How to Reduce Rear Bump Steer?
10-01-2006, 10:39 AM
#1
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How to Reduce Rear "Bump Steer"?
We all know the multi-link beam is great on smooth pavement but pretty crappy on rough surfaces. There is one road I drive frequently that has a big pavement seam in the middle of a turn and the rear wheels just skip what feels like 6 inches and unsettles the car a lot. And this is at probably 20 MPH! It's pretty pathetic.
I assume a stiffer strut setting in the rear would help reduce this, but it doesn't seem to help much. I don't have a RSB (which I think would make the rear bump steer even worse) and don't plan to get one. I have TEIN H-Techs and Illuminas. Just wondering if anyone has any tips or insight.
Old thread with some interesting discussion:
http://forums.maxima.org/showthread.php?t=167655
I assume a stiffer strut setting in the rear would help reduce this, but it doesn't seem to help much. I don't have a RSB (which I think would make the rear bump steer even worse) and don't plan to get one. I have TEIN H-Techs and Illuminas. Just wondering if anyone has any tips or insight.
Old thread with some interesting discussion:
http://forums.maxima.org/showthread.php?t=167655
10-04-2006, 06:25 AM
#2
That's not bump steer. It's just a momentary upset of grip at the rear, caused by the camber change induced in both wheels when only one hits a bump. The only way to even begin to make up for that would be to somehow increase both compliance AND grip at the rear.
In theory: Reduce unsprung weight, reduce spring rates (and damper firmness as appropriate), increase rear sway bar rate, and widen rear tires.
In practice, for your case... I have no idea. Lighter rims and calipers would be a start, but you wouldn't want to mess with any of the other stuff unless you were doing it all at the same time.
In theory: Reduce unsprung weight, reduce spring rates (and damper firmness as appropriate), increase rear sway bar rate, and widen rear tires.
In practice, for your case... I have no idea. Lighter rims and calipers would be a start, but you wouldn't want to mess with any of the other stuff unless you were doing it all at the same time.
10-04-2006, 10:32 AM
#5
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OK, I wasn't sure if it was actually what is meant by "bump steer" or not. I do have Ben's rear strut mounts so I've got a good amount of rear travel.
I've tried various Illumina settings and it doesn't seem to make much difference. I haven't tried anything about 3, though, because the roads around my house aren't good enough.
Actually, I think a lot of the unsettledness could be due to chassis flex. I remember a discussion somewhere about the 4th gen Maxima beam versus the G20 beam (which is pretty much identical) and how much better the G20 is said to feel and handle. The conclusion was that, contrary to Org lore, the beam itself can actually perform quite well on rougher surfaces if the chassis is stiff enough. The G20 has a much smaller and shorter chassis and it's probably better reinforced, both of which factors reduce chassis flex. The Maxima as a family car was designed to have pretty soft suspension and so the beam only starts to feel bad when you put stiff lowering springs on it. How does that sound for a hypothesis?
When I finally get some money I will installing subframe connectors without a doubt.
I've tried various Illumina settings and it doesn't seem to make much difference. I haven't tried anything about 3, though, because the roads around my house aren't good enough.
Actually, I think a lot of the unsettledness could be due to chassis flex. I remember a discussion somewhere about the 4th gen Maxima beam versus the G20 beam (which is pretty much identical) and how much better the G20 is said to feel and handle. The conclusion was that, contrary to Org lore, the beam itself can actually perform quite well on rougher surfaces if the chassis is stiff enough. The G20 has a much smaller and shorter chassis and it's probably better reinforced, both of which factors reduce chassis flex. The Maxima as a family car was designed to have pretty soft suspension and so the beam only starts to feel bad when you put stiff lowering springs on it. How does that sound for a hypothesis?
When I finally get some money I will installing subframe connectors without a doubt.
10-04-2006, 10:37 AM
#6
Chassis stiffness is definitely a factor, but just remember that as long as the two wheels are connected by a rigid beam, there is never a way around having one wheel get disturbed when the other one hits a bump. All you can do is try to make your car as solid and well-optimized as possible, which chassis stiffening will definitely help.
10-04-2006, 10:48 AM
#7
BTW, bump steer is when your suspension and steering geometries cause your wheel to toe in or out when the suspension compresses over a bump. Since only the front has steering, this only happens on the front wheels.
10-04-2006, 09:20 PM
#8
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Originally Posted by d00df00d
That's not bump steer. It's just a momentary upset of grip at the rear, caused by the camber change induced in both wheels when only one hits a bump. The only way to even begin to make up for that would be to somehow increase both compliance AND grip at the rear.
