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Double wishbone w/ pushrods

 
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Scooter



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PostPosted: Sun Nov 12, 2006 1:08 am    Post subject: Double wishbone w/ pushrods Reply with quote

I've been doing some research on the topic and I'm trying to figure out how feesable it would be on my S14. I've wanted to do this for a long time and I'm thinking if I can get a good design and parts list I'll give it a shot next summer. Trying to learn as much as I can now.

Here's my first set of questions:

1. what is a good length ratio for the upper and lower wishbones to achieve a good camber gain? I know the top needs to be a bit shorter, but don't know how much. looking for maybe a percentage here?

2. Are there any good uprights that people use when making a double wishbone setup? With good geometry already designed into it? Maybe FD uprights? I'm guessing that since this is for my drift car that I'm not going to find any for the front that have the geometry I desire. i.e. very little ackerman, and very little camber change in steering. Or is custom pretty much the only way to go?

3. Does anyone know where you can get single lug knockoff hubs? I've looked everywhere to no avail. (this is mainly for a buddy, I know I don't have the cash for these haha)

4. Are there any adverse effects of a rocker with a relatively large leverage ratio (Is that what it's called?)? Like say 1" input movement equates to 4" output.

Also I've heard that in McPherson strut suspension designs that having the steering rack in a different plane as the lca creates a lot of bumpsteer. Is this also true in a double wishbone system?

That's all for now. Sorry if that's too much to ask at once.

Thanks,
Scooter
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PostPosted: Sun Nov 12, 2006 7:42 pm    Post subject: Reply with quote

Alright, well I might not be able to answer everything right away, but I will get through what I can right now.

It's hard to give a number for the ratio between the upper and lower a-arms to get a good camber gain. First you would need to know what is a good camber gain and second it depends on the seperation of the arms both on th inboard and outboard. It also depends on if you want to make your own spindles or use another car's. I know this is part of the second question, but I'll just address it all together.

The camber gain is something you're going to want to mess around with. You'll also need to be thinking about roll center location and that will constrain you somewhat for your arm locations.

Deciding on a splindle might also be a good idea of something to get done with first. You'll need to find something that is rear steer unless you plan on moving the rack. And other than that, you can pick whatever, if you need to do different ball joints or some type of adapter, that shouldn't be much of a problem. Or you should decide if you're going to do a full custom spindle. This would be the way to go if time and money weren't really an issue. Making a custom upright is a lot of design work and a lot of fabrication depending on the method you plan on making it. A sheet metal upright can be very light and strong, but is very hard to make. Having something cast could be an option as well, but it could be expensive to get one or two spindles cast because they would have to make a new mold probably for each.

So if you're going to pick a spindle from a car that already has a a-arm setup, you might be starting off in a better position. A couple cars that come to mind is the FD which you already mentioned, but I don' tknow where the steering rack is located on that car, Z32, skylines. You could also consider making a piece that will bolt onto the stock strut mount and accept some type of ball joint. This might be the easiest way to do it.

Basically, you just need to get a good idea of what pieces you are going to use, so that you are aware of all the constraints on the system. So that way you don't run into problems further down the road.

As for the geometry that you want. Ackerman is something that you can change by messing with the steering arm and wouldn't be too bad to do. Pretty much something like the FC guys do. But as for the camber change with steering. That is both a function of the kingpin angle and caster angle. Most cars will have a decently high kingpin angle, caster angle is differnet for a lot of cars. But these two work to cancel each other out. Kingpin angle will have a positive caster gain on the outside wheel and caster will have a negative camber gain on the outside wheel. This is something that you might be able to get, but you'll have to mess around with it a lot. You probably could use a stock spindle with a custom mount and get a decently low kingpin angle. And then you could reduce caster a little and the camber change might not be too bad, but this isn't something I have looked too far into, especially on the S-chassis.

You can find single lug hubs from used race car parts in the UK. Around here it seems like you'll have better luck finding wide 5 hubs than anything. Of course you could just have them made. Even just from normal hubs. Just replace the studs with pins and then you could probably weld a sleeve onto the hub and thread it to accept one of those big wheel nuts. I don't know if welding the piece on would be the best idea though. But they're probably machined. It would be similar to making a regular hub, you would just need that extended piece and you wouldn't need to put studs in it. Can I ask what application this would be for? Also, why does your friend feel that he needs this? I mean personally, I think it would be awesome to have center lock wheels and hubs, but I can't really justify the cost for the hubs and new wheels which are going to be a lot more expensive that everything else anyway. But then again, you could probably even make a bolt on adapter to do something like this. But I also wouldn't really suggest doing that, but it would be the easiest way to get it on there.

