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| gkmccready | Bump vs Rebound? | ||
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Guru Newb
 
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Why do we use rebound to control the rate of weight transfer rather than bump?
Left hand turn, two options: 1. Slowing the extension of the left side shock with rebound to control the rate at which we load the right side tire. 2. Use bump on the right side shock to slow the compression of the right side shock? Theory: We use rebound to control roll rates, because we want to use bump simply to control, well, bumps. Also, controlling the rate of weight transfer is only part of the system, you still need to control forces applied to only a single wheel. If we used bump to control roll rate you'd have to run super stiff bump valving, but then if you hit a bump, it'd be very harsh, right? Shock valving seems like a huge compromise when you first think about it; the bump/rebound system to control weight transfer is much much different than the valving you would want to control individual actions on a single wheel. How does a shock tell the difference in its two roles? High speed vs low speed? Single, double, triple, quad adjustable? Anybody have a good book about shocks and valving? Bonus if it goes in to how the springs and swaybars come in to the system. The number of variables in the system seems huge and it feels an awful lot like the only way to put together something "right" is through a huge amount of testing on each individual platform. Throw in the option of limiting travel by limiting droop, or by limiting bump, and things get even more exciting. Enough rambling, anybody want to share their theories? :-) |
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Posted on: 2008/11/9 20:13
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| bogus | Re: Bump vs Rebound? | ||
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wow. ok. theories.
I emailed Gary at HardbarUSA.  As for my theory on suspensions: I prefer to have soft springs and firm shocks. It creates a more supple suspension that is much more controlled, but at the same time, able to keep traction by not losing the contact patch. If the tires hop, they ain't holding. It's the way I have set up my RC cars, too. I know, not much of a comparison, but it did work. Soft springs, firm shocks. Also slowed down the motions, making for a more predictable outcome. |
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Posted on: 2008/11/9 20:59
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| gkmccready | Re: Bump vs Rebound? | ||
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Guru Newb
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With the soft springs do you choose big anti-roll bars?
What role is each suspension component supposed to have? Aren't shocks simply there to control the springs? And springs to hold the car off the ground and provide roll resistance? Swaybars to add side-to-side roll resistance that can't [comfortably] be provided by the springs? If springs are too soft aren't you putting the shocks outside of their comfort zone? Using them to try to control the roll and not just the roll rate? I'm looking at this as an Optimal spring rate, and then a too stiff or too soft spring rate. It's all about compromise, right? Too stiff works only on smooth surfaces or with a crazy good driver. Too soft has a much bigger sweet spot for surfaces and drivers -- until you bottom out and go infinite on the spring rate. All this is outside of the whole bump/rebound and what each is supposed to control. Or how a bleed, shim stack, or blow-off come together to control things that cause different shock shaft speeds, of course. :-) I'm trying to understand the real compromise in valving; since you need to control a front/rear weight transfer, and a left/right weight transfer and those seem like they're very different things. Top that off since the two distinctly different roles of roll control (low-speed) and bumps (high-speed) ... I'm trying to come to grips with how the innards of a shock differentiate that. |
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Posted on: 2008/11/9 22:00
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| astock165 | Re: Bump vs Rebound? | ||
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Registered Vendor
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Mmmm, very interesting. My first question is would option 1:
Quote:
really help control the rate @ which the right side is loaded? I'm picturing T1 in the South Chicane @ NHMS (my home track) which is a left hander off the front straight. Even in my stock LT1 I can go into it @ about 100 mph. As I turn left the right side will be loaded regardless of the extension of the LR shock. In fact, if the reaction is so slow, I can picture picking up the LR wheel off the ground, which does nothing for grip. Given that, I'd say option 2 would be better than option 1, but still less desirable than controlling the weight transfer with the proper spring rate. That's a great question though, I'm looking forward to seeing where this thread goes. |
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Posted on: 2008/11/9 22:10
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| astock165 | Re: Bump vs Rebound? | ||
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Registered Vendor
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Quote:
I'd have to agree with that. I have the Z51 springs on my car and I can feel it being much stiffer and having less weight transfer but I know my shocks don't damp them enough. How? Because there is a particular turn (RH) that has a set of rumble strips in it. Every time I hit those rumbles the car becomes very upset and I hate to admit how many times I've spun there. I spoke to Gary about this and he suggested that the shocks can't damp the spring enough and after I leave the rumbles, the spring is still bouncing, causing the rear end to have a mind of its own, which makes sense to me. I think your above statement is dead on. Use springs to control weight and weight transfer, use the shocks to control the springs. |
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Posted on: 2008/11/9 22:17
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| ghoffman | Re: Bump vs Rebound? | ||
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Senior Guru
 
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The unloaded side is always the hardest to control.
