Solid rear axle mod?

[stadlxr]

So the other day I saw a post, can’t remember if on instagram or the forum of a modification to the rear axle. The user welded a plate on the under side of the rear axle. (Think of our axle as a U shape and then welding a plate along the open side)

I can’t remember where I saw this and want more info as I’m interested in doing this myself to reduce twist / torsion in the beam and add more stability. Any guidance or images would be appreciated

 

[stadlxr]

Bump?

 

[kivnul]

I have not heard of it being done, but it should work in increasing stiffness. I would be worried about long term longevity though: Currently the whole length of it flexes, if you were to stiffen up only part of it, it will focus the stress right where your added plate ends.

There are (at least in the USA) for sale stiffeners that bolt up inside the torsion beam that seem to work well and I have not seen any failures. An added bonus: if you do not like it, they are easy to take off. If you add your plate, you will need to do a very thorough welding job to get it to stay in place, making it rather permanent.

Option 1
Option 2 (what I use)

Previous Conversation on this exact topic

 

[Dialcaliper]

I’d be fairly cautious going that route on this car. It was fine on older cars with fairly wimpy anti-roll in the torsion beams, but given this car actually has fairly stiff roll control for a factory vehicle, you’ll end up with something a lot stiffer than you’re probably expecting. The open U-section is what makes the beam flexible enough to act as a sway bar, but actually boxing the whole thing in will essentially give you similar effect to a tubular sway bar the diameter of the beam, so imagine installing a 75mm sway bar on the car.

The stock ST “U” beam stiffness is roughly equivalent to a 20-22mm bar. The stiffest available aftermarket rear bar (22mm) roughly doubles the stiffness. Due to the effects of cross section scaling as roughly a 4th power of diameter, welding the whole thing solid would (theoretically) would be over 100x stiffer. In reality you won’t quite achieve that, especially if you just tack a plate at the ends and a maybe a few places in the middle, but besides inducing fairly excessive oversteer, you’ll essentially be moving closer to matchbox car suspension in the rear, meaning it will be hard to keep the inside rear tire on the ground over any kind of road disturbance, making the car fairly unstable in the rear. Oh and ride like crap.

That is to say what you’re suggesting worked on older cars with very small torsion beam connections and poor roll control, but is not a great idea here.

 

[NOVA_Ranger]

I'll pass on trying to outsmart Ford engineers on this one, haha.

 

[LilPartyBox]

^ what @Dialcaliper said. Baaad idea. I'm running the tiny torsion bar inside the beam and anti-sway bars. It's all we need on these cars if ur looking for a more playful rear end. I don't even wanna imagine what a handful it would be if it were way stiffer than it already is.

 

[Capri to ST]

I'll pass on trying to outsmart Ford engineers on this one, haha.

I agree with you on that point. Here's a suspension overview video with one of the Ford Performance engineers who worked on the car. As I watched it I was impressed with the amount of thought and detail that went into the design of the car and the modifications they made from the base Fiesta.
Here's a quote from it that I pulled out showing specifically what their intentions were for the twist beam. The fact that they pretty carefully designed it the way they wanted it would make me very reluctant to do the modification that you're talking about with the dramatic increase in stiffness which @Dialcaliper described above.
"The [REAR] twist beam is about 75% stiffer, and the twist beam on a twist beam car it's like your stay bar, it's your roll stiffness. So the stiffer the Twist beam is, it's like putting a bigger stay bar on it. There’s no extra stay bar.. we did it all on the beam."

