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Learning how to build Lotus Seven replicas...together!
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PostPosted: September 24, 2016, 1:54 am 
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Location: Victoria B.C.


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PostPosted: September 24, 2016, 12:16 pm 
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What kind of set up do you have as far as suspension. I've got some QA1 shocks with Hypercoils, 350lbs out back and 200lbs up front. I did have any sawybar when I first build it but I have since build one for the front. I use roughly the same design as the caterham ones.


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PostPosted: September 25, 2016, 12:33 am 
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First more congrats on making a nicely finished car, it may need more work but it's not the finish quality...

The limited slip may be a bit more problem than I first though. Did you freshen it up when you put it in? Like new springs or whatever? I don't think you should give up on it or anything, but if it's setup for a heavier car it will be harder for a light car to make it slip. The RX7 was pretty light car ( I don't remember ), but your car is much lighter.

It's probably contributing to the understeer in your photos above too. And once you have the front wheels turned like that and you start applying the power it mostly just gets worse as you push the tires forward - then when it breaks loose it's always sudden. You will get much more used to that with just another day or two on a track. A slow track and tight turs are worst case for this too. Our autocross folks would know more about that.

All the powerful tracks cars (sedans) use various LSD and locker type diffs. These are solvable problems, but they are very real. Cars with live axles tend to use much higher front roll centers, because the live axle tends to have a high RC. That may be in your future. I'll try to find some stuff on this, it doesn't apply to me so I tend to sort of skim that stuff.

You're going to enjoy your next track days even more once this stuff starts to work better. Your going to be passing those cars that are getting by you now. :) :)

If you can get your driver's seat any lower, even if just for the track - I think you'll like that.

There's a coupe of issues with the hands. You know partly it's something can go wrong with your car and you don't get to choose when or what and that goes for other cars near you o the track. The more real thing deals with driver inputs that are unintended ( I think that's why you were holding the wheel at 12 OClock to help that ). With one hand on the wheel as you moving down the pavement and there are little dips etc. - the weight of your arm is not balanced by the other arm. So you can can get a little weave started. Then there is also a lot of air turbulence, especially getting up to 100 and above. This can also start oscillations. You have plenty on your plate, so these things don't help.

It can be boring on a straight, sort of, so it's tempting to relax a bit. I make it a mental exercise to make sure I drive as hard on the straight as I do in a turn. Make that car really be where you want it, not a foot or two left and right. Picture that line going into that next turn, think about something just stay on the ball so to speak. This gets much easier after a couple more events.

OK, so please don't let me lecture on this too much, but please don't hold on to the roll cage! :rofl: Maybe hold your shoulder belts. Try to buy yourself safety all the time with all these little things because boy when the time comes you're gonna want it all! I wear arm restraints and sort of hate them, but I do it. DIdn't use to, we just sat in our body shaped fuel tanks and loved every minute of it. Don't know if you can use arm restraints in these cars though. My formula car has a frame only an inch or two shorter than yours, but I mostly fit down inside, my shoulders hit the upper rail - and it scares me to be so high up, every single time I go out!

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PostPosted: September 25, 2016, 2:48 am 
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Gabriel wrote:
What kind of set up do you have as far as suspension. I've got some QA1 shocks with Hypercoils, 350lbs out back and 200lbs up front. I did have any sawybar when I first build it but I have since build one for the front. I use roughly the same design as the caterham ones.



350lbs rear might be a bit stiff? Depends on your set-up...but maybe try softer? You could even swap front to back if the coils are the same just to experiment....


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PostPosted: September 25, 2016, 3:13 am 
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Thanks Markus for the tips, I'm actually running independent rear suspension on the car. I know the Torsten diff from the Miata would fit in the Rx7 so I'm thinking that might be an easier fix than playing around with those clutches. I had the diff apart but haven't change anything. Actually my brother did the work for me at his shop. He took it apart to install the LSD from a different one and put new oil in there.