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Posted on: 2008/11/9 23:42
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| gkmccready | Re: Bump vs Rebound? | ||
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Quote:
Okay, RF will be loaded at some rate, controlled only by the RF bump damping, but the car is a platform on four wheels. If you can't "pivot" around the CG by extending the LF while you compress the RF what's going to happen? |
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Posted on: 2008/11/10 0:12
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| astock165 | Re: Bump vs Rebound? | ||
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You would lift the L side tires, which you already know is bad. You were correct in your first post about finding the optimum spring rate. Which, the more I see and discuss, is mostly likely a progressive rate. Then just use the shocks to control the spring motion. And from what Gary said elsewhere: sounds like max travel is a good thing. |
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Posted on: 2008/11/10 3:37
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| BillH | Re: Bump vs Rebound? | ||
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The Stig Moderator
 
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I'd suggest you pick up these: Carroll Smith's "Prepare To Win" And "Tune to Win" They were published in the mid 1970's but still have the info you're looking for. Shocks have come a long ways since then but the principles are still the same. Carroll also wrote "Engineer to Win" & "Drive To Win" Reading these,numerious times, will give you an understanding of how all the components work in a system. The questions you're asking here, while good, are about individual components and their effects. You really need to understand the whole suspension system first. Almost everything that's adjustable on the system will be affected by the other components. "Bump" adjustment is and isn't about control over bumps. It has to be used with the other components. And changing bump adjustment can be done for both high speed and low speed shaft travel. That's where you get into the valving, shim stacks,etc. The main thing you try to do is maintain the tire contact patch and keep the slip angle reasonable. |
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Posted on: 2008/11/10 15:49
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| gkmccready | Re: Bump vs Rebound? | ||
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I actually have Prepare & Tune to Win, but I don't really remembering them talking about how the shocks actually work, more about how to adjust them.
It's actually those books that made me wonder why everybody, especially Corvette folks, love to throw huge swaybars on the cars. At $1000 for the Pfadt Pfatty bars, you can put some much higher rate VBP springs on the car... |
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Posted on: 2008/11/10 16:02
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| ghoffman | Re: Bump vs Rebound? | ||
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Senior Guru
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What do Smith, Penske, Gordon Murray (McLaren) know anyway? All they have ever done is win NASCAR, ALMS, F1 and Indy car races!
