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    Shock placement in relation to Suspension movement
    #1
    Keeper of the Asylum K-fab's Avatar
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    I've been cruising around the site a lot recently - huge amounts of good info, lots of good pix and such too.

    One thing that I have noticed, though, is that a lot of the guys here that are building vehicles are really messing up on shock position in relation to
    the suspension movement.

    Too many of the cars that I see have front shocks that lay down WAY too much. You're actually building in a dramatically decreasing rate when you lay them down.

    Looks cool as hell - works like arse. The Chenowth Millenium Car was a perfect example of this. They laid the shocks waaaayyy over (man they
    looked killer), and the front suspension just blew through the travel. It ended up being the downfall of a killer car too.

    It's all about leverage ratios.

    First, Motion Ratios:
    Motion ratio is the ratio of wheel travel to shock shaft travel.
    12" wheel travel, 8" shock travel - 1.5:1 motion ratio
    12" wheel travel, 6" shock travel - 2:1 motion ratio.
    Easy enough...
    And the place to start.

    "In general", you want to work with a motion ratio that's somewhere between 1.5:1 and 2.25:1. You can go as extreme as 3:1, but when you get into this range, your spring rate and shock valving are usually very stiff and it can get hard to dial in/control the suspension movement. Linkaged suspensions (motocross bikes come to mind) have rising rates - they don't count in this discussion.

    Now for the design aspects.
    First off, at full compression you want the bottom of the car to have, at minimum 2" of ground clearance - if you can get 3 to 4 inches - especially
    when running ATV style tires it's even better. Tire flex can be quite large.

    Then again, if it's strictly a sand car, you can get away with the 2" or so clearance - sand absorbs a lot of a hit. - if you're running in rocky stuff - go large on clearance.

    Now here's where a lot of people mess up in design:
    At full compression - no matter what the motion ratio is, be it front end or rear end - you want the shock to be as close to perpendicular to the line
    that exists between the point where the suspension mounts to the car and the point where the spindle attaches to the suspension as possible.

    Full compression = shock perpendicular to suspension. This little rule of thumb should dictate the mounting location of your shocks on the chassis.

    Yea, it screws up the looks of the vehicle at times, but it's very, very important. Often, it's very hard to get this to work up front w/o making some fairly odd looking shock towers and most people do lean the shocks inward a bit, but you don't want to get anywhere near 45 degrees - maybe 15 to 20, but never 45. In the rear of the car, where the weight is usually located, it's quite easy to design in - roll cage or frame is generally in the vicinity and will allow for the proper geometry.

    Ever see any of the long travel beam cars? The tops of the shocks lay forward in relation to verticle - but they're perpendicular to the arms of
    the suspension at full compression. Looks weird, works wonderfully

    The reason being is that this (perpendicular) is where you get the best, most direct, mechanical advantage of the shock and spring. - It's where it works the best.

    quick note - Because suspension components don't move in a vertical only manner, you have to take into account the actual angular changes that take place as the suspension cycles. This movement about three points (shock mounting location, suspension pivot point & wheel pivot point) will vary the actual shock angle to suspension arm - but as long as you work towards perpendicular at full compression, all is good.

    Lay a shock down so that it's not perpendicular at full compression and the leverage that the suspension arm is able to exert against the shock goes up and shaft travel goes down. You are required to have a larger rate spring, and the valving has to be stiffer to deal with the higher forces, yet shaft travel goes down so you're trying to do more work over less distance. You loose shock efficiency.

    The stiffer valving and spring rate required to deal with the higher forces a laid over setup requires to do the same work usually translates into a stiffer initial setting (we're not going to throw in the by-pass shock - it's dialable for this) - so the ride quality suffers as a side effect too.

    Now we have to throw in spring rate for all this to make sense.
    Everyone knows what spring rate is, right?
    100 lb spring rate means 100 lbs moves the spring 1", 200 lbs moves it 1 more inch for a total of 2", so on and so forth. Each (additional) 100 lbs of input will cause 1" more spring compression.
    150 lb spring rate means 150 lbs moves the spring 1", 300 lbs - 2", so on and so forth.

    At full compression, perpendicular input is a direct 1:1 motion of the suspension mounting point on the arm to the shock shaft. - this multiplied by the motion ratio, at that point, will give you what's going on out on the wheel end of the suspension.

    Take a look at the graphs below next to the data chart.
    Notice the motion ratio in the 90 degree system versus the 45?

    Yes, the 90 degree system changes some as the suspension cycles - about 0.15 change. Ideally (for this writing) it would not change, but as mentioned earlier, since we're moving around a point, the angles change and you get a change in motion ratio.

    Now look at the 45 degree system - yikes! Goes from a very active (requires stiff spring rate) 5.21 down to 3.68 - a change of 1.53 - or TEN TIMES the 90 degree setup. - and the shock mounting location on the arm has not changed!!

