Do Suspension Lifts Reduce Flex? (1 Viewer)

[DirtNap]

I'm curious about the effect of a suspension lift on suspension flex and wheel articulation on our 200's. I've seen it mentioned in passing before but can't remember seeing it discussed in depth. (Apologies if I missed it!)

As I understand it, in a "typical" 1.5"-3" 200-series suspension lift (excl. body lift), the lift comes from some combo of spring rate, spring length, pre-load, and perhaps the length of the strut's fixed portion. The primary impacts on geometry and articulation/flex are:

• No change to total suspension travel / range of motion
• More up travel and less down travel vs. stock since the wheel/control arms sits "lower" in the suspension's range)
• More ground clearance (forcing wheels "lower" means body is raised higher; assuming same tires)

My question is, holding all else equal, does a lift reduce the usable flex of the suspension (e.g., as measured by RTI - Ramp Travel Index*)? Seems to me that it would but check me on this:

• If a higher spring rate (and/or pre-load) is used so the truck sits higher at rest: According to "F = kx" the deflection would be less for a given loading, so we would see less up travel before the force shifts the weight and the drooping tire lifts.
• If a longer spring is used: I'm foggy on spring design, but conceptually couldn't one have a longer spring so the truck sits higher, but the spring isn't stiffer? If so, then maybe it would not affect flex/RTI?
• Whether the rear is lifted by spring rate or length, it would have less remaining down travel, so would lift off the ground even sooner than stock

What got me thinking about this was seeing this YouTube video of an RTI test on stock 2020 LC and 2020 LX570. The LC had more flex (661 vs. 588), but he had to put the AHC to "High" so the front "Sport" spoiler would clear the ramp. Assuming AHC operates basically by increasing the effective spring rate, it didn't seem apples-to-apples, and an LC lifted a similar amount (like mine) would experience the same decrease in RTI. Am I right here?

Thanks,
DN

* Ramp Travel Index: (condensed from DrivingLine article): Ramp Travel Index (RTI) indicates the amount of suspension flex. Drive one wheel up a 20 degree RTI ramp until one of the tires lifts. RTI = f(how far up the ramp, normalized by vehicle wheelbase). Flex can be increased by:

• longer shocks
• longer control arms
• sway bar disconnect
• coil compression rate (softer = more flex)
• More fender clearance (if travel is body-limited)

 

[hickuptruck]

Yes, all things being equal. A lift will have higher spring rates from the coils, so there will be more weight needed for the spring's downtravel

 

[bloc]

If the lift includes shocks that allow more down travel you may end up with increased articulation.

I had the same questions though. I then talked to Ben at filthy motorsports and he assured me in the ranges we are talking about flex shouldn’t be impacted.

That said, it is entirely possible for a suspension to have the same or even lower spring rate and provide lift if those springs are longer. Thing is most people upgrading suspension also add weight in the form of bumpers, winch, extra battery, compressor, etc, so most lifts increase spring rate at the same time.

I recently upgraded suspension, and the struts themselves allow approx 2” more down travel per corner, but the front springs are 20% stiffer and the rears stiffer as well, though I can’t remember the number. I did flex it up before the lift and intend to do so soon with the new suspension.. will post when I get that done.

 

[DirtNap]

Thanks for the replies, Hickup and Bloc. Glad to know I am at least not missing something obvious, all else being equal etc.

Bloc, I was wondering (after I posted) about some struts that say extended travel. They can't really allow more up-travel owing to the bump stops (and or the body), right? SO they must offer more down travel or droop, which could be benefit. Is there any sort of suspension geometry limit on that? I know with the 100's they often include a different drop so the axles aren't over stressed a high angles. But that might be more of an issue if the axles are in that state more or less constantly, vs. the relatively very limited time you'd see on a 200 with extended travel suspension being at the extreme extra droop.

 

[TeCKis300]

I would agree with the premise, that all else being equal, a suspension lift can result in less RTI. As pointed out earlier, aftermarket suspensions generally increase suspension rate and neutral height. These two things may conspire enough to limit up travel at the two corners that would otherwise be at full stuff with a stock suspension which is key to high RTIs.

Downtravel is a thing I've been interested in too. Off-road, down travel can matter as much to keep tires on the track, equating to more mechanical traction. I've been interested in creating a database with max downtravel depending on suspension setup. I measured a .75" gain in droop travel with OEM front spacers installed. Too bad I didn't measure hub to fender to allow that to be comparable. I'll try to get that measurement next time I lift the car. Hopefully others will contribute too.

Some other things that can improve RTI
1) track width, i.e. lower offset wheels
2) lower tire PSI and more tire sidewall for additional compression
3) more droop travel

 

[bloc]

Thanks for the replies, Hickup and Bloc. Glad to know I am at least not missing something obvious, all else being equal etc.

Bloc, I was wondering (after I posted) about some struts that say extended travel. They can't really allow more up-travel owing to the bump stops (and or the body), right? SO they must offer more down travel or droop, which could be benefit. Is there any sort of suspension geometry limit on that? I know with the 100's they often include a different drop so the axles aren't over stressed a high angles. But that might be more of an issue if the axles are in that state more or less constantly, vs. the relatively very limited time you'd see on a 200 with extended travel suspension being at the extreme extra droop.

Yes, at some point there will be a functional limit on down travel. CV angle is one consideration, but if you think through what you brought up about the fact that the increased travel will be with droop, it is also true that that scenario usually presents itself when there is less ground pressure on that corner and as a result less traction. So less load on the CV. There are people running diff drops on 200s (I think), and while I know a lot less about 100s it seems their CV angles are already more aggressive at stock height than for 200s, necessitating a diff drop much more often. In my case I limited lift to about 2”, and that is where the truck spends most of its time. I’m assuming that won’t significantly increase CV wear, and if it does that is another price to be paid for the increased ground clearance.
Conversely to down travel, up travel hasn’t been changed, and that is when CVs typically see the most load.

@TeCKis300 does trackwidth alone increase RTI or only when that width comes in the form of longer arms and resulting increased travel? Meaning wouldn’t 3” of track width from longer arms on each side be very different than merely 3” of wheel spacers that shouldn’t impact actual wheel travel at all?

Also if we know the suspension lever ratios it should be pretty easy to compare down travel as a function of shock length numbers. I’ve found hub to fender to be less than ideal for this measurement because of the square fender lip profile and potential for very different positions the vehicle can be in with the front suspension dropped out. Moving your tape measure a few inches can impact that number..

 

[TeCKis300]

@TeCKis300 does trackwidth alone increase RTI or only when that width comes in the form of longer arms and resulting increased travel? Meaning wouldn’t 3” of track width from longer arms on each side be very different than merely 3” of wheel spacers that shouldn’t impact actual wheel travel at all?

Both.

It is a limitation of the RTI method as it doesn't explicitly consider track width like it does wheel base, which can give wider rigs an advantage all else being equal. Intuitively, the longer the distance between opposite compressed tire corners (Pythagorean of wheel base and track width), the lower the chassis averages the height that the two other drooping wheels are at.

To your point, longer arms should generally lead to more suspension travel. Droop particularly, which should also enable a better RTI score.