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Learning how to build Lotus Seven replicas...together!
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PostPosted: November 20, 2018, 8:17 pm 
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Joined: June 13, 2014, 11:55 am
Posts: 62
I would put my money on springs and shock settings. Chassis rarely need tweaking unless extremely soft due to twisting forces.
If the springs feel "right", most likely they are too soft.
Also, It is very strange to have shock settings so far apart. This makes me think that you need to revise your spring values, and start with medium settings on the shocks.
I´ve done a lot of reading, but I would be hard pressed to say what I remember.
But I can look it up on one of my books should you want...
If Harder springs are too uncomfortable, then you may gain some with an ARB.
Hope this gives you ideas..
PS: I have no experience whatsoever doing this. My comments above are based solely on what I remember. Again, I may look more detailed info should you want.
Best regards
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PostPosted: November 21, 2018, 11:57 pm 
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Cory,

I was looking through Carroll Smith's book, "Tune to Win" for information on anti-roll bars for my own build. I found some diagnostic information that might be helpful to you. He only has oversteer on entry and oversteer on exit as categories. You spoke of mid-turn oversteer as I recall, but here's what he has for entry, the better fit, I thought, since you're not out of the turn.
Attachment:
File comment: Diagnostic from "Tune to Win."
Carroll Smith Text.jpg
Carroll Smith Text.jpg [ 73.46 KiB | Viewed 496 times ]


Going back to the inclinometer and body roll, I understand 2 degrees is a good target for maximum roll angle, but less is better. I have it in my notes, but did not write down the source.

Knowing if the front and rear roll at different rates is helpful. The best explanation I've heard in layman's terms is (paraphrasing) "If you conceptually think of dividing the mass of the vehicle into two parts, front and rear, you want them to roll together at the same rate, not different rates."

If you envision a plane through your car's center of gravity, perpendicular to the roll axis, you can think of a rear mass handled through the rear suspension, and a front mass handled through the front suspension. If the resistance of one end is significantly different (easier, say) that the other, the the stiffer end will receive forces that should be handled by the other, "easier" end of the vehicle. That should show up in the roll angles front and rear, if the imbalance is actually there.

I found that explanation appealed to me, anyway.

Cheers,

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

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PostPosted: November 22, 2018, 1:16 am 
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Carroll Smith's "Engineer In Your Pocket" book says for Mid-Corner Oversteer:

- Driver threw car at corner to get through initial understeer - only cure is to educate driver and/or decrease understeer
- Excessive rear tire pressure
- Excessive relative rear ride and/or roll stiffness
- Rear suspension bottoming in roll
- Insufficient rear droop travel (non droop limited cars only)
- Very loose rear anti-roll bar linkage

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PostPosted: November 22, 2018, 8:35 am 
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My Legends car has the same suspension set up. It took a lot of mods to go from a top 20 to a top 3 car (autocross)
The major design flaw was that the rear trailing arms (3 link) were too short. This caused a lot of rear toe change with small amounts of roll. The only way to stop it was super stiff springs and raising the panhard bar. This caused the car to bounce all over the place and be undriveable

Once I changed to long links, it became much more forgiving, and tune-able. The original lower links were 12" - changed to 26 or so. Also made the upper link adjustable (i made 3 hole on the rear to add angle - no slots, slots are bad) Also make the pan hard as long as possible and adjustable.

On the front, you need an ARB. Super stiff springs or raising the RC causes other issues. As mentioned by others, the solid rear has a high RC - need the front ARB to balance. I also added some anti dive, as when braking hard and turning, the car would dive a lot (nose hit the ground) but otherwise felt really good in a skid pad type situation. Antidive fixed this. I made a bunch of ARBs - 1/2, 5/8, 3/4 - i cold bent normalized 4130 (from Mcmaster) Worked great, never bothered to get heat treated. My weights are different than yours but i ended up with Wheel rate about 1/2 of sprung corner weight and the ARB about 50% of wheel rate. It layman's terms, the suspension compressed 2" from the time the spring starts compressing. I use non adjustable Bilstein shocks. I tune the suspension with the rear panhard height. Once you get the chassis dialed in, very small panhard adjustments (1/8") make a noticeable change in balance. I set my tire pressure by pyrometer and do not use tire pressure to adjust balance.

I suspect that your car is understeering going into the turns, then snapping into oversteer. Once i made all the mentioned mods, i could both "drive" through the corners or drift through them. When friends drive the car, they are amazed at how easy it is to control for a car with a 73" WB. Riding with a friend with a perfect handling car to see his driving lines made me understand what i needed to make my car do.


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PostPosted: November 24, 2018, 12:35 am 
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Joined: June 8, 2010, 8:02 pm
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Location: White Rock, BC, Canada
As always, thanks for the help guys.