    Also, take a look at the vertical aspect of the chart - this is the actual rate of vertical push that the shock will put into the suspension arm. Since the shock is not perpendicular to the suspension through out the entire motion, the angular loads play in (more trig!). The horizontal aspect (not shown) is put into the suspension components and is the pull that the mounting points to the frame will feel as the suspension moves.

    The other thing that the graphs show is how the motion ratios, in relation to the amount of spring pressure diverge as the suspension compresses - the more this diverges the more your rate changes - and, as you can see, the 45 degree system diverges much more than the 90. Decreasing Rate!!!

    You can see this in the two data charts - the difference among the spring rate versus the vertical aspect of spring pressure versus actual wheel pressure and motion ratio.

    Let's say the top chart shows a good setup - 100 lb rate spring - and the car's able to be held up with this setup. It starts out fairly soft, the motion ratio is fairly low and the wheel pressure is roughly 2 times spring pressure - it stays pretty consistent.

    If we were to take that same spring and put it on the other system - the car would blow through the travel quickly - motion ratio (which is leverage) is very high - just a little input on the suspension is going to be multiplied by more than 5 - and it's really soft (yes, I know - we can use a larger rate spring - hold onto that thought) - and, as the suspension compresses, the motion ratio decreases - gets SOFTER in mechanical advantage. - uhm, you want your suspension to get STIFFER...

    Okay, so let's just make it stiff enough by changing out springs for a heavier rate. If we put a 200 lb rate spring into the 90 degree system, you actually start out with a softer initial wheel pressure and a stiffer compressed wheel pressure - but the big issue is that you've done a trememdous amount of work over a droping rate system and - over a much shorter distance, shock travel wise, than the 90 degree system. 9.4 inches of shaft travel versus 5.4 inches.

    Trying to dial in a shock that works well at a 5:1 ratio is horrible - things don't like being in this range - 3 is about the max. You start breaking shim stacks and such. You have to valve very stiff to deal with the motion ratio- yet this is the soft end of the system, spring wise. The system's already fighting with itself and you're standing there wondering why the front end feels so stiff, yet blows through the travel.

    Get to the other end of motion - you've just pushed that very stiff valving, over a short distance and the motion rate has softened - so you need to have a softer valving setup - DOH!!!

    Set up so that you have good damping at full compression and, well, that darn mechanical advantage takes over down low and it just blows through the shock movement.

    You can't win!!

    The longer (more time) you can stroke a shock, the easier it will be to tune. It all boils down to making the big, bottoming out hit move over as much distance as possible for the same amount of time required to make the movement.

    Shaft speed will be higher on the 90 degree system, relative to the 45 degree setup, but it spreads this higher speed load over a longer distance - you can valve much softer and have better, smoother control over movement.

    The cool thing about shocks is that the harder you hit 'em, the harder they fight back - it's the whole idea. If you can valve light, the little stuff found at ride height will be plush - and as you increase shaft speed the valving gets stiffer and meaner. You can see how the energy put into a system during the big bottoming out is put into the shock when you think of it this way. Big hit - fast, sudden movement of the shaft, valving fights back to control the rate of compression, spring just catches the whole thing.

    Take all this and relate it to the 45 degree setup and, well, hopefully you can see the issue. Stiff valving initially, not much travel, decreasing rate.

    Yup - laid over shocks look cool, but they don't work well.






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    Good post
    #2
    Admin Gene's Avatar
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    Good post. Let's see if I learned the lesson taught here.

    First my car before changes had laid down shocks (~45*) mounted near the spindle. Shocks had 8" stroke and travel was advertised at 20. Ratio was 2.5:1 and because shocks were laid down spring rate and valving becomes increasingly difficult.

    New suspension design has at least 20" travel, maybe 22. Depends on how much camber change or bump steer occurs above 20. Replace old shocks with 12" stroke and achieve ratio of 1.67:1. Locate upper mount on frame by going to full bump and compressing shock. Install at 90* at full bump. Wherever it falls on the A-arm is the right place. Right?

    Now let's look at the rear A-arm setup. I have to mount the shock on the wheel bearing carrier. This will require me to locate the lower mount in much the same way. Full bump, compress the shock, get as close to 90 as possible, then locate upper and lower mounts.

    Right?
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    Super Moderator rowycoracing's Avatar
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    Good explanation K. Tim
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    Vendor yoshi's Avatar
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    Like almost every builder out there, I gave up some functionality for cosmetic appearances. You just can't get the perfect angle on the shocks without having huge towers or an extreamly wide front end. Is it technically correct? no, but it rides smooth. I've ridden in rails that had the shock angle as far as mine and ones that were worse and literally could not tell you were driving over rough whoops. Maybe it would ride smoother if they were "technically" correct, but they felt really smooth to me. It's all about compromise, I would give up a little comfort to look good, especially if it rode smooth as is.