View: https://www.youtube.com/watch?v=M7YKS9CnpJg


When he talks about a stay bar, I think he's referring to what I call a sway bar

 

[stadlxr]

Genuinely appreciate all the responses gents. Got a white line ARB and a swave summit style mounts being made right now

 

[Fusion Works]

Ford increased the beam stiffness of the Mk 8, substantially over the Mk 7 to 1400NM/deg. (still looking for where I found the OEM beam stiffness for the Mk7 cars

 

[Dialcaliper]

Ford increased the beam stiffness of the Mk 8, substantially over the Mk 7 to 1400NM/deg. (still looking for where I found the OEM beam stiffness for the Mk7 cars

I managed to find info to back calculate equivalent sway bar rates for the different years of torsion beams based on various article sources of "increased by X% over the previous". I'd have to hunt again to find the original sources. I'm sure that introduces some error somewhere, but it comes out in the right ballpark

The increases over Ford's various revisions are as follows:
Base Fiesta -> Early Mk7.5 ST = +70%
Early Mk7.5 ST -> ST200/Late Mk7.5 = +10%
ST200/Late Mk7.5 -> Mk8 ST = +8%

For a point of reference, a 22mm rear "bolt-on" bar like the adds roughly 55-65% to the Mk 7.5 ST's rear beam stiffness. The "torsion beam stiffnener" like the Pierce is harder to quantify, but my best estimate is that it adds on about another 20-25% to the "stock" beam

 

[Dpro]

I managed to find info to back calculate equivalent sway bar rates for the different years of torsion beams based on various article sources of "increased by X% over the previous". I'd have to hunt again to find the original sources. I'm sure that introduces some error somewhere, but it comes out in the right ballpark

The increases over Ford's various revisions are as follows:
Base Fiesta -> Early Mk7.5 ST = +70%
Early Mk7.5 ST -> ST200/Late Mk7.5 = +10%
ST200/Late Mk7.5 -> Mk8 ST = +8%

For a point of reference, a 22mm rear "bolt-on" bar like the adds roughly 55-65% to the Mk 7.5 ST's rear beam stiffness. The "torsion beam stiffnener" like the Pierce is harder to quantify, but my best estimate is that it adds on about another 20-25% to the "stock" beam

This is a cool reference and is interesting though according to Ford themselves if I recall correctly our rear beams on late MK 7.5 are actually 30 percent stiffer not 10% like you calculated . Also the front sway went from a 19mm to a 21mm which is easily a 30% stiffer bar as well and this was in direct relation to softening of the dampers and springs for the sake of ride comfort after numerous complaints of stiff ride on both sides of the pond. So ya I question the 10% calculation. That seems drastically minimal on Fords part especially when they upped the original beam from the base Fiesta by 70% by your calculations.

 

[Dialcaliper]

This is a cool reference and is interesting though according to Ford themselves if I recall correctly our rear beams on late MK 7.5 are actually 30 percent stiffer not 10% like you calculated . Also the front sway went from a 19mm to a 21mm which is easily a 30% stiffer bar as well and this was in direct relation to softening of the dampers and springs for the sake of ride comfort after numerous complaints of stiff ride on both sides of the pond. So ya I question the 10% calculation. That seems drastically minimal on Fords part especially when they upped the original beam from the base Fiesta by 70% by your calculations.

I'm not 100% sure of the 10% rear as I've found different articles saying different things about the ST200 changes - specifically the torsion beam at 10%, 27% and 33% and none that seem to agree. In either case it "softened" up the car a bit. Softer springs, stiffer sway bars, Net effect being less harsh ride with less body roll and slightly tamed (but still present) liftoff oversteer - the earlier cars are more stiffly sprung and a bit more tail happy, but with slightly more overall body roll.

I found "27%" from a "Ford media" article here, so I may need to review my numbers (the 10% might be "overall" increase in the rear):
https://media.ford.com/content/ford...hatch-arrives-in-europe--fiesta-st200-di.html

Also, reading carefully the "1400 Nm/deg" claims for the Mk8, I'm not actually seeing anywhere where Ford is claiming an increase over the previous generation...

https://media.ford.com/content/ford...ffers-limited-slip-differential-and-debu.html

Either way I might need to revise my numbers slightly

Keep in mind that roll stiffness as it affects understeer/oversteer is also influenced by weight distribution - with 60% of the weight on the front axle, adding roll stiffness in the front has less effect, and sway bars only tell part of the story

Once you put the "whole package" together it kind of makes sense - attached below is what I come up accounting for springs, sway bars and also weight distribution as well as roll centers and cg height.

