Bumpstop engagement and it's affect on spring rate

Trying to have the best of both worlds is a problem.   I'm a slave to fashion, but still want some functionality.  E36, it sits right where I want it.  I should raise it up a little, but, by golly I like it where it's at.

The GC kit I got for it is 375/430 (the giant lever arm of the rear suspension means it's just dandy about town, not too stiff at all).  The front is just a tad stiff for my liking - honestly, I think that combo is for the six cylinder car, mine's a 4 cyl, but I digress -  but, I can't go too much softer or the car will bottom too often.  And before I trimmed the bumpstops, ride was a little too firm up front since I was getting in to the bumpstops  too often at the current ride height (which, I don't plan to raise!  If I can help it...).

Which begs the question - aren't bump stops basically adding to my spring rate?  

Which begs the next question, couldn't I use a lighter spring to improve ride quality, and maybe use a longer/lighter or progressive bumpstop to make up the spring rate difference and keep the car from bottoming?

For instance, Let's say I make the spring rate 325lb/in, with a bumpstop rated at 50lbs/in.  Let's say first inch of travel is "free" at 325lb/in (spring only).  At that point the bumpstop is engaged. 

At 1" plus of travel, between spring and the 50lb/in bumpstop, isn't the "effective" spring rate is back to 375?  At less than 1" of travel, I reap the benefit of a softer spring, right?

Or do bumpstops not work that way?

Bumpstops are extremely progressive in rate. 

What is generally used in application like you seek is helper springs. Well, yes and no, see the link...

https://raceland.com/news/springs-101-helper-spring/

I mean, yes, you can use bumpstops as suspension springs (trust me, I race an F500 car), but there are more durable, comfortable, and effective solutions. Helper springs will also be more predictable in nature.

F500 rubber pucks are replaced about every season on a more rarely driven pure race car, so durability is a factor as well. (Using pure rubber)

I purpose to no longer trim bumpstops - I cross-drill them to make them softer.  Yes, I'm hitting them sooner, but their addition to the spring rate is more gradual than abrupt when they are softer.

OldGray320i said:

Which begs the next question, couldn't I use a lighter spring to improve ride quality, and maybe use a longer/lighter or progressive bumpstop to make up the spring rate difference and keep the car from bottoming?

You've just described the stock suspension design on the NA/NB Miata

Apexcarver said:

Bumpstops are extremely progressive in rate. 

 

What is generally used in application like you seek is helper springs. Well, yes and no, see the link...

 

https://raceland.com/news/springs-101-helper-spring/

 

I mean, yes, you can use bumpstops as suspension springs (trust me, I race an F500 car), but there are more durable, comfortable, and effective solutions. Helper springs will also be more predictable in nature.

F500 rubber pucks are replaced about every season on a more rarely driven pure race car, so durability is a factor as well. (Using pure rubber)

Um, helper springs are just to keep the main spring seated in droop.

Thats what that article says as well.

I like to think of bumpstops as a way to keep the spring rate slightly progressive at the end of bump travel of a suspension.  Without bumpstop, the spring rate will go infinite when there is hard contact under full bump.

I have heard some spec miatas tune suspension through bumpstop, but I am not familiar enough to know if that is because they are "handcuffed" by the rules, or because it is a good way to do it.

OldGray320i said:

Which begs the next question, couldn't I use a lighter spring to improve ride quality, and maybe use a longer/lighter or progressive bumpstop to make up the spring rate difference and keep the car from bottoming?

Congratulations, you're now qualified to be a suspension engineer for any of the major manufacturers.

It does work pretty well when executed correctly. But it's also really not that hard to make a stiff-ish car ride just fine with the appropriate dampers.

I have tried a dual spring setup where both springs were active at static ride height. That's basically what OldGray is envisioning. I'd have to dig up what the secondary rates were but it was in the hundreds.

It gave a nice plush ride on small movements, but was erratic because of how the rate would rise when the primary kicked in. It also had a fairly significant amount of body roll - it would roll until it would sit on the primary spring, then take a set. Because there were two different spring rates at play, it was basically impossible to get the damping right. After I'd run them for a while, I found out that usually this setup is accompanied by a limiter that keeps the softer spring locked out at static ride height. It was an interesting experiment but I ended up pulling it all off and going back to a more traditional spring.