In theory: Reduce unsprung weight, reduce spring rates (and damper firmness as appropriate), increase rear sway bar rate, and widen rear tires.
In theory: Reduce unsprung weight, reduce spring rates (and damper firmness as appropriate), increase rear sway bar rate, and widen rear tires.
I will just add that unsprung weight is relative. The correct way is to upgrade to lighter components to decrease momentum to allow the suspension to react quicker to absorb bumps rather than transmit them to the chassis. But that's expensive, can only go so far, and is sometimes impossible. The cheap and dirty way is to add weight to the chassis. Coming back from the grocery store with a few cases of bottled water in the trunk always makes my ride silky smooth. Sad, but true.
10-05-2006, 04:07 AM
#9
I see what you're saying. You're talking about increasing the ratio of sprung weight to unsprung weight by adding sprung weight rather than decreasing unsprung weight. But, while that may work for ride quality, it won't work for handling. Sure, the ride will be smooth if you add a bunch of sprung weight, but hitting the same bump in the same corner will be worse: since the car now weighs more, it has more momentum, and that momentary loss of grip will make the rear end more likely to swing out and harder to bring back into line.
10-05-2006, 10:21 PM
#10
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I don't think that's right. The heavier the car, the less upset it is going to be by a bump of a given size. It seems to me that if I have a 150 pound load in my trunk then the rear feels much more planted and less likely to swing out.
10-06-2006, 04:33 AM
#11
Really? Because that would work over some bumps, but not all. Keep in mind, how the car feels and how the car is acting are not always the same, ESPECIALLY with this car. Your rear end could be moving around just as badly without your feeling it, especially since weight slows down and irons out the car's reactions.
The key is how much grip you have relative to how heavy your car is, and how much grip is taken away from you in the bump. If you hit one that takes your rear tires off the ground enough that you no longer have enough grip to hold the lateral load, your rear is going to skip sideways. Then, when your tires come back down, they will have a harder time bringing things back into line if there is a lot of weight back there.
This is making me think, though... Maybe the problem is that your car is skipping off because your rear end is getting tugged sideways due to the location and orientation of the pavement seam, and not because of a loss of grip. If that's the case, it might be a little more difficult to resolve because tramlining might be a factor.
Post a picture?
The key is how much grip you have relative to how heavy your car is, and how much grip is taken away from you in the bump. If you hit one that takes your rear tires off the ground enough that you no longer have enough grip to hold the lateral load, your rear is going to skip sideways. Then, when your tires come back down, they will have a harder time bringing things back into line if there is a lot of weight back there.
This is making me think, though... Maybe the problem is that your car is skipping off because your rear end is getting tugged sideways due to the location and orientation of the pavement seam, and not because of a loss of grip. If that's the case, it might be a little more difficult to resolve because tramlining might be a factor.
Post a picture?
10-06-2006, 03:37 PM
#12
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That's a good point. So I guess there is a distinction: when you hit a bump in a straight line, the heavier car will feel better and not be as unsettled since there's no lateral aspect, but when you hit a bump while turning, the heavier car has more weight to move around the turn and it may handle worse. But the bump will still not feel as bad inside the cabin.
I know my tires (235/45/17 Kumho Supras) tramline pretty bad on some rutted roads around my house, but this bump runs transversely across the road. When I hit it I am finishing the turn and probably hit it at 20 degrees off straight. And it's not just this bump (though it's the worst); there is a railroad crossing on a sharp curve that I didn't used to notice but after lowering I can definitely feel the car kicking out in the fraction of a second. First the front tires, then the rear are a bit worse. It's a fairly smooth railroad crossing but I take the turn pretty fast...maybe 40 MPH...haha I guess I take it a lot faster than I used to so that's another factor.
I know my tires (235/45/17 Kumho Supras) tramline pretty bad on some rutted roads around my house, but this bump runs transversely across the road. When I hit it I am finishing the turn and probably hit it at 20 degrees off straight. And it's not just this bump (though it's the worst); there is a railroad crossing on a sharp curve that I didn't used to notice but after lowering I can definitely feel the car kicking out in the fraction of a second. First the front tires, then the rear are a bit worse. It's a fairly smooth railroad crossing but I take the turn pretty fast...maybe 40 MPH...haha I guess I take it a lot faster than I used to so that's another factor.