"Front Lateral Load Transfer Distribution" is the most informative here - basically the ratio of front/rear roll stiffness compared to front/rear axle weight distribution. There are error bars here, but generally, FLLTD between 0.9 and 1.1 is a more or less "neutral" setup. Less than 0.9 is steady state oversteer, and greater than 1.1 is steady state understeer. All cars also have a tendency under liftoff to unload the rear axle, resulting in "liftoff oversteer"

What I come up with for the stock setup is ~0.79 for the early cars, and 0.84 for the ST200 - both still in the realm of "mild" oversteer and a fairly subtle difference in behavior between the two. Popping a stiff rear sway bar on either version brings you down to 0.7 or lower, into the realm of fairly notable oversteer bias which some people like and some people don't. In no case do most spring and sway bar adjustments result in actual steady state understeer, with the exception of throwing on a big honking 25mm front sway bar.

 

[Dpro]

I'm pretty sure about the 10% rear as it was quoted in an article about the ST200 changes that "softened" up the car a bit. Softer springs, stiffer sway bars, Net effect being less harsh ride with less body roll and slightly tamed (but still present) liftoff oversteer - the earlier cars are more stiffly sprung and a bit more tail happy, but with slightly more overall body roll.

Keep in mind that roll stiffness as it affects understeer/oversteer is also influenced by weight distribution - with 60% of the weight on the front axle, adding roll stiffness in the front has less effect, and sway bars only tell part of the story

Once you put the "whole package" together it kind of makes sense - attached below is what I come up accounting for springs, sway bars and also weight distribution as well as roll centers and cg height.

"Front Lateral Load Transfer Distribution" is the most informative here - basically the ratio of front/rear roll stiffness compared to front/rear axle weight distribution. There are error bars here, but generally, FLLTD between 0.9 and 1.1 is a more or less "neutral" setup. Less than 0.9 is steady state oversteer, and greater than 1.1 is steady state understeer. All cars also have a tendency under liftoff to unload the rear axle, resulting in "liftoff oversteer"

What I come up with for the stock setup is ~0.79 for the early cars, and 0.84 for the ST200 - both still in the realm of "mild" oversteer and a fairly subtle difference in behavior between the two. Popping a stiff rear sway bar on either version brings you down to 0.7 or lower, into the realm of fairly notable oversteer bias which some people like and some people don't. In no case do most spring and sway bar adjustments result in actual steady state understeer, with the exception of throwing on a big honking 25mm front sway bar.

I can attest to at least throwing on a Hotchkis rear bar it created sudden or snap oversteer in high speed corners with camber plates . I am sure without it would have reacted at lower speed which is probably why an autocross guy without camber plates might like it and why one with camber plates might not feel it’s needed. As it is I do run camber plates for grip in corners that allow me higher corner speeds without sliding . Sliding is fun but not the fastest way around a corner especially at higher rates of speed. I find the way the stock suspension slid was fun again though when going for faster corner speeds it seems like most these day are running negative camber in the rear be it through shims or plates and are not running rear sways. The debate still goes on about rear sways but I think had Ford felt them as needed with the amount of time they took trying to tune our stock suspension they would have included them if needed. Plus if one changes to coilovers it totally jiggers your figures and averages. Anyways interesting stuff and I honestly do not feel our rear beams need any more stiffening . If one feels they do toss on a swaybar and see if you like it. I prefer my tail wagging to be underpower not off power aka RWD drifting. It’s just nice that FiST drives more like that kind of vehicle than other FWD cars.

