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 Post subject: Re: New Build
PostPosted: February 10, 2012, 1:15 pm 
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BARBAPAPA wrote:
mjalaly wrote:
i might be able to angle the tabs on the chassis and the UBJ mount on the upright and keep the control arm straight.


That would be ideal. I have the joints on my front uprights 'inline' along the KPI and the rears are also machined at the angles needed but the chassis tabs in my case were done first and it would have been a nightmare to try to angle them because of my overall strategy.


Rods in bending!!! Oh no!!!! :boxing:

Just kidding. Yeah i will probably just angle the tabs.

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 Post subject: Re: New Build
PostPosted: February 10, 2012, 1:38 pm 
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So this is interesting...

I sent my suspension layout to Dennis at Palatov Motorsports to see if he had any suggestions and he came back with this...

First some clarification on his part.

first i need to explain the sketch a bit - it's not totally intuitive because it was designed for my own purposes. it is set up for analysis IN ROLL, even though the driving dimension is 'bump', i.e. vertical displacement on the wheel from neutral rideheight. it is intended to get a general feel for what the suspension is doing and not an all-encompassing dynamics analysis.

the angled ground plane is the approximate roll angle corresponding to the displacement (and assuming similar but opposite displacement on the other side, which of course is an approximation but suffices for the analysis). the driven dimension is the angle between wheel and road surface, basically the effective camber - in reality it will be somewhat less than indicated because the off-side wheel doesn't really droop as much as the loaded side compresses.

the reason to focus on camber in roll is that this is what affects cornering grip. camber change in straight up bump doesn't matter much because there is no lateral load on the tire at the time. the only time it does come into play in in dive/squat (braking/acceleration) and that's where the tradeoffs come in. a suspension with little camber gain will favor braking/acceleration grip, one with a lot of camber gain will favor cornering. neither is fundamentally 'right' or 'wrong', just depends on what kind of car you're building and what aspects of performance you want to optimize.

as designed, your suspension has very little camber compensation. this is evidenced in the three pictures 'mj susp X bump/drop'. the angle dimension shows the resulting camber. if your suspension is very stiff and/or tires are very forgiving in camber (or if accel/braking is more important than cornering) it's fine. also note that in 2" drop the suspension arms are nearly parallel, which means any further drop will 'flip' the virtual swingarm to the outside and really build adverse camber on the tire. this is a characteristic of unequal-arm suspension.
by changing the relative length of the arms you can adjust where and if that point occurs in the range of travel.

the second set of pictures, 'mj mod susp xxx', show the difference by lowering the inboard pivot of the upper arm by 1.75". should be self-evident. this would be my recommended setup as a reasonable all-around compromise but it's by no means the only one. there is no single 'right' solution.


My set up...
Attachment:
mj susp 2 droop.jpg


Attachment:
mj susp 2 bump.jpg


Attachment:
mj susp 4 bump.jpg



His change
Attachment:
mj mod susp 2 drop.jpg


Attachment:
mj mod susp 2 bump.jpg


Attachment:
mj mod susp 4 bump.jpg


This kind of goes against suggestions that the arms should be kept parallel (and yes i know it is a bit more complex than that). Any thoughts? Any other suggestions? I will try to attach the suspension sketch i sent him later today here.


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 Post subject: Re: New Build
PostPosted: February 11, 2012, 11:43 am 
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It all boils down to the goals for your suspension design. Suspension geometry is just the tool we use to achieve the desired set of compromises and priorities appropriate for the car. "Right" is nothing more than meeting those goals as best we can within the constraints given.

That said, if you don't understand exactly what your suspension will be doing and when, and what that means to the car's actual driving dynamics, it is entirely possible to build a "very fast" car that spends much of it's time just spinning, sliding and squealing instead of translating the engine's power into usable motion. Even then it should still be light enough and have enough contact patch to still go like heck, even if it does handle at far less than optimum efficiency.

Even then, you can stiffen up the springs, damping and sway bars and adjust the air pressure in the tires to the point where the suspension will work fairly well on smooth surfaces, at least in theory.