I would be concerned about the damping problem if a set of extremely long bumpstops was used, but their progressive nature would make them a little more consistent.

Spec Miatas used to spend so much time on the bumpstops that they were effectively used as the main spring. That's less of a problem in the Penske suspension used now.

wvumtnbkr said:

Um, helper springs are just to keep the main spring seated in droop.

 

Thats what that article says as well.

 

I like to think of bumpstops as a way to keep the spring rate slightly progressive at the end of bump travel of a suspension.  Without bumpstop, the spring rate will go infinite when there is hard contact under full bump.

 

I have heard some spec miatas tune suspension through bumpstop, but I am not familiar enough to know if that is because they are "handcuffed" by the rules, or because it is a good way to do it.

You can configure dual springs in more than 1 way.  It can be a really soft helper spring just to avoid unseating in droop.  Or it can be stiffer where you have some travel left before one spring binds and stiffens the rate.  However, getting a dual (or triple) rate setup damped correctly is more challenging. 

In reply to Keith Tanner :

Even on off road stuff there's usually a crossover ring that locks out the secondary spring a few inches into compression travel. The secondary springs are mostly just there to absorb hard landings. There's no reason you couldn't make it work out on a street car but for the travel and rates it'd probably be way easier to just find an appropriate progressive rate spring.

OP:  Is there a way to increase bump travel for E36?

On Miatas for example there is a rear shock mount that will allow for a bit more bump travel (but you sacrifice the same amount of droop travel).  It can help when you are running the car low and hitting the bumpstop all the time.

On a strut car you could sink the insert further down in the tube (if using an insert).  Or you could weld the ears on higher so the suspension sits lower without changing travel.

In reply to dps214 :

They're used in circle track as well - and yeah, you lock them out. I'll have to look to see if I still have enough parts to put it back together, I think I do. Might be worth revisiting. I think I may have run out of real estate to put the lockout in place which is why I never went further.

If you start searching circle track bump stops there is a whole other world of options. Different rates, the math on how to add them, all of that. The "good" part of using bumpstops is that since they're separate from the spring stack they're purely additive in rate. That lets you tune the bump stack separately from the primary spring stack. 

Define to stiff: Does it crash over sharped edge surfaces? Skitter over wash board surfaces? Is it slow to compress and rebounds rapidly? What tire pressures do you run on the street?

It think Carroll Smith's Tune To Win book as a section on bump stop shapes and their effect. 

Those of us with F500 in effect have cars that run bump stops for suspension; the key on our cars is we have the ability to change the motion rates.

My concern would be how will this effect the balance of the car.

No clue if it's possible but I'd see if you could find slightly softer springs, say in the 350-360 range rather than 325.

I seem to recall something about 'helper' springs technically being low rate just to hold the spring in place, while 'tender' or 'assist' or 'secondary' springs have a meaningful rate contribution that may go into full-bind during the course of normal operation.

While bump stops are progressive by nature, many (most?) are actually reasonably linear (and 'low' rate) up to about 50% compression, where upon they largely knee up and go asymptotic up.

Metal coil 'bump springs' also exist.

As noted, your premise is also not wrong. In addition to many stock suspensions, I have seen more than one person go to longer/softer bump stops and improved the overall suspension function with their particular setup, after having started out shorter/stiffer (or cut down) bump stops.

I don't know about E36, but on my 128i BMW calls them 'auxiliary shock absorber' and not bump stop. That's because there is very little travel before they engage in front, and none in the rear. So they're a frequent contributor to the suspension. However, when I purchased the car, my fronts were cracking and falling apart and my rears were notably absent. So while they're presumably great for tuning the suspension within the warranty period, generally speaking I'm more in the 'just get the right spring/damping rates' camp... Which is why I've got custom spec'd coilovers waiting to be installed.

It's all a balancing act that starts with a good set of measurements and a bit of math to understand how all of the components all interact with each other.

ProDarwin said:

OP:  Is there a way to increase bump travel for E36?

On Miatas for example there is a rear shock mount that will allow for a bit more bump travel (but you sacrifice the same amount of droop travel).  It can help when you are running the car low and hitting the bumpstop all the time.

 

On a strut car you could sink the insert further down in the tube (if using an insert).  Or you could weld the ears on higher so the suspension sits lower without changing travel.