10-06-2006, 06:52 PM
#13
Originally Posted by VQuick
That's a good point. So I guess there is a distinction: when you hit a bump in a straight line, the heavier car will feel better and not be as unsettled since there's no lateral aspect, but when you hit a bump while turning, the heavier car has more weight to move around the turn and it may handle worse. But the bump will still not feel as bad inside the cabin.
Originally Posted by VQuick
I know my tires (235/45/17 Kumho Supras) tramline pretty bad on some rutted roads around my house, but this bump runs transversely across the road. When I hit it I am finishing the turn and probably hit it at 20 degrees off straight.
Here's the question: Is your rear end squirrelly at all after the bump, or does it just stick when it lands (despite having landed 6 inches off)?
10-06-2006, 07:59 PM
#14
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Well I took the turn about 15 minutes ago. My Illuminas were on 3/3. I didn't take it particularly fast this time but I seemed to have a little bump steer in the front wheels. I know it's unsettling the front suspension because I was accelerating in second gear and I can hear the engine blip as the front wheels lose traction! And then when the rear wheels hit the bump, the rear kicks out to the right and that in turn makes the front feel like it's turning left. It definitely does not just stick when it lands. It's more of a dip/hump combo than just a plain bump.
10-08-2006, 10:31 AM
#15
That's not bump steer either.. bump steer is when the wheels actually toe in/out as the wheel runs through its travel. on a 4th gen, you will only have it in the front, and generally only if you've lowered the car more than an inch or so. The rear axle is fixed in toe, so you simply can't have bump steer.
on an unequal-length arm independent suspension like U13 Altima, 240SX, 300ZX, 350Z, etc you can have bump steer in the rear because of the way things pivot, but on the 3 gen Max and the B13 sentra with Parallel links, bump steer is almost nil because it can't change toe through the travel. Camber will change in the rear on the 3 gen and B13, but the toe changes very very little, if any at all.
What you're seeing in your car is just the simple fact of the car bouncing over a bump due to the suspension being too stiff and driving too fast over that particular bump.
the only way to fix that is to go to tires with a larger sidewall (use smaller wheels), drop your tire pressure, lower spring rates, and soften both compression and rebound damping.
basically, you want to go to a 4x4 type setup over a low, wide, grippy track car setup.
on an unequal-length arm independent suspension like U13 Altima, 240SX, 300ZX, 350Z, etc you can have bump steer in the rear because of the way things pivot, but on the 3 gen Max and the B13 sentra with Parallel links, bump steer is almost nil because it can't change toe through the travel. Camber will change in the rear on the 3 gen and B13, but the toe changes very very little, if any at all.
What you're seeing in your car is just the simple fact of the car bouncing over a bump due to the suspension being too stiff and driving too fast over that particular bump.
the only way to fix that is to go to tires with a larger sidewall (use smaller wheels), drop your tire pressure, lower spring rates, and soften both compression and rebound damping.
basically, you want to go to a 4x4 type setup over a low, wide, grippy track car setup.
10-13-2006, 09:16 AM
#17
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You don't have bump steer.
What you do have rear axle roll steer and rear axle compliance steer. The first is a function of the orientation of the suspension links and the effective height of the Scott-Russell link, while the latter is mostly a function of the fore/aft compliance in the trailing arm bushings (and perhaps a little in the brackets themselves). Rear toe will change, relative to the car centerline, which you might well feel. Variation in total rear toe is possible (nothing is infinitely rigid), but this should be negligible/minimal under most conditions.
Depending on the bump, I'm wondering if it is possible that the Scott-Russell link is "bottoming out" laterally as a combined consequence of lowering and bump motion and that this is what you're experiencing? Is it equally bad at the same speed and loading taken in opposite directions along the same line?
Kinematically, this suspension is not tremendously different from a stick axle RWD suspension in which the rear sta-bar is bolted directly to the LCAs instead of being U-bolted to the axle and using endlinks back to the chassis. You just don't have the drive components, anti-squat considerations, or nearly as much unsprung weight.
Stability and ultimate grip are loosely related here, rather than being tied together in a direct relationship. More sprung mass will improve the grip under transient conditions following a bump (the wheel moves down relatively more while the chasssis moves up relatively less, which affects the other three wheels' grip less). What you do sacrifice is ultimate grip and behavior (read: slip angles) as you approach that point. So as long as the additional loading is not pushing the tires noticeably further from linear response (and you're not pushing it much past the tires' linear range), you should see gains in stability. Pressed for a number, I'd guess that's at about 0.4 lateral g.