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 Post subject: Re: New Build
PostPosted: February 11, 2012, 2:10 pm 
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Quote:
This kind of goes against suggestions that the arms should be kept parallel (and yes i know it is a bit more complex than that)


Hi MJ- I might be mis-reading your comment, but... I read your statement above as saying that you think the upper and lower arms should be parallel.

The "typical" designs for Locost front end geometry are that the lower control arms be parallel to the ground when the car is at rest. The upper control arms should angle downward slightly from spindle to frame, even when the car is at rest on a level surface. It's hard to tell exactly from the drawings, but I think that's the kind of arrangement Mr. Palatov came up with.

If I'm "preaching to the choir" here and you were already on-board with that bit, I apologize. If not, I hope I helped things somewhat. Either way, keep after it, looks like you're making progress. And, like Martha says, "That's a Good Thing."

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 Post subject: Re: New Build
PostPosted: February 11, 2012, 3:38 pm 
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And as clarification, i don’t think the changes are wrong, they are just quite different than what I had done originally. Since my suspension knowledge is fair and his layout is shown differently than what i would do, i wanted to make sure that this change is really worth the mechanical issues that i would need to compensate for. I have posted my original suspension below for comparison. Mainly i wanted to make sure others had a chance to see differences and why.

Attachment:
Suspension.jpg


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 Post subject: Re: New Build
PostPosted: February 12, 2012, 5:22 am 
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It's really hard to talk about suspension in terms of absolutes.

Everything starts with the tire. It helps to have some idea of how the specific tires you plan to race on will behave when side loaded and under acceleration and braking. Knowing the tire manufacturer's recommended setup information for your chosen tire (right down to any specific quirks in your chosen size & sidewall if possible) is also helpful.

Then you need to have some idea of your car's sprung and unsprung weights, and how the car will be driven. If you can sort out a ballpark CG from here even better. All of those factors, plus wheelbase and track width go into figuring out weight transfer. Once you know that, you have some idea of what your (starting) spring, damper and sway-bar choices will be based on your preferred degree of roll stiffness and chosen suspension frequency.

Then you can work out roughly how much roll you will see in a given corner with a given side load. Factor in the weight transfer for acceleration and braking events and you are back to optimizing the suspension geometry to move so it best fits the tire's characteristics. Torque/HP also matter, as they help to determine if you need to prioritize acceleration and braking over steady state cornering.

I think it's great that you're sharing this information, but I guess I'm struggling a bit to put it into context as part of how you plan to use your car. While you have quite a bit of detail on parts of your plan and build, I didn't see anything talking about tires sizes, engine, planned power, etc. Of course it's possible that I missed it when I re-skimmed the thread.

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 Post subject: Re: New Build
PostPosted: February 12, 2012, 2:34 pm 
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erioshi wrote:
It's really hard to talk about suspension in terms of absolutes.

Everything starts with the tire. It helps to have some idea of how the specific tires you plan to race on will behave when side loaded and under acceleration and braking. Knowing the tire manufacturer's recommended setup information for your chosen tire (right down to any specific quirks in your chosen size & sidewall if possible) is also helpful.

Then you need to have some idea of your car's sprung and unsprung weights, and how the car will be driven. If you can sort out a ballpark CG from here even better. All of those factors, plus wheelbase and track width go into figuring out weight transfer. Once you know that, you have some idea of what your (starting) spring, damper and sway-bar choices will be based on your preferred degree of roll stiffness and chosen suspension frequency.

Then you can work out roughly how much roll you will see in a given corner with a given side load. Factor in the weight transfer for acceleration and braking events and you are back to optimizing the suspension geometry to move so it best fits the tire's characteristics. Torque/HP also matter, as they help to determine if you need to prioritize acceleration and braking over steady state cornering.

I think it's great that you're sharing this information, but I guess I'm struggling a bit to put it into context as part of how you plan to use your car. While you have quite a bit of detail on parts of your plan and build, I didn't see anything talking about tires sizes, engine, planned power, etc. Of course it's possible that I missed it when I re-skimmed the thread.


Erioshi,

Those are pretty easy questions to answer to some extent.

The car is going to be 70% road and 30% track. The power train is a 5mt with a limited slip, B20Vtec with 8k rev. and output should be around 190ft/lbs and 250Hp.

