How does a rally suspension get extra travel?

I'm not buying a rally suspension, I'm just curious how they work. How is all of the extra travel achieved in a rally setup vs. a factory setup?

Smaller spring diameter means the tire can clear the spring and thus be a lot longer? Something vastly different going on inside the shock?

Where are the factory shortcomings?

It depends a lot on the car. In some cases, the geometry will handle more travel with just a spring / shock change. In other cases, you've gotta deal with control arm angles getting excessive or moving components to allow more up-travel without parts colliding, etc.

Raised top hats and trimmed bump stops where possible, attached external reservoirs (rules in many rally classes disallow remote reservoirs) and these days they sometimes use a shock type where the shaft passes all the way through the body (like a motorcycle steering damper). Saw an article a while ago saying that the limit to front upward suspension travel on modern rally cars is often the back of the headlight housing.

You start by finding out what limits your travel, then you address that thing I know, that sounds ridiculously oversimplified but that's really all it comes down to. It's actually really interesting looking at a well-designed suspension at the limits of travel, you discover the reasons for all sorts of little curves and divots. And then you look at aftermarket parts for that same suspension and you realize that the designers of THOSE parts have never looked at travel limits.

Oh, another thing to consider is that the extra travel jammed into the shocks mostly gets spent on extended downward travel with the assistance of helper and/or tender springs and limiting straps. That's good for soaking up jumps and keeping all the tires touching the road as much as possible. There's never much room to increase upward travel vs. stock.

If you look for pictures of a current WRC car suspension you'll notice that the shock center line is offset from the axle centerline. The shock body passes in front of the axles so that there's more available stroke. That, plus suspension arms that are longer than the stock model the racer is based on ,gives them longer travel, typically 14 inches. As stated earlier, much of that travel is extension to keep the wheels on the ground more.

Keith Tanner wrote: You start by finding out what limits your travel, then you address that thing I know, that sounds ridiculously oversimplified but that's really all it comes down to. It's actually really interesting looking at a well-designed suspension at the limits of travel, you discover the reasons for all sorts of little curves and divots. And then you look at aftermarket parts for that same suspension and you realize that the designers of THOSE parts have never looked at travel limits.

Except FM, of course.

GameboyRMH wrote: Oh, another thing to consider is that the extra travel jammed into the shocks mostly gets spent on extended downward travel with the assistance of helper and/or tender springs and limiting straps. That's good for soaking up jumps and keeping all the tires touching the road as much as possible. There's never much room to increase upward travel vs. stock.

I was able to realize increased droop in the rear of my rallyx e30 by simply spacing down the upper shock mount. compression is limited by a bump stop w/in the separate beehive spring (not a coilover). droop was limited by the shock travel. I added an inch by using some basic bushings on the mounting bolts. It's held up well for the last half of last season and the first half of this season so far. I was kind of surprised it was that easy, and it seemed to help enough I noticed.

edit: mostly my point is to reinforce what Keith said. Just look at what you're dealing with and think about it.

In reply to DeadSkunk:

DeadSkunk wrote: If you look for pictures of a current WRC car suspension you'll notice that the shock center line is offset from the axle centerline. The shock body passes in front of the axles so that there's more available stroke.

This is one of those things that has multiple purposes.

You can also get a lot of anti-dive under braking when the strut is off-axis like that. Compression wants to rotate the knuckle rearward, so the brake caliper trying to rotate the knuckle forward drives the suspension into the ground.

They also need this to counteract the high amount of pro-squat engineered into the suspension with the control arm. The forward pivot is a lot higher up than the rearward, so forward force on the knuckle also drives the wheel into the ground.

I haven't seen a WRC suspension that had high-offset struts that didn't also have high angularity on the control arm...

wvumtnbkr wrote:

Keith Tanner wrote: You start by finding out what limits your travel, then you address that thing I know, that sounds ridiculously oversimplified but that's really all it comes down to. It's actually really interesting looking at a well-designed suspension at the limits of travel, you discover the reasons for all sorts of little curves and divots. And then you look at aftermarket parts for that same suspension and you realize that the designers of THOSE parts have never looked at travel limits.