Alright, well that's a start, I'll have more later tonight.

Tim
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Scooter



Joined: 12 Nov 2006
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PostPosted: Sun Nov 12, 2006 9:49 pm    Post subject: Reply with quote

Tim thanks for the Help!

That's a lot of useful info in there. Can't wait to see what else you have to say. One thing I was thinking about the rocker question is if the leverage ratio of the rocker would have an effect on the spring stiffness you'd need. Like say you'd normally need a 5k spring, would you now need a 20k spring (with a 1:4 ratio on the rocker)?

I've found with my suspension now that the less caster I have, the less my wheel flops over (as you indicated). But my bandaid for the current is a ton of static camber (like 4*) to help out with it not flopping over too much. But that essentially increases the KPI, right?. So why does it help the wheel not flop over? Sorry for the extra questions, you haven't even had a chance to answer all of my original ones. haha I'll quit for now.


As for my buddy who's looking for the center lock hubs, they're for an FC. We're thinking of tube framing a good bit of his car, doing a double wishbone pushrod setup too, and he's wondering if we can find some single lug hubs while he's at it. His car will be time attack tho so it would be nice to have them. Also from a mechanical standpoint, center lock with the pins and such is far superior to standard 4/5/6 bolt setup. And as fanboy as it sounds, it'd be baller.
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PostPosted: Tue Nov 14, 2006 7:44 pm    Post subject: Reply with quote

This place has some old centerlock stuff from an Accord or something.
http://www.touringcarspares.com/editable/transmission.htm



You could make something like this, I mean it would be a pain in the ass, but it's probably the easiest way to get a single lug hub. You just need a piece coming out of the middle off and the rest of the hub could probably be pretty similar to a normal hub. But if he's just looking for a cheaper option, then calling teams that raced FC's and used center lock stuff would be his best bet.

Alright, back to the first set of questions.

The leverage ratio or motion ratio or installation ratio, it's all the same thing, it's just the shock or spring or roll bar movement relative to the wheel movement. You have to be careful, some people specify it as shock/wheel and other specify wheel/shock. It still means the same thing it's just a different number. I usually use shock/wheel. So after you know the installation ratio, you will be able to find out the wheel rate based on what springs you're running. The wheel rate is the amount of force it takes to move the wheel one inch. So it's rated the same as the spring would be, it just takes into account the leverage the wheel has on the spring.

So to go from spring rate to wheel rate, you multiply your spring rate by the installation squared. So if you have a 600lb/in spring and you installation ratio is 0.8, you're wheel rate is 600*0.8^2 = 384lb/in. The installation ratio is squared for the spring rate and the damping rates, but it's not squared for displacements and velocity. I forget the whole reason for this so I don't want to try to explain it and have to correct it later, but if you want to know why, I can find it in my notes later.

So in terms of what installatio ratio you want, I would say the closer to one would be the best. Having a great difference in the movement of the shock/spring versus the wheel can cause some problems. It's not as bad if the springs or roll bars have a low ratio because they are pretty linear and you can account for that. But with the shock, you would have to valve it funny to get the shock to act the way you wanted for the speed ranges that the wheel will see. Like if you have a 0.25 shock/wheel installation ratio. Then the wheel would be moving 4 times as fast as the shock. So you would have to adjust the breaks in the damping curve to reflect this in order to get the damper to behave like you would want it to. If you had it the other way, like you mentioned in the first post with a 1" input equals a 4" output, it would be the opposite. And in that case, you would need a damper with a ton of travel.

So if you're doing this all custom, you should be able to pick any installation ratio you want. So I would look at what parts you would want to run and where you want to package them and then figure out what the closest installation ratio to 1 you can get. You would also need to do the kinematics and make sure that you installation ratio is linear through the range of travel. If this is not linear you will get a progressive effect from the spring/damper/arb which you may not want.

With all of that said, I don't think it's a good idea. I don't know what your reasons for wanting a pushrod suspension are, but I just don't think it's worth it. It would be cool to do and awesome if done right, but the amount of work and custom parts you would need may not be worth it. I do think converting to SLA in the front would be a great idea, but I would keep a conventional shock setup up there. It's just with the pushrod part, there are tons of places where any error in your calcutions or fabrication will be amplified.

