R6 bellcrank. Perhaps it's the angle of the camera, but just by eye balling the angle of the pushrod to bellcrank and bellcrank to damper, I would have a hard time believing this would give 4"+ of wheel travel with a 200 to 300 lb/in wheel rate at the rear. Perhaps I'm overlooking something?

Anyhow, I started mocking up my rear suspension:
Attachment:
Rear left suspension mockup.jpg
Rear left suspension mockup.jpg [ 982.2 KiB | Viewed 2683 times ]

The hard points are: outboard wheel center is 11.25" high. Inboard radius rod pickup point to triangulation member within main roll hoop. Lower control arm is to remain horizontal at rest. Upper control arms are still to be fabricated. (Waiting on parts.)

I plan on placing the OEM rear dif, as well as a spare bare engine block in the jig, and go from there. One disheartening thing I found is that if I lower the engine, the transmission must be lowered with it, which increases the angle of the CV joints and reducing the applied power to the wheels. By raising the engine and transmission, the axle can remain horizontal, but then the raised engine increases the CG of the car.

The options here I see are:
-clock the engine 90 degrees forward to raise the inboard axle pickup point (necessitating a dry sump)
-design around 13" wheels with a 10.2" wheel hub centerline
-run the CV joint horizontal to outboard chains/gears to allow for engine lowering. (Not likely to happen).

So the trade off here is high CG with a better CV axle angle, or a low CG with a bad CV angle. Any thoughts?

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PostPosted: September 16, 2014, 8:49 am 
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That shock had a little stronger spring, 554" IIRC, and 1.625" of travel plus a 1" bumper. I have a bunch if you or anyone else is interested. No bellcranks though.

The install ratio is a litle hard to judge. The bellcrank ratio looks like less than 2:1, so maybe not much more than 2:1 overall.

3" travel used to be considered a minimum for a Locost. One dealer, GTS Tuning, sold a lot of coilover packages. I believe the rear shock (intended for solid axle) had 3" travel (if that) and the front was 2".

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PostPosted: September 16, 2014, 10:53 pm 
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Gents,

I took this evening to start mocking upper rear A-arm positions. The factory damper fork is centered about the LCA, and the upright LCA pickup point is centered about the damper. The splay of the UCA will account for the shock body.
Attachment:
Upper A-Arm.jpg
Upper A-Arm.jpg [ 763.76 KiB | Viewed 2633 times ]


Visualizing the lengths and directions of the upper A-arms:
Attachment:
A-arm and toe link planning.jpg
A-arm and toe link planning.jpg [ 113.67 KiB | Viewed 2633 times ]


Note the toe link will be parallel with the LCA in both the horizontal and vertical planes. Furthermore, the length of the toe link will be the same as the LCA such that bump steer (or roll steer) will be kept to a minimum. Also note the 10 degree inward angle of the UCA. The UCA will be relatively short (~9" and 8" tubes), which will attach to the longitudinal member starting reaward of the main roll hoop.

The top of the shock tower lines up well with the first bend in the main roll hoop. The problem I see here is that longitudinal tubes will end up being placed high. The high placement of a longitudinal member results in the X-brace tubes to be positioned more horizontally, and load the X-brace tubes in bending rather than compression.
Attachment:
shock mount to roll hoop.jpg
shock mount to roll hoop.jpg [ 909.26 KiB | Viewed 2633 times ]


Also note the Koni shock with the fork is over 23" long, which is 10 inches longer than the GSXR1000 shock. I think I will use the GSXR1000 spring/shocks for the front, as I could get away with a lower IR and lower wheel rate.

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