Except FM, of course.

Let's just say that we've changed some of our design requirements

Besides, travel testing makes for fun pictures. This car has 7.5" of rear shock shaft travel and 5" in the front.

heh heh... shaft travel

That coil-over offset in front of the halfshaft is cool. Wonder what it looks like behind the brake rotor. Is the knuckle shaped different to allow for that, or is the strut mounting bracket different than the traditional ears?

Keith, I get your comment. I'm just curious where companies are commonly finding the room for extra travel. I would think, assuming the same ride height, that there really isn't a way to get additional travel from a shock of the same length. You could sacrifice compression travel for droop travel or vice versa, but inside a shaft a piston can only move so far, right? This would lead me to believe that either A) body contact is a common factory limiter (but prevented by springs/bumpstops) or B) that there is internal shock travel left on the table (again prevented by springs/bumpstops).

When we replace the struts in our rallyx car, I intend to move it through its travel without the spring in place to have a look at everything, but its unlikely that would change our path anyway. Only so much you can do on a tiny budget with a car with no aftermarket support

The uprights are designed with offset built in.

In reply to ProDarwin:

From what I have seen, the struts don't have ears, they are pinched down by a 4 bolt clampy thing (engineering term). Kind of like an XR4Ti/Sierra/Taurus/Tempo/Escort/Contour/Focus (notice a trend? Friggin Ford of UK) pinchy thing but less pinchy and more clampy.

Also, Ford at least seem to service the strut/upright/axle as an assembly. Interestingly, from much YouTube investigation, the axle cup stays part of the trans, they peel the inner CV boot back and yank the axle/rollers out of the cup, then get a caulking gun full of CV grease to pump new grease in the cup when they replace the axle and just sort of roll the inner CV back over the mess.

I bet the cars are NASTY after a rally from all of the un-clamped CV boots flinging grease all over the place.

My limited knowledge pertains to GRMish privateer Mac-strut front, four/five-linked solid-axle coil-sprung rears...like most RWD Toyotas. The usual solution on the rear is a turreted setup that involves fabricating a new upper spring/shock mount a lot higher than stock (in the pic below, the factory put them just above floor height):

The front is usually a much smaller diameter and much longer spring than stock, plus different shocks with a special shaft and custom strut tops and bump stops:

vs:

ProDarwin wrote: Keith, I get your comment. I'm just curious where companies are commonly finding the room for extra travel. I would think, assuming the same ride height, that there really isn't a way to get additional travel from a shock of the same length. You could sacrifice compression travel for droop travel or vice versa, but inside a shaft a piston can only move so far, right? This would lead me to believe that either A) body contact is a common factory limiter (but prevented by springs/bumpstops) or B) that there is internal shock travel left on the table (again prevented by springs/bumpstops).

You're right, you can't get more overall travel without increasing shaft travel. Sometimes there's room in a stock size shock body for more shaft, sometimes there isn't. For example, the front shocks on an ND Miata usually (should) have room for another 2.5" or so of shaft travel - but the suspension can't take advantage of it. The rears are fully utilized.

So, if you can't get more shaft travel out of the shock dimensions, you have to make room for a longer shock. That's where you get into the turreting that ae86andkp61 mentioned. When we do Fox suspension for a new platform, for example, we find out what the limits of the suspension linkages are, then figure out how to package a shock with the shaft travel we need to reach those.

Side note - any time you're shopping for coilovers and they tout "adjustable body length!" or "independent spring preload adjustment!" - you're looking at a generic shock cartridge with an adapter screwed on the end and crap shaft travel. Stay away.

ae86andkp61 wrote: My limited knowledge pertains to GRMish privateer Mac-strut front, four/five-linked solid-axle coil-sprung rears...like most RWD Toyotas. The usual solution on the rear is a turreted setup that involves fabricating a new upper spring/shock mount a lot higher than stock (in the pic below, the factory put them just above floor height):

Mikko Kauppinen's old Starlet!