Norm
What you do have rear axle roll steer and rear axle compliance steer. The first is a function of the orientation of the suspension links and the effective height of the Scott-Russell link, while the latter is mostly a function of the fore/aft compliance in the trailing arm bushings (and perhaps a little in the brackets themselves). Rear toe will change, relative to the car centerline, which you might well feel. Variation in total rear toe is possible (nothing is infinitely rigid), but this should be negligible/minimal under most conditions.
Depending on the bump, I'm wondering if it is possible that the Scott-Russell link is "bottoming out" laterally as a combined consequence of lowering and bump motion and that this is what you're experiencing? Is it equally bad at the same speed and loading taken in opposite directions along the same line?
Kinematically, this suspension is not tremendously different from a stick axle RWD suspension in which the rear sta-bar is bolted directly to the LCAs instead of being U-bolted to the axle and using endlinks back to the chassis. You just don't have the drive components, anti-squat considerations, or nearly as much unsprung weight.
Stability and ultimate grip are loosely related here, rather than being tied together in a direct relationship. More sprung mass will improve the grip under transient conditions following a bump (the wheel moves down relatively more while the chasssis moves up relatively less, which affects the other three wheels' grip less). What you do sacrifice is ultimate grip and behavior (read: slip angles) as you approach that point. So as long as the additional loading is not pushing the tires noticeably further from linear response (and you're not pushing it much past the tires' linear range), you should see gains in stability. Pressed for a number, I'd guess that's at about 0.4 lateral g.
Norm
10-13-2006, 07:45 PM
#18
here's a better idea, call the city roadway dept and voice several loud complaints about the condition of that particular section of road.
Once the city knows of a section of road that is substandard, they are obligated to fix it. in fact they are required by law. That's part of the reason you pay taxes.
I only mention this because it sounds like a massive disruption in the pavement you are hitting with your car.
Good luck.
Once the city knows of a section of road that is substandard, they are obligated to fix it. in fact they are required by law. That's part of the reason you pay taxes.
I only mention this because it sounds like a massive disruption in the pavement you are hitting with your car.
Good luck.
10-14-2006, 05:41 AM
#19
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Even though that would be the ideal solution, fixing the roads is somewhat outside the scope of this particular forum and sidesteps the issue of a possible limitation on this individual suspension arrangement and tuning. If there's anything to uncover suspension-wise in all of this, an understanding of whatever it is it may have application in other situations.
Norm
Norm
10-14-2006, 09:01 AM
#20
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Put it this way, I never noticed the bump until I lowered the car. As I think I mentioned, I probably take turns faster on average now, so those are two factors. The bump is more of a dip/hump combo, but there's some sort of seam in there also. Maybe I'll try to take a photo but I'm not sure how useful it would be or how much I care at this point.
Thanks for the technical info, Norm! I've never studied the Scott-Russell linkage mechanism but I will when I have time.
I forgot to mention that I have 15mm wheel spacers on the rear wheels, anyone think that would make a difference?
Thanks for the technical info, Norm! I've never studied the Scott-Russell linkage mechanism but I will when I have time.
I forgot to mention that I have 15mm wheel spacers on the rear wheels, anyone think that would make a difference?
10-15-2006, 05:55 PM
#24
Originally Posted by Norm Peterson
Idle curiosity - do the trailing arms run "uphill" from the axle to the pivot centers at the chassis brackets or do they run "downhill"?
Norm
Norm
10-19-2006, 07:40 PM
#25
Norm, you certianly know your stuff when it comes to suspension and tuning of it. Fantastic. I have learned a lot from this thread allready.
With regards to my suggestion of fixing the road, I was just making a suggestion and I know that it will only fix the road, not his issue with the suspension. Some people don't know that their city has a roadway dept and they just put up with horrible roads.
Where I live, we have minimum on 100 frost cycles/year and that wreaks havoc on the asphalt. Frost heaves are a common occurance and "what the city don't know, the city won't fix".
I hope that VQuick is able to solve his suspension issue and I will continue to check this thread as it is an important issue to me also.
Again, thanks for the info.
With regards to my suggestion of fixing the road, I was just making a suggestion and I know that it will only fix the road, not his issue with the suspension. Some people don't know that their city has a roadway dept and they just put up with horrible roads.
Where I live, we have minimum on 100 frost cycles/year and that wreaks havoc on the asphalt. Frost heaves are a common occurance and "what the city don't know, the city won't fix".
I hope that VQuick is able to solve his suspension issue and I will continue to check this thread as it is an important issue to me also.
Again, thanks for the info.
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