    There are other things that are better that I didn't go with for cosmetic reasons that would be better if done, the v bracing in the windshield like trophy trucks run, x bracing across the entire back, more confinded cockpit area with a less padded seat that fit snug around your body, etc. If I was building a rail purely for offroad racing and wasn't trying to make money selling them, I would have everything "technically" correct because looks are not the important thing in that field but i'm not, i'm selling a product to a market that most people buy into because of looks, just like almost every other market that has consumers buying products for intertainment.

    I get so many calls from people saying how awsome my rail looks and people actually buying one based off looks alone. I have to go through and tell them what I designed into the rail to make it safe, durable, ride good, but that's not what they care about. So yes, I knowingly made the front end of my rail less functional by having them slant more but the rail still rides amazing, the 2 seater will ride even better with the added shocks....
    www.SinisterSandSports.com
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    Super Moderator rowycoracing's Avatar
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    :shock:
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    #6
    Keeper of the Asylum K-fab's Avatar
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    Gene - You got the idea.

    Yoshi - I hear you. - and just for the record, no, I wasn't pointing at your car, even slightly. - but now that I take another look... :shock: (you're not the only smartass here... :lol: )

    As I've said before - you're rail's a killer looking vehicle - top notch in, what appears to be, every way. It's obvious that you've spent a lot (two words!) of time designing and sorting it out.

    I agree - there's a point where function versus form meets and you can fall on either side of that fence. A laid down shock can work - it just takes a lot more compromising of setup. You've done one hell of a job making the two meet.

    Technology in shocks (Fox internal bypass comes to mind) will help in making the "less than perfect" designs work very well. Look at KTM and their linkless MX and Off road bikes. They're using some pretty trick stuff inside the shocks to make the valving think it's a rising rate.

    Without pushing the edge, trying new ideas and setups, nobody would ever get anywhere. - As Mario Andretti says in the movie 'Dust to Glory' - "If it can be imagined, it can be made." (or something like that). It just takes some out of the box thinking at times.
    I don't like being inside the box. Well, a refridgerator or washer/dryer box is sort of fun...

    You guys have to realize - I come from the function side of the fence. Darn Mechanical Engineer in me... I tend to over build versus make it light (desert racing mentality), play with the theory and formulae of design and such - I guess just to keep myself challenged and not go brain dead.

    I have no idea what prompted me to post what I did... It's been sort of lingering in the hollow spot between my ears for a couple days and last night, I sat down and started writing.

    I'm in the process of designing a short course car to race in the CORR Super Buggy class next year. I guess I have to get thoughts and ideas out on paper to help me direct what/where I'm heading with design.

    I'll be sharing more as I get deeper into the project. I'm hoping that I'll be able to start building this winter.

    I'll probably post more on suspension soon enough. I have a fairly different, yet sound, idea for the rear setup.

    Just got Autodesk Inventor 5 installed this afternoon. Now I have to get the basics down so I can start taking my ACAD drawings and making them into full 3-D, kinetic drawings.

    This stuff's FUN!!!!
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    Vendor yoshi's Avatar
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    I didn't think you were talking to me specifically. I just felt that anyone who read this and looked at my rail would put 2 and 2 together, and I didn't want anyone to think I didn't already know about it. It wasn't a mistake in design or an accident I did the front end the way I did, I just made a compromise, and even with that compromise I think it's gonna handle as good, if not better, than any of the top mini builders. I prob. spent more time researching suspension design than any other aspect of the build. There's nothing you said I didn't know, well, maybe the charts are a little outta my league, i'm A.D.D in case you didn't know and highly technical stuff like that is a little more than I can input sometimes. I had to read my suspension books over and over and over again before I started to understand what I was reading, it's a good thing there were pic's or I might never have picked it up :roll:
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    Post came at the right time for me..........thanks
    my shocks lay down way to far and have been wanting to change that for some time,infact I just had a conversation with belmore rail today on this same subject and will be sending him picks later tonight so we can come up with a plan

    It will be in the shop for a diff change and what better time to make the other changes (shock layout,beefier a-arms, x bracing, B pillar, V the window and corosponding braceing,etc....)

    This post helps me understand it a lot better so I will hopfully have some valued input to the design,because damn it I like the looks of the laydown but functionality is seriously compromized in my set up.
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    Vendor yoshi's Avatar
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    If you could figure out a way to make a cantilever system that didn't weigh a ton, make the a-arms look like crap and got over 20 inches of frame travel on top of the car bottoming out 3.5 to 4 inches off the ground, you would be onto something. My problem is I kept changing my desgin to improve it and I was never getting it built in the begining. I finally just had to stop myself and produce something. I can always work on other designs later and improve, but you won't have any fun if you spend 6 years designing and building one rail instead of sticking with something that works, and playing with other ideas for later projects........
    www.SinisterSandSports.com
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    I think we all agree that correct can be ugly. Suspension design has as many sound engineering ideas and applications as there are builders. Each has there own merits. That having been said I thoroughly agree with K-fab in his post even tho as yoshi said we all make compromises for whatever reason. All that ultimately matters is that they perform as expected even if not optimised.
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