A guide to damper tuning can be found on pages 15-17 in the Penske manual on my website: http://hardbarusa.com/PenskeManual.pdf |
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Posted on: 2008/11/10 16:28
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| BillH | Re: Bump vs Rebound? | ||
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The Stig Moderator
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Quote:
OK, got ya. Do a search and download the Penske manual, it's really good. Diagrams for fluid travel, valve stacks and adjustment procedures. As to why they're putting on the big bars, probably because they're rubbing the tires on the fenderwells, I've seen this a lot. That dosen't say it's the correct thing to do. I think that Vettes are undersprung and I'm not a fan of soft springs and big bars. If you look at pics of the T1's in corners, they're picking up the inside rear wheels. |
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Posted on: 2008/11/10 16:35
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| CentralCoaster | Re: Bump vs Rebound? | ||
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Senior Guru
 
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But keep in mind the car is so upset to begin with because the shock was too stiff to react quickly to the rumble strip. Instead it jolted the entire car up and over the bump, and then you come crashing down, then the suspension has to bring that into control. I see a softer setup that can soak it up and allow more wheel movement leaving you better off from the rumble strips, (but it will hurt you in other places.) Maybe just run stiff and stay off the rumble strips entirely unless you need them to throw the rear end over to complete the turn, but I don't think that's going to get you consistent laps or control. |
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Posted on: 2008/11/10 17:37
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| astock165 | Re: Bump vs Rebound? | ||
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Registered Vendor
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Quote:
After comparing my shocks to others I've seen this season, it's hard for me to believe ANY part of my shock is too stiff. Could it be that the shock does not offer enough (or any) initial reaction to the rumble strip? And certianly not enough after I leave it? |
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Posted on: 2008/11/10 17:53
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| ghoffman | Re: Bump vs Rebound? | ||
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Senior Guru
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When you get it right, the rumble stips really are not there anymore. For years I maintained that a spring is a spring and defended the leaf springs. I now have to say after having them back to back with the same shocks, with a coil in the front and the leaf in the rear, the coil definitely works better. This was just for testing purposes and now I run a dual rate coil over on our Hardbar/Penske 8700's with our clevis mounts. Here is the way the car acts over rumple strips at NHMS. Note the lack of upset the car sees over the rumble strips at 3:10 and at 2:48 in the video.
http://s67.photobucket.com/albums/h28 ... =view¤t=NHIS3-1.flv |
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Posted on: 2008/11/10 20:19
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| BillH | Re: Bump vs Rebound? | ||
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The Stig Moderator
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Quote:
Is that with the 8770's ? Knocking the high speed bump back a little? |
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Posted on: 2008/11/11 0:03
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| BillH | Re: Bump vs Rebound? | ||
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The Stig Moderator
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Quote:
What brand of shocks, Tom? |
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Posted on: 2008/11/11 0:08
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| astock165 | Re: Bump vs Rebound? | ||
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Registered Vendor
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Quote:
Now from my car last year, Leaf springs with inadequate shocks. My vid starts about the same place as Gary's, going into T1: |
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Posted on: 2008/11/11 0:25
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| ghoffman | Re: Bump vs Rebound? | ||
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Senior Guru
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They are 8700 3 speed Penske's, and as you mentioned the high speed bump circuit allows the rumble strips to disappear while the low speed bump is nicely controlling the body roll. The more I dyno other brands (name any, foreign or domestic) the more I respect Penske shocks.
BTW, when I did a spell check on this thread, my Mac had a very funny suggestion for "Penske's"! |
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Posted on: 2008/11/11 13:35
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| gkmccready | Re: Bump vs Rebound? | ||
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Guru Newb
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Do all shocks have high- and low-speed bump and rebound circuits? Meaning, does a non-adjustable have all four circuits? or a single? or double? or do you really need to get in to triple and quad for the shock to actually carry all those features?
I ask because a non-adjustable, or single-adjustable will obviously have a bump and rebound. Couldn't they be built with high- and low- speed as well without adjusters? I'm starting to get the impression they're not typically? |
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Posted on: 2008/11/11 16:34
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| ghoffman | Re: Bump vs Rebound? | ||
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Senior Guru
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Well, yes and no. With a Penske 7500 or similar non adjustable shock, you can basically replicate any curve that a 8300 or 8700 could do, but dyno's do not tell the whole story. The 2 and 3 speed shocks are analogous to having a carburetor with idle, main jets and power valves, in that they have a wider bandwidth. For a single condition, single track situation, a non adjustable can be optimized for those conditions, just as a carburetor on a single speed generator engine does not need any more than a single carb jet. This is what most F1 and Cup teams do. They can characterize the track with dozens of springs and adjustable shocks, get it correct in testing and then go with the single point solution for the race. For those that have broader circumstances, there is only so much that that can be done with a non-adjustable, albeit with the 7500 it is a very good solution for a large % of the people out there.
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Posted on: 2008/11/11 17:22
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| BrianCunningham | Re: Bump vs Rebound? | ||
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Ouch! Turn 3 is not a good place to get loose! |
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Posted on: 2008/11/24 0:20
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