About bumpsteer, the whole idea is to have the arc of the tie rod follow the arc of the wheel. Things can be out of plane with each other and this can still happen depending on the inboard and outboard position of the tie rod. The easiest way would be to have the tie rod and steering rack in plane with either the upper or lower control arm, but this usually doesn't happen.

Alright from the second post. If you had a 1" wheel to 4" shock installation ratio, and you would have a 5k wheel rate, you would need a 5/4^2 = 0.3125k spring, which would be about 17.5 lb/in spring. When the installation ratio is less than 1, the spring rate will be stiffer than the wheel rate. When it is greater than 1, the spring rate will be softer than the wheel rate.

About the caster/camber settings. I also run a lot of static camber and one of the main reasons is the camber curve in bump. With a very low car, there is a positive camber gain during bump travel. So when you compress the front corner, the wheel could go from 4* camber to 2*. I don't know the exact numbers, but you get the idea.

For the kingpin angle to change it depends on where you're getting your camber. If you use the camber plates, then you're kingpin will increase. But if you adjust down at the strut mount on the spindle, your kingpin will not increase. I'm not sure on the amount of camber gain you get based on camber angle or the amount of camber you lose based on kingpin, so they could me relatively small based on the amount of camber you're running. And then small increase in KPI could be much less than the increase in static camber even at full lock.

Let me know if I missed anything or you have any more questions.

Tim
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PostPosted: Tue Nov 14, 2006 10:57 pm    Post subject: Reply with quote

Wow Tim you rock!

Thanks for all the input!

That's the one thing I didn't think about, the installation ratio throughout the whole wheel travel. I know there are a ton of kinematics software programs out there that can do it for me, but are the equations simple enough to compute by hand? Or do you know of a free kinematics software that is simple enough for me to figure out?

The whole reason I want to do this IS because of the fab work and effort I'd have to put into it. I love this kind of stuff. I had fun doing it with my ITB's and plenum and such, and now I want to do it with my suspension.

Bang for the buck, what would you say is the best 2-way shock out there? Any other companies I should look into besides Moton, Penske and Ohlins?

I was looking at the Ohlins ST44's and they look pretty bitchin'.

Oh and is there a "standard" center lock hub size? I have seen quite a few different sizes but don't know which is the most common/has the widest variety of wheels available.

Thanks dude!
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PostPosted: Wed Nov 15, 2006 2:36 am    Post subject: Reply with quote

You can do the kinematics for the bell crank by hand, but it's a lot of work. The work isn't hard, but to model it through out the range of travel, you have to iterate it a bunch of times.

I think most CAD software has kinematic analysis stuff in it. So if you don't have something like that already, you can probably find a copy pretty easily. There are other suspension software programs that you could probably use if you want to model everything. Two of the simpler ones are susprog3d and peformance trends suspension analyzer.

I understand how you like the fab work. I just still think that the pushrod/bell crank setup is a lot more work than it's worth. Where are you thinking of putting the dampers? And what type of sway bar setup are you going to run? If you keep it off the LCA, then it won't be a big deal, but if you try and run it off the bell crank it may be hard to keep everything in plane.

Have you thought anymore about what your plans are for the SLA setup? Are you going to use the stock spindle and make some type of adapter or are you thinking custom?

Whatever you do, I want to see tons of pictures.

About the shocks, all the ones you mentioned are pretty expensive. If you go with the pushrods, you have a ton of freedom to pick anything. All the companies you listed are very good and well worth the money. You could also look at the Koni stuff. I've had good luck with Bilstein stuff, but they seem to be much better at the non-adjustable, but easily revalvable stuff.

The ST44, I think, is the damper they use on the Audi R10 Le mans cars and stuff like that. I think the cost on them is about the same as a full set of coilovers for an s-chassis. But I had the chance to use some mountain bike dampers with Ohlins stuff inside on our FSAE car. They were 4-way, twin tube dampers and every click did something. It was both felt by the driver and seen on the dyno. They were awesome.

Of course, I'm also working on some stuff. Right now it's stuff that will bolt up. But if you want to work on something, I would love to be more involved with this project. I'm going to be getting a dyno in the near future, so valving and everything can be done in house. But it really depends on which way you want to go. The stuff I'm working on right now is going to be non-adjustable at first. But I'm looking at rebound adjustment options and compression adjustment would be handled by a Penske remote reservoir. But cost starts to get expensive when you add the Penske stuff on there.

