Standard Suspension Bump Travel

[86•BRZ]

Ok I have been trying to work this out...

The OEM front struts have ~150mm of shaft exposed on the shock when removed from the car... You cannot extend them anymore than that off or on the car.
-this tells me the total travel is ~150mm, right?

The bump stops are ~60mm tall.
-the tell me the total usable travel for the car is ~90mm, right?

If you jack the car up, the total droop is ~85mm, front & rear, obviously this doesn't reflect the travel on the rear as the shock is nearer to the middle of the lower control arms.
-on the front though, with a near 1:1 motion ratio, that takes up most of said travel?

In my mind, ~85mm droop taken out of total ~90mm travel leaves 5mm before you are touching the front bump stops, surely this isn't right??

what am I missing with my monkey maths??

[Suberman]

Strut angle. Works the same on a McPherson strut as on a separate shock.

Wheel travel is greater than shock travel even for a McPherson strut. Otherwise you'd get no camber change.

[86•BRZ]

Cheers,
I figured there might be a few mm in that, as I said it would be close to 1:1 motion ratio.
The struts are pretty much vertical though and there is little camber increase on compression.
There can't be too much difference though??
how much do you reckon?

Example picture below shows a worst case mcpherson strut.

In my mind there is still next to no bump travel before the bump stop are hit?? especially for car with lowered springs

[86•BRZ]

simple question then... how much bump travel does the FR-S have?

[7thgear]

just wanted to add a little bit to this bit

"The bump stops are ~60mm tall.
-the tell me the total usable travel for the car is ~90mm, right?"

the bumpstops are not solid, they are somewhat compliant and will act a spring when compressed.. a spring with an exponential rate that reaches infinity after maybe 25-30mm of compression.

[Wepeel]

Originally Posted by 86•BRZ

Ok I have been trying to work this out...

The OEM front struts have ~150mm of shaft exposed on the shock when removed from the car... You cannot extend them anymore than that off or on the car.
-this tells me the total travel is ~150mm, right?

The bump stops are ~60mm tall.
-the tell me the total usable travel for the car is ~90mm, right?

If you jack the car up, the total droop is ~85mm, front & rear, obviously this doesn't reflect the travel on the rear as the shock is nearer to the middle of the lower control arms.
-on the from though, with a near 1:1 motion ratio, that takes up most of said travel?

In my mind, ~85mm droop taken out of total ~90mm travel leaves 5mm before you are touching the front bump stops, surely this isn't right??

what am I missing with my monkey maths??

Thanks for taking those measurements. I think the train of thought is correct. For front strut/wheel motion ratio, you can assume around 0.96.

As 7th said, the bumpstop is compressible, and can probably compress down to half or even 1/4 of it's free length before the rates really start to increase to infinity.

If you have those same measurements for the rears you can use 0.78 for the ratio shock motion to wheel motion.

[Racecomp Engineering]

This car is very bumpstop active....

You're very close to our measurements....about 5mm of bump travel before you start touching the bumpstops.

Drop the car an inch and you're about 20mm into the bumpstop at rest. The spring rate of the bumpstops is progressive and ramps up by that point to be fairly substantial. This is why a lot of cars with big drop springs, OEM bumpstops, and soft-ish front rates lead to understeer. The coil spring rate ratio (soft front / stiff rear) doesn't mean all that much if you are rolling in progressively firmer and firmer bumpstops and quickly overloading your outside front tire.

EDIT: this is for the front, I don't have the rear numbers off the top of my head but yes you need to use the motion ratio to get correct numbers.

- Andy

[7thgear]

what kind of a theoretical effect would trimming the hard-end of the bumpstop do on an otherwise stock setup?

[86•BRZ]

I understand the bump stops are progressive. I just wouldn't have though you would be into them after so little travel. Wow.

[mike156]

Many cars use bumpstops as a second spring now (many put the car on the bumpstop at normal ride height even). It lets them use a soft spring for normal driving then the spring package goes very progressive over larger bumps. Modern springs are wound in a manner to offset the bending load put on the strut. I wouldn't be surprised if the bumpstop approach is a way to keep strut friction down, along with soft ride rates, but then the ability to soak up big stuff.

[86•BRZ]

Thank you. I guess I always saw them as a last resort to stop the strut breaking and that they wouldn't hit that much in everyday driving.

[Suberman]

Originally Posted by 7thgear

what kind of a theoretical effect would trimming the hard-end of the bumpstop do on an otherwise stock setup?

Not theoretical but real, the shock could bottom out which blows them up eventually.

[Suberman]

Originally Posted by mike156

Many cars use bumpstops as a second spring now (many put the car on the bumpstop at normal ride height even). It lets them use a soft spring for normal driving then the spring package goes very progressive over larger bumps. Modern springs are wound in a manner to offset the bending load put on the strut. I wouldn't be surprised if the bumpstop approach is a way to keep strut friction down, along with soft ride rates, but then the ability to soak up big stuff.

Oh I don't think so. Riding the bump stops in a corner could be very exciting indeed.

You will know when you hit the bump stops on any car. If you hit them in a corner the results will be pretty exhilarating.

[Suberman]

Originally Posted by Racecomp Engineering

This car is very bumpstop active....

You're very close to our measurements....something like 5mm of bump travel before you start touching the bumpstops.

Drop the car an inch and you're about 20mm into the bumpstop at rest. The spring rate of the bumpstops is progressive and ramps up by that point to be fairly substantial. This is why a lot of cars with big drop springs, OEM bumpstops, and soft-ish front rates lead to understeer. The coil spring rate ratio (soft front / stiff rear) doesn't mean all that much if you are rolling in progressively firmer and firmer bumpstops and quickly overloading your outside front tire.

EDIT: this is for the front, I don't have the rear numbers off the top of my head but yes you need to use the motion ratio to get correct numbers.

- Andy

really? I've got to check this out, it would explain the weird handling quite nicely. Oversteer arrives at ridiculously low cornering loads. I've never experienced a car which hits its bump stops in normal driving.