 

 

I believe that's confined to raising the car, but sounds like I'm on the right track in my thinking.

Tom1200 said:

Define to stiff: Does it crash over sharped edge surfaces? Skitter over wash board surfaces? Is it slow to compress and rebounds rapidly? What tire pressures do you run on the street?

It think Carroll Smith's Tune To Win book as a section on bump stop shapes and their effect. 

Those of us with F500 in effect have cars that run bump stops for suspension; the key on our cars is we have the ability to change the motion rates.

My concern would be how will this effect the balance of the car.

No clue if it's possible but I'd see if you could find slightly softer springs, say in the 350-360 range rather than 325.

It feels a little out of kilter with the rear stiffness and feel, and not by that much, and otherwise has no real issues. 

On a huge bump at highway speed it bottomed a little harshly, but otherwise performs just fine. 

I just have particular goals and aesthetics for what I think a particular car should be,  and spend too much time and effort to achieve it. 

But, as Hannibal Smith of  The A-Team used to say, I love it when a plan comes together. 

Driven5 said:

I seem to recall something about 'helper' springs technically being low rate just to hold the spring in place, while 'tender' or 'assist' or 'secondary' springs have a meaningful rate contribution that may go into full-bind during the course of normal operation.

I have seen tender and helper both defined in both ways (in direct contradiction), so I just use primary for the high rate and secondary for the low rate :)

Driven5 said:

I seem to recall something about 'helper' springs technically being low rate just to hold the spring in place, while 'tender' or 'assist' or 'secondary' springs have a meaningful rate contribution that may go into full-bind during the course of normal operation.

While bump stops are progressive by nature, many (most?) are actually reasonably linear (and 'low' rate) up to about 50% compression, where upon they largely knee up and go asymptotic up.

Metal coil 'bump springs' also exist.

 

As noted, your premise is also not wrong. In addition to many stock suspensions, I have seen more than one person go to longer/softer bump stops and improved the overall suspension function with their particular setup, after having started out shorter/stiffer (or cut down) bump stops.

I don't know about E36, but on my 128i BMW calls them 'auxiliary shock absorber' and not bump stop. That's because there is very little travel before they engage in front, and none in the rear. So they're a frequent contributor to the suspension. However, when I purchased the car, my fronts were cracking and falling apart and my rears were notably absent. So while they're presumably great for tuning the suspension within the warranty period, generally speaking I'm more in the 'just get the right spring/damping rates' camp... Which is why I've got custom spec'd coilovers waiting to be installed.

It's all a balancing act that starts with a good set of measurements and a bit of math to understand how all of the components all interact with each other.

 Ok, so I'm on the right track, then. 

Given the packaging constraints of the E36 struts, what I'm thinking is the best way to achieve it. 

It's got Konis on it, so I can play with the rebound a little to fine tune it, and I think the balance and handling of the car will be not terrible at all. 

If what I'm thinking is right, it would have good turn in initially and understeer a bit when pushed. 

Should be just fine for a nice daily. 

I would also recommend some zip tie tell tale experimentation. How much travel are you really using? Just driving around? Hard cornering? 

In reply to mazdeuce - Seth :

In dirt bikes we use o-rings but it's the same principal...............totally forgot about this.

If the car is really only getting into the bump stop by 1/4" to 1/2" it would be worth raising the car. On my son's LS400 we raised the front of the car 10mm and it's the perfect balance between aesthetics and performance.

Tom1200 said:

If the car is really only getting into the bump stop by 1/4" to 1/2" it would be worth raising the car.

 

Never. 

In reply to OldGray320i :

You slave to fashion you.

Tom1200 said:

In reply to OldGray320i :

You slave to fashion you.

OldGray320i said:

 Ok, so I'm on the right track, then. 

Maybe. Maybe not. That depends on whether what you think is happening is what is really happening. It may be that the bump stop is stiff enough and/or your travel great enough that you're feeling it starting with initial contact, and that a the right amount longer/softer bump stop might help. It may be that the bump stop is still soft enough and/or your travel small enough that you're feeling the 'knee' as you simply get too deep into the bump stop, and that a longer softer bump stop (with the 'knee' occurring further up in the total travel) might actually hurt. 

How much free travel do you have before contact with the bump stop at ride height? How long/firm are your current bump stops?