Basically, you have a ton of options. I would try try and figure out where you're going to put the damper to at least get an idea of what overall length you can fit and then look at what is offered and go from there.

On the centerlock stuff, I really have no idea. I would just make a couple calls and see what they say. Just for a general idea, I think HRP sells centerlock wheel nuts, so that may give you an idea of what you're looking at.

I'm glad I can help out.

Tim
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PostPosted: Thu Nov 16, 2006 12:58 am    Post subject: Reply with quote

alright, I've got a few more questions.

If my installation ratio is 1, then my shock is going to have to be as long if not a little bit longer than my wheel travel right? Just making sure.

What would you say is a good camber gain to shoot for? like 1*, 2*? So maybe static of 2.5* gaining to 4* at the end of wheel travel? I know it's car/driver/track specific usually but I'm just looking for a ballpark.

You can induce some anti squat and anit dive characteristics into the suspension by having both of the wishbones off axis a bit right? Like a few degrees off horizontal. Would that even be worth it? If it's not really necessary I'd like to avoid it because that would really throw a wrench into the mix for me.

Also, I believe the shock you're talking about that you used on your FSAE car is a Cane Creek Doubble Barrel. I looked into them and they're $650 a piece. Is there any reason you chose to get those instead of something else? Because for that price you can pretty much get a set of Ohlins dampers right? Or maybe you know of a good deal on them?

Also, do you see any reason why I shouldn't run streetbike rear shocks for my first set of dampers? They're dirty cheap. You can get factory Showa dampers from like an 02' F4i that are 2-way adjustable for about $50 a piece. I was talking with a two-wheel nut at work and he said that the damping adjustments work quite well, and have a pretty wide range. What do you think?

Thanks Tim!
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PostPosted: Thu Nov 16, 2006 2:08 am    Post subject: Reply with quote

If the installation ratio is one, then the stroke of the damper will have to be as much as the wheel travel. But depending on how heavy your car and how stiff a spring you run, that travel won't be much. I mean we're talking maybe +-2 inches of total travel. And most dampers you'll find for cars have at least 5 inches of travel. It's the shorter ones that are meant for Formula type cars that have less travel.

In terms of camber gain, you would really want to look at your roll camber gain. But for this, you'll want to have an idea of how much the car will roll and this will be based off of your springs and bars. So let's just say that you're shooting for a 1.5deg/g roll gradient. So this is the amount the car will roll under 1g of lateral acceleration So for this 1g of lateral acceleration, let's say the wheel moves 1". So let's say the wheel gains -1deg of camber for each inch of wheel travel. So if you start with -2deg static camber.

So for a 1g turn. The car rolls 1.5 degrees, so if there was no camber gain, the camber relative to the road would be -0.5degrees. But there is a camber gain of 1 degree per inch of wheel travel. And the wheel moves an inch, so now you're at -1.5 degrees. So you're roll camber gain is going to be +0.5 degrees per g. This of course is camber relative to the road. The camber gain relative to the vehicle coordinate system, will still be negative.

So you need to find out how much you're wheels are going to be moving and how much the car is going to roll, so you can have a better idea of what amount of camber you're going to need. Also, you're camber gain is related to your roll center. So depending on what you are doing with the rear, you want to be careful of where you put this. You want your front roll center to be lower than your rear roll center.

Another thing about camber gain in jounce versus camber gain in roll. You have to consider that you're braking performance might suffer slightly with a higher camber gain in jounce. But you also need to make sure you don't end up having positive camber on your outside wheels under cornering loads.

So I think you're probably starting to realize how much each variable relies on the other and how this is a very iterative process.

About anti-squat and anti-dive, I wouldn't suggest working any of them in. They are based off the side view instant centers and the moments around the CG. So if the arms are parallel to each other there won't be any anti's, but there can also be no anti's if the arms are angles. A little bit of an anti probably wouldn't be too big of a deal to have in the suspension. I don't have enough experience with the anti's to make a suggestion either way. One thing to consider is the anti's will load up the suspension members versus transferring the load directly to the spring.

Roll center stuff and roll moments is basically the same as the side view anti's. It's just the roll center stuff is front view and the other anti's are side view.

We did use the double barrel. It was expensive and if I had the chance to do it, i would buy them all over again. There were several other dampers that where in the same price range, but most of the ohlins stuff that was applicable was modified and revalved snowmobile stuff which still ended up being too long. Packaging was a driving factor in the decision. The cane creeks were 7" with a 2" stroke and were easy to put where ever we wanted them. And the other stuff in the price range wasn't 4-way adjustable. There were other 2-way adjustable options that may have been cheaper, but we had dealt with them and didn't like them. They sucked on the car, the dyno plots weren't great. And we were trying to step things up this year.