So I was cleaning the garage the other day and came across the factory struts for both the rally Bentra and my DD Saturn. Surprisingly the Saturn has more travel up front than the Nissan by about 3/4" despite having a the same size strut body. Unsurprisingly, it has 2.5" more travel in the rear - the Nissan has a silly beam axle setup with very short dampers. This is ignoring motion ratios, but on both cars the suspension attach very close to the wheel so the ratio is very close to 1:1.

Anyway, also on both cars, the strut body seems significantly longer than the shaft. 6 or 6.75" of stroke, but a 13" long body. What's a typical piston height inside the body?

Its clear after measuring that the big way to gain travel would be to put different struts in there. Assuming no coil-bind, pretty much any other mechanical interference on the rallyx car could probably be resolved with a sawzall, hammer, etc. So where does one get a strut of roughly the same size with extra stroke?

Nissan front drivers tend to have extremely low front travel, for whatever weird Nissan reason. It isn't uncommon for them to be a half inch off of the bumpstop at rest.

The strut body can't be all stroke, because there needs to be internal room for bushings (or, sometimes, roller bearings!) to guide the shaft. More bushing length means more strut strength and lateral stiffness, as does more diameter, which is why all decent rally suspension is upside-down struts (the bushing rides on the OUTSIDE and the shaft sees no side load) and IIRC they are up to 70 or 80mm now for the high end stuff and 60mm for clubman stuff.

You can't use 2.5" springs with this, obviously, but that's fine because larger diameter springs will offer more travel for the same length and rate.

I know we're mostly talking about high-end custom stuff here, but the problem i have seen lifting things with front CV axles is the passenger axle dragging on the inner control arm mount area. Not as much of a problem with cars with intermediate shafts and equal length CV axles.

Anyway, regarding axles be careful about changing stock travel. If the inner joint either extends or compresses too much, it's going to fail. I actually blew up an inner CV joint at 70mph WOT (boooost) bottoming out through a dip in one of my Caravans and the broken axle slapping around broke my transmission case. All because i had modified it to allow more up-travel than stock. Also the same reason i have torn my oil pan open but that's more of a lack of skidplating issue. #lowlife

Knurled wrote: Nissan front drivers tend to have extremely low front travel, for whatever weird Nissan reason. It isn't uncommon for them to be a half inch off of the bumpstop at rest.

Many modern cars are designed to drive on the bumpstops in hard cornering, like the Mazda2...but 0.5" seems incredibly short.

GameboyRMH wrote:

Knurled wrote: Nissan front drivers tend to have extremely low front travel, for whatever weird Nissan reason. It isn't uncommon for them to be a half inch off of the bumpstop at rest.

Many modern cars are designed to drive on the bumpstops in hard cornering, like the Mazda2...but 0.5" seems incredibly short.

A WRX STi is touching the bumpstops at rest...

Vigo wrote: I know we're mostly talking about high-end custom stuff here, but the problem i have seen lifting things with front CV axles is the passenger axle dragging on the inner control arm mount area. Not as much of a problem with cars with intermediate shafts and equal length CV axles. Anyway, regarding axles be careful about changing stock travel. If the inner joint either extends or compresses too much, it's going to fail. I actually blew up an inner CV joint at 70mph WOT (boooost) bottoming out through a dip in one of my Caravans and the broken axle slapping around broke my transmission case. All because i had modified it to allow more up-travel than stock. Also the same reason i have torn my oil pan open but that's more of a lack of skidplating issue. #lowlife

People who pay attention will pull the inner CV boot off and stroke the suspension travel to see if the axle needs to be lengthened/shortened. I'd imagine you could rejigger your control arm mounting points to minimize plunge but if people do this in the real world at the clubman level, I haven't heard of it yet.

^On that note... That's a problem I could solve when I encounter it, or add limiters back to the suspension to prevent it from happening.