The tires i have right now are Kumo Ecsta 205/50-R15 tire for set up then i am switching to a Advan A048 tire in the same size.
Shocks will be Koni 30 5325 with spring rates of 350rear and 300front.

Target weight is 1500lbs and i will post the estimated unsprung in a bit. No plans for sway bars yet but 2°-3° would be ideal.. i think. :?

Hope this clarifies.

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 Post subject: Re: New Build
PostPosted: February 13, 2012, 9:55 am 
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This is what i came up with the eliminate the suspension issue (with the help of you guys). Worse to machine but it will allow 4" bump.

Attachment:
New upper link.jpg


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Last edited by mjalaly on February 13, 2012, 3:48 pm, edited 1 time in total.

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 Post subject: Re: New Build
PostPosted: February 13, 2012, 11:53 am 
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Looks much better to me. Keep in mind too tha with 6-9deg caster, you may want to tilt that upper mount by the same. This will maximize the available range for bump and droop.

If you normally expect 2/3 range in bump and 1/3 in droop, you may also want to design the angle accordingly.

Good work.

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 Post subject: Re: New Build
PostPosted: February 13, 2012, 2:57 pm 
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Assuming you will be running the shock from the lower arm, I would plan on at least putting a helicoil into the threaded blind hole for the LBJ bolt. Ideally you want a nut on the back though.

Most of the weight on that corner of the car will be trying to pull the bolt from those aluminum threads. Plus the steering motion will be trying to unscrew the bolt.

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 Post subject: Re: New Build
PostPosted: February 13, 2012, 3:43 pm 
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a.moore wrote:
Assuming you will be running the shock from the lower arm, I would plan on at least putting a helicoil into the threaded blind hole for the LBJ bolt. Ideally you want a nut on the back though.

Most of the weight on that corner of the car will be trying to pull the bolt from those aluminum threads. Plus the steering motion will be trying to unscrew the bolt.


Right now i have a Helicoil in every hole that is treaded. I will see if i can pocket out an are and put a nut in. Thanks.

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 Post subject: Re: New Build
PostPosted: February 13, 2012, 3:47 pm 
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Have any of you guys run across a really clever way to adjust caster? I was just planning on making an inboard mount that could be adjusted with shims. The static caster will be 5°.

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 Post subject: Re: New Build
PostPosted: February 13, 2012, 10:03 pm 
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So if I plot out my caster to 5°, there is also a .25 forward movment that will affect my camber. Is this critical? Plan on putting 5° caster into the upper inboard mount design and hahing the ability to shim it up from there. Looks like most of the designs i have seen keep both upper legs equal and just shim or lossen a rod end.

Attachment:
Untitled.jpg


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Last edited by mjalaly on February 13, 2012, 11:47 pm, edited 1 time in total.

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 Post subject: Re: New Build
PostPosted: February 13, 2012, 11:45 pm 
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rx7locost wrote:
Looks much better to me. Keep in mind too tha with 6-9deg caster, you may want to tilt that upper mount by the same. This will maximize the available range for bump and droop.

If you normally expect 2/3 range in bump and 1/3 in droop, you may also want to design the angle accordingly.

Good work.


It looks like with the caster I still have enough room without tilting the upper mount, up to 8°

Attachment:
Caster.jpg


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Last edited by mjalaly on February 14, 2012, 11:01 am, edited 1 time in total.

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 Post subject: Re: New Build
PostPosted: February 14, 2012, 11:00 am 
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a.moore wrote:
Assuming you will be running the shock from the lower arm, I would plan on at least putting a helicoil into the threaded blind hole for the LBJ bolt. Ideally you want a nut on the back though.

Most of the weight on that corner of the car will be trying to pull the bolt from those aluminum threads. Plus the steering motion will be trying to unscrew the bolt.


Moore,

After some research your bolt suggestions looks like the right design choice. I change the layout to incorporate it. Thanks.
Now i can machine some parts!

Attachment:
Thru Bolt.jpg

Attachment:
Thru Bolt 2.jpg



Picked up a Corbeau 3" 5 point cam lock harness (new) form Craigslist for $50!


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