The bike rear shocks, well, the valving would be off. Because even if you have the same spring rate that is on there, the masses of the sprung and unsprung will be different and therefore so will the damping coefficients. I also wouldn't trust them under the higher loads they would see on a car. I'm just assuming this, I don't think that the loads on the bike would be as high as the loads a car will see. I mean there's no lateral load transfer, so the shock can't be loaded more than the total weight of the bike plus rider. And that's probably close the static corner weight. So once you get into different loading scenarios, it's going to be loaded much greater than what it would normally see. Also, the stroke on those is pretty small, isn't it. It looks like bikes run a low installation ratio and use pretty stiff springs. I just don't know if it would be a good idea.

I would look at all possible options and design around what you want to run. I wouldn't make any compromises in the design to run bike shocks, but if they did fit up without modification, then you could try them out and see how they do. I would just be worried about having them on there.

Tim
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PostPosted: Fri Nov 17, 2006 2:23 am    Post subject: Reply with quote

Tim, why do you want your front roll center lower than the rear? I've heard this before, but don't know why it is. How much lower are we talking? a couple inches?

I think you're right about the motorcycle dampers. I didn't think about the load capacities they are designed to take. I'm just going to buck up and get the Cane Creek's. 3" stroke so I know that I won't bottom them out.

I'm off to Spain for a week so more questions will ensue after I get back.

Thanks again Tim!
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PostPosted: Fri Nov 17, 2006 5:40 pm    Post subject: Reply with quote

You will pretty much always see the front roll center lower than the rear. The amount is based on your spring and bar rates and the dimensions of the car. One of the reasons is to try and keep the roll axis somewhat parallel to the mass centroid axis of the car. This is just an imaginary axis greated by slicing the car up and finding the cg of each piece. The main goal you want to get to with your roll center heights is based on the lateral load transfer distribution and how it happens. You would want the front and the rear to be pretty close.

Some of the Mark Ortiz articles that I have posted up go a little more into detail on this stuff. I think there's a few about the mass centroid axis idea.

So you want to try and use the Cane Creek's on the car? I think that's a worse idea than using the motorcycle dampers. The cane creeks are meant for mountain bikes. And a 3" stroke is probably not going to be enough. That does depend on what your spring rates and installation ratios end up being, but you would need a pretty stiff car to only have 3" of total travel. I would look for something with 4-5" at least. But like I said, it depends on what springs and everything else you are running.

Have fun in Spain, we'll talk more about this when you get back.

Tim
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PostPosted: Thu Nov 30, 2006 12:53 am    Post subject: Reply with quote

Well after a lot of thinking, I can't afford to do this project right now. So I guess I'll just make a new set of lower control arms and get as much roll center adjustment as I can out of them. The problem with that tho is the angle in the uprights for the lower ball joints doesn't allow for much spacing, as it puts the lca in the brake rotor. Any ideas on this Tim?

Also, wouldn't correcting my roll center (in the rear) with the LCA's and not messing with my ruca/traction rod mounting points change my camber arc in the rear?

I guess I'm in the same boat as AceinHole (forget his name). looking for good application specific dampers (2 or 3 way). However I don't think any of them save for maybe RS*R will allow me to go as low as I would like.

Sigh.
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PostPosted: Mon Dec 04, 2006 2:21 pm    Post subject: Reply with quote

I'm sorry to hear that.

As for the LCA, I haven't looked into it too much, but I know there are clearance issues with the rotor. One option that would probably end up being too expensive would be new calipers with brackets that would allow you to space the rotor a little further outboard. Also, you could relocate the inner pivots and raise them up a little. Just make sure you do both the LCA and the TC rod otherwise you could be adding some anti-dive to the front suspension.

In the back, it will affect your camber curves by adjusting the LCA for a better roll center. I'm not sure how much, but it probably won't be too bad.

AceinHole is PJ. I'm not sure what he's planning as of right now for dampers. We've talked a bunch on it and I'm not sure what, if any, conclusions he's come to so far. I really wish I had a better idea of when I am going to get my stuff done and had an idea on pricing, but all the little side projects have distracted me from finishing up the damper drawings. Hopefully soon I will be able to finalize everything and get an estimate on pricing and time frame.

Tim
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