I was briefly conversing with someone who used to do professional level race stuff and his guess was maxing out the rear inner shock on extension. Being that I only have around 2" of extension, and antisquat, with these soft springs it seems very plausible. I find it odd that the handling improved with maxing out the shock setting full hard F, and full soft R but maybe the F is having a more significant effect making softening the R null.

It's also entirely possible I have multiple factors here which will make diag tough. On the plus side, I have a few cameras I don't care about so can get some suspension footage next season.

Cheers!

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PostPosted: November 24, 2018, 12:37 pm 
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You're at the first pass level of tuning so I don't think shock extension is a problem if you do have the 2" of droop.

Quote:
I find it odd that the handling improved with maxing out the shock setting full hard F, and full soft R but maybe the F is having a more significant effect making softening the R null.


This would put understeer into your corner entry. While the car is rolling you will have a stiffer front relative spring rate.

In our cars the frame can be effectively a torsion spring between the front and rear suspensions. It should not be the case that we are talking about different amounts of roll in front and rear suspensions. If this is the case the use of anti roll bars, spring rates and roll centers will have limited affect. The car is effectively a loosely coupled assemblage of parts that will behave according to it's center of gravity and it's contact patches. You tune it with tire pressures and camber angles etc., not weight transfer. You can still do quite well.

Without stiffening modifications a Locost may be that flexible.

JD was right about describing how people use shoe polish on tires. In your case though the evidence is till there. Maybe you could get us a close up picture of the edge of the tread/sidewall of your tires? I guarantee there is a story there.

Can you describe how you set the rear up for anti dive? I looked thru a few pages of build log but didn't find it yet. If your lower links are angled upward I think that would give bump oversteer as the car rolled. I just watched the video where you spun out and it kind of looked like the tire edge rolled under? It seemed like the last time I went out in my Formula Ford on old and slightly over inflated slicks.

You could look at or measure your bump steer by having someone stand on the side of your cockpit and measure how much the rear wheels move forward and back. Just an eyeballing will get your attention if there is a problem there. No need to wait until next season for this type of stuff or staring at tires.

Good luck, I think you'll get better results quickly...

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PostPosted: December 10, 2018, 1:02 am 
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Location: White Rock, BC, Canada
Sorry for the delayed response. I was able to finally get to my shop and take some (low precision) measurements. Looks like my F RC is 4.6" and R is 12.25". RC seems reasonably stable. Antisquat is less than 20% (it's at it's min setting). I can't see being at shock extension limits at any time at autoX. I do have a mark where a bolt head on my lower link is scraping a frame tube. Only on one side. I can't see it being the problem, but will trim the bolt head down for clearance.

Sean: my rear links are 15". I don't doubt for a second you are right and I need longer links, but don't really see a viable option to lengthen on this car at this point.

Marcus: The tires show nothing at this point. Two wet event and the drive home made them look like new. No scubbing/wear other than where the tire rubbed the fender. The rear antisquat is with two lower links, parallel to ground, and one upper link with it's front lower than rear. As mentioned, it is minimal right now so I can't see it being a problem. The video where I spun was from a throttle issue. It was jumping from 0-10° from the pedal geometry. It caused me to be off line well before the spin (you can see me in the marbles), and didn't get back on line before a hump in the ashphalt while dodging a cone. Car was basically undrivable that event. I put weight on the car, and bounced it to look for bump-steer. I saw zero movement to the tire. Front bumpsteer is good, I checked it while it was on the alignment machine when first built. Didn't check rear but will next Spring.

So I think the plan is still to add F&R rollbar mounts, and go from there.


Thanks as always guys.

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PostPosted: December 10, 2018, 1:04 am 
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Location: White Rock, BC, Canada
Vsusp link if it helps. measurements are not overly precise, but should be close.