 

[Dialcaliper]

I can attest to at least throwing on a Hotchkis rear bar it created sudden or snap oversteer in high speed corners with camber plates . I am sure without it would have reacted at lower speed which is probably why an autocross guy without camber plates might like it and why one with camber plates might not feel it’s needed. As it is I do run camber plates for grip in corners that allow me higher corner speeds without sliding . Sliding is fun but not the fastest way around a corner especially at higher rates of speed. I find the way the stock suspension slid was fun again though when going for faster corner speeds it seems like most these day are running negative camber in the rear be it through shims or plates and are not running rear sways. The debate still goes on about rear sways but I think had Ford felt them as needed with the amount of time they took trying to tune our stock suspension they would have included them if needed. Plus if one changes to coilovers it totally jiggers your figures and averages. Anyways interesting stuff and I honestly do not feel our rear beams need any more stiffening . If one feels they do toss on a swaybar and see if you like it. I prefer my tail wagging to be underpower not off power aka RWD drifting. It’s just nice that FiST drives more like that kind of vehicle than other FWD cars.

I edited the previous post, as I'm no longer sure about the 10% rear thing and might need to redo some numbers. The actual case may be a 27% increase from early to ST200, and maybe no actual change to the rear beam itself with the Mk8 (possibly a mild change in spring rate)

In any case, I had a similar experience with the rear sway bar - hotchkiss 22mm rear (early 2016 with small front sway bar) by itself was too squirrelly and I spent too much effort just wrestling with the rear. After upgrading the front to a 22mm (7/8" not 21mm), I've got something that's feeling pretty good now. Throttle steer, but without the unruly rear.

In any case, one surprising thing I've noticed from the "math" side is that even stock, roll stiffness is exceptionally high for a factory vehicle. So much so that trying to affect understeer/oversteer by changing spring rates is actually pretty ineffective and its all in sway bar tuning, meaning spring rates are mainly for tuning ride frequency.

 

[Dpro]

I edited the previous post, as I'm no longer sure about the 10% rear thing and might need to redo some numbers. The actual case may be a 27% increase from early to ST200, and maybe no actual change to the rear beam itself with the Mk8 (possibly a mild change in spring rate)

In any case, I had a similar experience with the rear sway bar - hotchkiss 22mm rear (early 2016 with small front sway bar) by itself was too squirrelly and I spent too much effort just wrestling with the rear. After upgrading the front to a 22mm (7/8" not 21mm), I've got something that's feeling pretty good now. Throttle steer, but without the unruly rear.

In any case, one surprising thing I've noticed from the "math" side is that even stock, roll stiffness is exceptionally high for a factory vehicle. So much so that trying to affect understeer/oversteer by changing spring rates is actually pretty ineffective and its all in sway bar tuning, meaning spring rates are mainly for tuning ride frequency.

Ya that 27% is much closer and more inline with the 30% I read and seems to make more sense. Yes the roll stiffness of our cars stock is insanely high it’s why the car lifts a rear wheel in the air like a dog peeing under hard corners. My buddy managed to tune the ride frequency to ideal rates with his MeisterR Zeta3’s. He did his own corner weighting.

 

[dhminer]

I edited the previous post, as I'm no longer sure about the 10% rear thing and might need to redo some numbers. The actual case may be a 27% increase from early to ST200, and maybe no actual change to the rear beam itself with the Mk8 (possibly a mild change in spring rate)

In any case, I had a similar experience with the rear sway bar - hotchkiss 22mm rear (early 2016 with small front sway bar) by itself was too squirrelly and I spent too much effort just wrestling with the rear. After upgrading the front to a 22mm (7/8" not 21mm), I've got something that's feeling pretty good now. Throttle steer, but without the unruly rear.

In any case, one surprising thing I've noticed from the "math" side is that even stock, roll stiffness is exceptionally high for a factory vehicle. So much so that trying to affect understeer/oversteer by changing spring rates is actually pretty ineffective and its all in sway bar tuning, meaning spring rates are mainly for tuning ride frequency.

Which front did you go with? I have a Hotchkis rear bar but not yet installed it. Sounds like I need to do it and the front at the same time.

 

[Fusion Works]

A stock beam axle Volkwagon will lift a wheel like that as do most FWD platforms it as little to do with the roll stiffness of the suspension. Its a FWD thing.

