Help with spring rate.

[V4641]

Hello,

I'm having a little trouble with a car I'm trying to setup for autocross. The issue is, I'm getting into the bump stops under braking which is hurting my braking performance. I think I've even hit the stops on hard cornering as well.

Vehicle Specs:
Total weight 3970 pounds (all fluids & driver)
Wheelbase is 112 inches
CG is 13 inches
Weight distribution is 54/46
Front unsprung mass is 117 pounds
Rear unsprung mass is 124 pounds
Maximum braking is ~1.1g
Front coilover travel is 5.5 inches and has a 1 inch bump stop

The coilover allows the ride height to be adjusted independent of spring preload.


Can someone help me out by going through the math and showing what the minimum spring rate/preload is needed to keep the car from getting into the stops under maximum braking?

I've been setting the car up so that I'm ~50% into the stoke at static ride height. I do this by setting the preload to get me in the middle of the stroke, then adjust the height of the coilover to set the ride height. Is this the correct approach with a MacPherson Strut?


Thanks,

James

 

[BrianPetersen]

So if I understand correctly, you have 2.75 inches of available compression travel of which there is a 1 inch bump stop included in that (so 1.75 inches of compression available before hitting the bump stop)?

I will assume that the geometry of the front suspension does not include any antidive. Most MacPherson suspensions don't have much.

So you have 2143 lbs on the front, of that 117 is unsprung (is this for both sides?) so 2026 lbs spring, 1013 per side.

Under braking you have a weight transfer of 1.1 x 3970 x 13/112 = 507 lbs comes off the rear and onto the front (split between both sides) so 253 lbs per side.

This is not all that much. MacPherson struts have a motion ratio of close to 1 (only affected by the angle off vertical, normally negligible). 253 lbs per side / 1.75 inches = 144 lbs per inch, is the minimum required to avoid bottoming, under static conditions. That is not very much spring rate at all.

I suspect you really have something else going on.

Nevertheless, coming from the motorcycle world, we normally set static ride height to use only about 30% of available travel, not 50%. The consequences of hitting the compression bump stop are a lot worse than the consequences of topping out. Raise it up by adding more spring preload. Try between 1.5 and 2.0 inches of static sag (this is how we measure it - distance from fully extended to nominal rider-aboard height). This will give you a lot more room before hitting that bump stop.

 

[Tmoose]

what is the rate of your current springs?

 

[V4641]

Brian - Thanks for the quick reply.

The CG height was a typo. The CG height I estimate to be 20 inches. The unsprung weights are per corner.

My front corner weights are: LF: 1095, RF: 1048


Tim, the spring rate is 325 lb/in. with ~ 0.5" of preload.


My calculations were as follows for the left side (rounded up to the nearest pound):

1095 (Sprung Weight) - 117 (Unsprung Weight) = 978 pounds

325 lb/in spring * 0.5" preload = 163 pounds

978 - 163 = 815 pounds

815 / 325 lb/in = 2.5"

5.5" Travel - 2.5" = 3" of total bump travel, 2" of travel before the bump stop.

 

[BrianPetersen]

OK, so now the weight transfer from braking is:

1.1 x 3970 x 20/112 = 780 lbs total = 390 lbs per side

so the front springs will compress about 1.2 inches under steady state heavy braking. While that's still clear of your bump stops, it doesn't leave a lot left for pavement irregularities. Also, if you are braking simultaneously with corner entrance (which we do on roadrace motorcycles all the time), you have braking load combined with cornering, and this could easily use up what you've got left.

Give it about half an inch more preload so that you have 2" in droop and 2.5" in compression available before the bump stops. The consequences of hitting the bump stop in compression is worse than that of topping out.

 

[V4641]

It feels like I'm hitting the stops, and I'm not convinced I'm not getting into them, but now I'm wondering if I could just be overloading the front tires without getting into the stops.

I see spring rate isn't taken into account for calculating the weight transfer. Help me understand why a stiffer front spring wouldn't reduce weight transfer to the front wheels under braking.

 

[Tmoose]

The change in angle of the car under braking dive has a pretty small effect on "weight" transfer, despite how it looks.

Usually something like this -
\

http://www.rallystuff.com/images/puhn_p13.gif

 

[BrianPetersen]

V4641 - Draw a free-body diagram including the moments. The spring rate doesn't come up.

Technically, yes, the movement of the suspension will shift the location of the center of gravity - but it will only affect the moments to the extent that it changes the height and fore/aft location of the center of gravity and thus that changes the instantaneous static weight distribution and the moments in that free-body diagram ... but unless you have extremely excessive suspension travel, the effect is too small to be meaningful to a first-order analysis. Front suspension compresses, rear suspension extends, first-order approximation of the effect on C of G height ... nil.

 

[V4641]

Brian, I'm not an engineer, so I have no idea how to draw a free-body diagram.

I do see that there is no spring rate variable in the equation, but it's hard for me to understand how the springs have no affect. That is, if I replaced the springs with a solid rod, would the car stop in the same amount of time as it does with my front springs? If the answer is no, then, how do you take into account the effect that different spring rates would have on braking performance?

Also, can there be a momentum affect that would allow the front to travel further into compression than the calcs would suggest if going from hard forward acceleration to hard braking?

That is, if the back of the car is 1.2 inches into compression due to hard forward acceleration, then the front suddenly swings through to 2.4 inches of compression in the front due to hard braking, would you expect the front of the car to go further into the travel than the 1.2 inches calculated?

 

[BrianPetersen]

You have to understand the difference between an idealized first-order analysis, and a full dynamic simulation including the effects of time and momentum, and "the real world".

