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Learning how to build Lotus Seven replicas...together!
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PostPosted: May 18, 2018, 11:20 am 
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@CodySimonson

You can find successful examples of both the soft-sprung and hard-sprung cars in racing. But, I think for a 7-like street car, you need to have the stiff-chassis/soft-spring setup to be happy over any significant driving distance. That's just my opinion, of course. Your pain tolerance levels may be very different from mine. :lol:

The nice thing about building these cars is that you can pretty much change anything you want later on if you don't make an ideal choice the first time around.

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: May 18, 2018, 5:29 pm 
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I could be mistaken, but I don't believe Nick was talking as much about chassis flex as he was chassis displacement. Think of it this way, if you roll over a speed bump at a crawl, the chassis displaces 100% of the speed bump height while the suspension effectively displaces 0% of the speed bump height. As the speed increases, the suspension displaces more and the chassis displaces less...To a point though, and it's not a direct correlation. Even the softest of suspensions cannot have the combination of suspension and chassis displacement be less than the height of the bump itself, and the stiffer the suspension the more chassis displacement biased it will be.

So the springs and dampers work in conjunction to naturally resist suspension motion and translate it to chassis motion. To illustrate just the damper part of the effect, look at my suspension displacement vs time plots in the linked thread and notice what happens to the suspension displacement when it goes from undamped to damped: https://www.locostusa.com/forums/viewtopic.php?f=5&t=891&start=90

As such, when combined with the spring effects, I would anticipate that on the vast majority of low slung sports cars, even on the more 'softly' sprung side, the suspension will typically still be stiff enough that the chassis will be displaced by bumps nearly as much as, if not more than, the suspension does.

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PostPosted: May 18, 2018, 8:48 pm 
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I don't know if there are any official wheel rate figures for the earlier 7s, but the Elan's wheel rate was 67 lb/in according to Norbye. Note the "S2" drawing floating about the net shows a ground clearance of 6.5" for the 7.

*But* those cars had hard bias-ply tires narrower than most modern motorcycles, let alone cars. They probably couldn't get enough sidebite for body roll to be a big issue. They could run a very soft wheel rate and lots of ground clearance and not have a big problem with the wheels hitting the bump stops. Indeed, looking at the drawings, unless there was something internal to the shocks, they didn't even *have* bump stops. No snubbers on the shock shafts either.

The old Shelby Mustangs and Cobras were notedly good handlers back in their day. In 2018, shod with modern tires, they're ten pounds of excrement in a five pound bag, assuming all else is original. The traction available with the original tires wasn't enough to show the faults of the suspension geometry. Heck, look at some of the open-wheel racers of the 1950s and 1960s and you can see some crazy suspension geometry. It wasn't sekrit sqrrl F1 tech; the geometry simply didn't matter with narrow, treaded tires. Jim Clark reportedly ran an entire *season* on the same set of tires back in the day.

Caterham has been steadily beefing up their chassis because they're not using 1957 tires any more.


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PostPosted: May 19, 2018, 10:28 am 
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Any suspension design starts with the tire. Problem is, getting info from the mfgr for less than normal weights and pressures.without that IMO, we are left to those who went before, educated guesses and wild a$$ guesses.

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“Any suspension will work if you don’t let it.” - Colin Chapman

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PostPosted: May 19, 2018, 11:41 am 
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Driven5 wrote:
I could be mistaken, but . . . .


Justin. You're not going to spoil this argument with diagrams, facts and figures are you? You're no fun. :mrgreen:

Interesting material, of course.

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: June 5, 2018, 1:09 am 
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CodySimonson wrote:
nick47 wrote:
I went through this about 5 years ago. I ended up with springs a lot stiffer than necessary. But it turns out that when you hit a bump, the suspension doesn't soak up the full height of the bump. Not by a long shot. The chassis takes up easily half of it (i.e. you get jarred), so the suspension only sees half the compression you'd expect, or less. I have about 4" of travel in front and 2" in back. I have never hit the bump stops. Not once, not even close, in 45K miles. And I've been down some seriously bumpy roads at speed over the years. My springs are 340#/140# F/R.


Have you by chance done any Aussie mods? I'm wondering because I'm building this chassis to be as stiff as possible, and I've already surpassed the book chassis stiffness by 50-75% in my model.

I would assume then that the suspension would then take the brunt of the force before the chassis.


I wasn't talking about chassis stiffness. These cars are light. Hit a two inch bump, the spring compresses maybe half an inch and the car jumps an inch and a half in the air. Your unsprung weight would have to be close to zero to get the spring to compress the full two inches.


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