What is the thickness change of the beam material over the life of the Fiesta models? Base model? 2013-2016, then late 2016 onto Mk8?

I know there is a reference for the thickness changes in the tech specs? Early cars were 7.5mm thick blade, later model US spec was 8.5mm, what was the final on Mk8? The mid year change was an 11.7% increase in beam stiffness.

 

[Dialcaliper]

Which front did you go with? I have a Hotchkis rear bar but not yet installed it. Sounds like I need to do it and the front at the same time.

I used Ultra Racing’s 22mm front bar (Originally for meant for the Mk6 ST, but the suspension components are identical).

If you’ve already got a 21mm front bar, the swap might not be worth it (dropping the front subframe). 21mm OEM is a viable option. I believe the base Fiesta also uses a “true” 22mm bar (not 7/8”) that might fit but I didn’t manage to locate one or try it.

The rear is an easy swap, I’d install the Hotchkiss first and try it out - some people like it alone but I was not a fan. If you don’t like it, it’s an simple removal - doing the front is a big job.

If you don’t feel like tackling the front and have a 19mm front sway bar, I would recommend at most using a 19mm rear bar like UR/Hardrace or DNA’s fixed bar (nonadjustable) instead or perhaps just a beam stiffener (Pierce/TB, etc) - for an even milder stiffening in the rear

Specifically what I didn’t like is that I couldn’t trail brake at all, and needed to be on light throttle before even starting turn-in or the rear end would step out and tail-wag, sometimes actually breaking loose. A very unforgiving transition going into the corner unless you’re trying to “drift” a FWD car.

End result was slower corner entry and less lateral grip - having to be on-throttle too early in the turn.

With the current setup, I feel like I can carry more speed into the corner and brake later. Still enough oversteer to throttle steer, but the rear is stable enough that I get to decide the timing to add throttle and steer with my foot. The rear end just feels more stable.

 

[dhminer]

I used Ultra Racing’s 22mm front bar (Originally for meant for the Mk6 ST, but the suspension components are identical).

If you’ve already got a 21mm front bar, the swap might not be worth it (dropping the front subframe). 21mm OEM is a viable option. I believe the base Fiesta also uses a “true” 22mm bar (not 7/8”) that might fit but I didn’t managed to locate one or try it.

The rear is an easy swap, I’d install the Hotchkiss first and try it out - some people like it alone but I was not a fan. If you don’t like it, it’s an simple removal - doing the front is a big job.

If you don’t feel like tackling the front and have a 19mm front sway bar, I would recommend at most using a 19mm rear bar like UR/Hardrace or DNA’s fixed bar (nonadjustable) instead or perhaps just a beam stiffener (Pierce/TB, etc) - for an even milder stiffening in the rear

Specifically what I didn’t like is that I couldn’t trail brake at all, and needed to be on light throttle before even starting turn-in or the rear end would step out and tail-wag, sometimes actually breaking loose. A very unforgiving transition going into the corner unless you’re trying to “drift” a FWD car.

End result was slower corner entry and less lateral grip - having to be on-throttle too early in the turn.

With the current setup, I feel like I can carry more speed into the corner and brake later. Still enough oversteer to throttle steer, but the rear is stable enough that I get to decide the timing to add throttle and steer with my foot. The rear end just feels more stable.

Appreciate the details! I have a later 2016 with sync 3 so I’m thinking it’s the 21mm but will have to get the ol caliper out to verify.

 

[Dialcaliper]

Appreciate the details! I have a later 2016 with sync 3 so I’m thinking it’s the 21mm but will have to get the ol caliper out to verify.

You might be able to guess initially by the build date on the sticker in your drivers doorsill. The suspension changeover was mid Feb 2016 (02/16 or later on the sticker, 2016 models were built July 2015 to June 2016 or something like that).

Some of the changes were rolled out earlier - mine is a 2016 with a Jan 2016 build date. It has (had) the updated 2-port evap line and Sync 3, but still came with the older model suspension, and no wind deflectors by the side mirrors