Foxtrot, that's not a bad idea but my spring would be to long to fit in the back.
I might just get a set of 300lbs just to see what happens, they are not very expensive.


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PostPosted: September 25, 2016, 3:31 am 
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Even the dampener adjustment can make a huge difference....I run mine very soft....

Quick search on "locost rear spring rate" seemed to come up with lots of hits under 200...really depends on your suspension set-up though...


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PostPosted: September 25, 2016, 12:47 pm 
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Just make sure you're comparing wheel rates and not spring rates. Comparing spring rates is comparing apples to oranges unless the installation ratio is identical.

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PostPosted: September 25, 2016, 1:41 pm 
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I took a quick look in Wikipedia, but I'm still pretty ignorant about the details of these cars. Is this what you have (from Wikipedia):
Quote:
The DTSS worked by allowing a slight amount of toe-out under normal driving conditions but induced slight toe-in under heavier cornering loads at around 0.5 G's or more; toe-out in the rear allows for a more responsive rotation of the rear, but toe-in allowed for a more stable rear under heavier cornering.


With your lighter car, it sounds like it would always be in toe-out mode. I don't know how this works, but I suggest a very small amount of toe-in in the rear. Jut enough to make sure you don't toe-out when you have defection of bushing etc.

The comment above is about 2nd gen RX7, which had IRS.

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PostPosted: September 25, 2016, 3:06 pm 
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Here a couple pictures of the rear suspension set up. You can see where the DTSS bushing would be, I replaced it with a solid aluminum bushing and made a bracket so I could have a bolt going straight trough.

Looking through my notes, I found the spring rate calculator I used from http://www.hypercoils.com/spring-rate-calculator/

At the back I ended up with a effective wheel rate of 164.43lbs.


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PostPosted: September 25, 2016, 4:24 pm 
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It looks like you're doing the right stuff in your rear suspension. The brakes and any offset of the coilover (forwards or backwards from the contact patch ) will put torque forces into the suspension mounting arms. The only thing resisting this is the strength against twisting of your lower arm. We found this on a car I worked on when it was noticed the jam nuts for those outboard rod ends would not stay tight. The rod end can't rotate because of how it is bolted on, but the jam nuts always backed off. We did the math and realized the arm twisted nearly 2" under braking and that worked the nuts loose.

I recommend a dot of nail polish between the jam nut and the control arm to witness any struggling there. :)

Are the rod ends carefully bushed and shimmed so the ball is tight between the mounting bosses? This makes sure the movement is in the bearing and not on the bolt shaft and also prevents any freedom of movement that is not desired.

Careful inspection of the clevis in the DTSS boss is also in order to make sure there is no movement, there is a good leverage there. Small amounts of play here under high loads could be giving you a 1/8" toe change...

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PostPosted: September 26, 2016, 10:24 am 
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All those rod ends are shined and tight, that nail polish is a great idea though. I'm wondering how much flex it would need to crack the nail polish. It would be nice to find a way to see if there is any movement backward and forward on high braking and acceleration.


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PostPosted: September 26, 2016, 12:32 pm 
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We use paint pens at work for exactly this.

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PostPosted: September 26, 2016, 2:07 pm 
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I did not elaborate, but the nail polish is part of diabolical plan to get a girlfriend involved... :rofl:

What's life without diabolical plans? Every day I start with a fresh plan.

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PostPosted: September 26, 2016, 3:30 pm 
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horizenjob wrote:
I did not elaborate, but the nail polish is part of diabolical plan to get a girlfriend involved...
Don't think the wife would go for that idea... She might get a diabolical plan to bonk me on the head with a lug wrench... :ack:

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PostPosted: September 28, 2016, 2:23 am 
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So, I'm trying to find some rear tires for the car but with not much luck. Nobody seams to carry any sort of r compound tire around here. Where do you guys get your tires. I'm looking for a bit of in odd size it seams, 245/45R16, the only one I can see would be the R888.


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