## Help with spring rate.

## Help with spring rate.

(OP)

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

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

## RE: Help with spring rate.

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.

## RE: Help with spring rate.

## RE: Help with spring rate.

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.

## RE: Help with spring rate.

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.

## RE: Help with spring rate.

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.

## RE: Help with spring rate.

Usually something like this -

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

## RE: Help with spring rate.

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.

## RE: Help with spring rate.

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?

## RE: Help with spring rate.

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".

## RE: Help with spring rate.

## RE: Help with spring rate.

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.

## RE: Help with spring rate.

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?

## RE: Help with spring rate.

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.

## RE: Help with spring rate.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Help with spring rate.

## RE: Help with spring rate.

## RE: Help with spring rate.

## RE: Help with spring rate.

## RE: Help with spring rate.

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.

## RE: Help with spring rate.

## RE: Help with spring rate.

A strongly progressive linkage causes other problems, so in reality, this effect is not large within the normal range of suspension travel.

## RE: Help with spring rate.

Could you please elaborate on this? I think I disagree but need more information to understand. Thanks.

## RE: Help with spring rate.

Most modern forks use long, soft top-out springs which have the effect of raising the spring rate towards topping out so that the suspension has at least some compliance during acceleration instead of simply being hard-topped-out against the stop. But when cornering, it is operating within the softer main spring rate.

## RE: Help with spring rate.

## RE: Help with spring rate.

left overwhile braking, unless you only hit the brakes on silky smooth pavement and never do it while setting up for a turn. This is they type of thing you might as well just go out and tune at the track. There are so many different forces acting on it that all you can do at your computer is get it kinda close. You're dealing with driving style, track conditions, weight transfers, lateral grip, Center of gravity moment arm, compression dampening, rebound dampening, suspension geometry, chasis rigidity, camber, castor, anti roll bar, wind resistance, driving style, course layout, and a bunch of other things that nobody on here can get enough info about. By the time you analyze all of that stuff and start calculating the finer points of it, your competitors already have dust on their trophies"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

## RE: Help with spring rate.

Lowering also causes more of the lateral load transfer to be resisted by the springs and sta-bars, which results in greater suspension compression/extension than you'd get with the same springs and bars with the ride height set higher.

Norm

## RE: Help with spring rate.

## RE: Help with spring rate.

Norm