If you have insufficient front compression damping, and you suddenly apply the brakes, yes, the forces involved will lead to downward initial acceleration of the front end, and with insufficient compression damping, that downward momentum will overshoot the amount of compression predicted by a static calculation, and the overall downward force at the tire contact patch won't be a constant (as predicted by the simplified, idealized first-order calculation) but will rather cycle up and down as the suspension collapses and extends, oscillating around the "average" which will be close to the number predicted by the first-order analysis. The driver will not be happy, and it's quite possible that on the unloaded phase of this oscillation, it will exceed the available traction and prematurely lock up the front wheels.

A simple first-order calculation cannot account for these dynamic effects. But, if you fix your suspension (by increasing the amount of damping), the driver will be much happier, the car will behave more consistently, and the amount of suspension compression will more closely approach what the first-order calculations predict, without overshooting, and stay there.

On the other hand, if you replace the suspension springs and dampers with solid rods, these compliance effects will not be present, and in the idealized world of a smooth surface, the weight transfer will instantly go to the amount predicted by the first-order calculation and stay there. Your driver will not be happy if there is the slightest bump or dip, though. The car will be forced to follow the bump, then on the other side of that bump, the upward momentum will unweight the wheel and the brake will lock it prematurely. Again, bad real-world effect, unhappy driver.

Doing a full dynamic simulation requires MUCH more detailed information than has been made available.

The real world of engineering calculations often means that a full analysis is not practical due to insufficient information - nor is it necessary when the first-order calculation produces an outcome that is "good enough".

 

[V4641]

Brian, thank you.

 

[Overrun]

A bit of a stretch, but if you’re using the height adjustment to lower the front end for competition, you’re changing camber gain, caster-trail and possible dialing in anti-dive. Anti-dive trends to stiffen the action of the suspension and may lock it up.
Most competition tuning tends to be empirical rather than math modeling –at least at my level. If you put a bit of modeling clay between the spring coils spring bind will be apparent –or not.

 

[V4641]

I don't lower the car for competition, but since we're on that subject, with a strut like the RideTech, where you can adjust ride height independent of pre-load, what is the correct way to set the ride height?

Do you set the pre-loads the same and adjust the height of the strut? Or do you set the struts the same and use preload to set the ride height? Or does it matter?

 

[BrianPetersen]

In the motorcycle roadracing world, we measure the rider-aboard sag - which is the difference in suspension height between completely unloaded (bike lifted to take the wheel off the ground), and the height that it settles to with the fully-dressed rider in his normal riding position and with normal/average fuel load. Our normal target is for this to be approximately 30% of available travel subject to fine-tuning (about 5% either way). Sag is changed by changing the preload on the springs.

The ride height has to be set to get the geometry correct. The rear swingarm has to have a down-angle in a certain range for it to behave correctly on corner exit, i.e. the correct amount of anti-squat. The steering head has to have a certain geometry for the steering feel to be correct (bike neither wanting to fall in and try to turn tighter and "wash out" the front end, nor wanting to stand up and go straight). Front ride height is set by sliding the fork tubes up or down in the clamps. Rear ride height is either by adjustable-length clevis on the shock or by adjustable-length linkages. The spring rates have to be correct so that the geometry is correct not only when the bike is going straight but also when leaned way over.

I've never worked on racing cars, but you generally are going to have antidive and antisquat effects to consider in the side view, and instant-center effects to consider in the front view, plus bump-steer and roll-steer effects that us motorcyclists don't have.

Still, the concept of having the suspension operate in the correct range of travel (sag, affected by preload) and with the correct geometry (affected by direct alteration of ride height) ought to remain applicable.

Good motorcycle racing suspensions always have separate adjustment of spring preload and ride height.

 

[GregLocock]

Brian - if you work to a defined sag from zero spring tension you are effectively setting a ride frequency target. It's easy to work out for a single spring system, not so easy for two springs

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[BrianPetersen]

Perhaps, but we've never concerned ourselves with the frequency, just to avoid bottoming and preserve the geometry in a suitable range. The frequency that this corresponds to is whatever it is, as long as we stay off the bump stops.

 

[Tmoose]

yeah, For dirt bikes of the 70s/80s at least it was a preload setting, not a spring rate setting.

 

[BrianPetersen]

You can change preload and ride height with adjustments that are built into good suspension components, but changing the spring rate requires changing hardware!

 

[V4641]

It looks like it does matter. Changing the height of the strut changes the suspension geometry. Intuitively, there must be a proper height for the strut, and the ride height set with spring preload.

 

[BrianPetersen]

Yes, and that is why good suspensions allow the ride height to be set independently of the preload.

Think of it this way; by lowering the ride height but then increasing the preload so as to achieve the same static rider-aboard geometry, you have essentially not changed the geometry but you have changed the distance to the bottom and top bump stops.

Did precisely that on one of my street bikes ... steering felt good in normal riding , but rider-aboard sag was too much, and I know it was bottoming when braking. So I established that the forks could be slid up in the clamps by about 15 mm while still ensuring that fender and wheel were not going to collide with anything with the suspension completely compressed - so I did that, and made new spacers for the springs that preloaded them by 15mm more. Result is same rider-aboard geometry, to preserve steering feel, but now the springs have 15mm further to compress before they start getting into the compression bump stops - so it doesn't happen as much. Of course, it is now closer to the topped-out travel limit, but this is of no real consequence.

My car both rides and handles better with Bilstein coil-over dampers and their matched springs, which are much stiffer than standard ... because the stock wobbly setup had the struts slamming into the bump stops all the time, and it no longer does that.