http://vsusp.com/?tool=2d#0.8%26project_name%3ASeven%26trim%7Bbody_roll_angle%3A0%7Cfront.left_bump%3A0%7Crear.left_bump%3A0%7Cfront.right_bump%3A0%7Crear.right_bump%3A0%7D%26front%7Bframe.susp_type%3A0%7Cframe.bottom_y%3A10160%7Cframe.center_to_upper_mount_x%3A30479%7Cframe.bottom_to_upper_mount_y%3A30479%7Cframe.center_to_lower_mount_x%3A16827%7Cframe.bottom_to_lower_mount_y%3A11430%7Ccontrol_arms.upper_length%3A25400%7Ccontrol_arms.lower_length%3A45085%7Cknuckles.hub_to_upper_x%3A15747%7Cknuckles.hub_to_lower_x%3A10033%7Cknuckles.hub_to_lower_y%3A10794%7Cknuckles.hub_to_upper_y%3A13335%7Cknuckles.hub_to_strut_axis%3A14000%7Cknuckles.strut_incl%3A8000%7Csteering.active%3A0%7Csteering.hub_to_outer_tie_rod_x%3A7620%7Csteering.hub_to_outer_tie_rod_y%3A7620%7Cwheels.offset%3A4191%7Cwheels.diameter%3A1500%7Cwheels.diameter_expl%3A35000%7Ctires.size_convention%3A0%7Ctires.section_width%3A20500%7Ctires.aspect_ratio%3A5000%7Ctires.diameter_expl%3A50000%7Ctires.width_expl%3A7620%7Ctires.compression%3A0%7D%26rear%7Bframe.susp_type%3A0%7Cframe.bottom_y%3A9200%7Cframe.center_to_upper_mount_x%3A28500%7Cframe.bottom_to_upper_mount_y%3A24000%7Cframe.center_to_lower_mount_x%3A17000%7Cframe.bottom_to_lower_mount_y%3A2400%7Ccontrol_arms.upper_length%3A24800%7Ccontrol_arms.lower_length%3A37500%7Cknuckles.hub_to_upper_x%3A15000%7Cknuckles.hub_to_lower_x%3A13000%7Cknuckles.hub_to_lower_y%3A13000%7Cknuckles.hub_to_upper_y%3A13000%7Cknuckles.hub_to_strut_axis%3A14000%7Cknuckles.strut_incl%3A8000%7Csteering.active%3A0%7Csteering.hub_to_outer_tie_rod_x%3A7620%7Csteering.hub_to_outer_tie_rod_y%3A7620%7Cwheels.offset%3A4000%7Cwheels.diameter%3A1500%7Cwheels.diameter_expl%3A35000%7Ctires.size_convention%3A0%7Ctires.section_width%3A19500%7Ctires.aspect_ratio%3A4500%7Ctires.diameter_expl%3A50000%7Ctires.width_expl%3A7620%7Ctires.compression%3A0%7D%26pref%7Bdiag1.px_per_mm%3A200%7Cdiag1.front_or_rear%3Afront%7Ctab.active%3A0%7Cunits%3A0%7Cshow.f%3A1%7Cshow.ca%3A1%7Cshow.k%3A1%7Cshow.st%3A1%7Cshow.stl%3A1%7Cshow.w%3A1%7Cshow.t%3A1%7Cshow.rc%3A1%7Cshow.rcl%3A1%7Cshow.ic%3A1%7Cshow.icl%3A1%7Cshow.fvsa%3A0%7Cshow.tl%3A0%7Cshow.kpil%3A0%7Credraw_during_drag%3A1%7Cchart.x_axis_center%3A0%7Cchart.x_axis_window%3A10%7Cchart.x_axis_num_steps%3A21%7Cchart.x_axis_field%3Atrim.body_roll_angle%7Cchart.y_axis_fields%3A%5BFR%5D.general.roll_center.y%7D

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PostPosted: December 10, 2018, 7:51 pm 
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Make the bolt holes on the frame far enough apart to accommodate the largest bar od and bush you might use.

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PostPosted: December 10, 2018, 9:58 pm 
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Before i made the long links, i increased the rear roll resistance as much as possible - mix of raising panhard and increasing spring stiffness. Then stiffened the front with a mix of ARB, spring rate and raising front RC. This worked well on smooth pavement, but the car would be hard to drive on rough stuff (we have a lot of that in CT)

AS far as the rear upper link - i kept increasing the angle until i got wheel hop under braking (sounds like a jack hammer) - then lowered a little. Really helps improve traction off the line and out of slow corners.

FYI - if the front RC is low and the rear is high, the car will want to lift the inside front wheel coming off turns - makes for cool pics, but not the fastest way around the course. I think it is called the roll couple angle


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PostPosted: December 11, 2018, 9:35 pm 
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Location: White Rock, BC, Canada
Miatav8,MstrASE,A&P,F wrote:
Make the bolt holes on the frame far enough apart to accommodate the largest bar od and bush you might use.


Thanks. Good input. I'd intended to do that. Still not sure where the front ARB is going to end up. Rear one is easy enough I think.


Sean: Good info. Always good to know what has/hasn't worked for someone else. I'd say our pavement is pretty good here, but has some humps/dips for drainage. Course designers avoid the dips for the most part, but enjoy putting the humps in middle of sweeper or transitions so I should plan to maintain a fair bit of wheel travel.

I'll post some pictures of what I have in mind for ARB routing/mounting soon. If anyone is still feeling helpful I am always open to input

Cheers!

Cory

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