Bump Steer Kits...Are They Necessary and When?

[Mark Bedel]

Hello All:

I think I may have aggravated an ongoing feud between two after market Mustang suspension suppliers with a few phone calls the other day. Anyway, let me start by saying that I'm relatively new to tracking my 2017 Mustang GT with PP. I thought I'd have a local performance shop who races Mustangs and has had lots of experience enhancing performance with other brands both foreign and domestic, add some suspension upgrades. I did some research and I think someone from this group may have suggested a supplier based in Texas for custom offset wheel-set that fit flush to the fender openings and would accommodate an extreme tire fitment of 305-30-19's in a square set up. I spoke with their head engineer several times, so I purchased this wheel-set, after researching the wheel supplier, and was told I should also purchase their camber plates to insure that the fronts would clear the fender openings, wheels are 11" x 19", and also provide camber adjustment options which I would want when tracking the car.

So, I purchased both items. Now, I must inform, that I only plan on attending three to four track events a season...at least for the immediate future...Chin Track Days, AutoInterest and HOD, etc...and most of the time this car will be on the road. My goal is to try and make the car the best handling unit I can without breaking the bank. I read also that if you're going to have camber plates installed, this would also be a great time to install different springs. Makes sense...so I did some research and settled on a set of progressive rate sport springs which lower the car about an inch, after to talking to one of this company's techs, and that they provide rates similar to a GT 350 spring when compressed, but don't beat you up during cruising. Purchased those. Of course, an email soon followed acknowledging this spring purchase and recommended the purchase of a bump steer kit, and spelled out the reasons for this.

So I did some research, and truth be told, the further I dug, the more confused I became. I get the theory of changing toe during suspension travel, etc., but when the discussions turn to suspension geometry and move more toward engineering, they kind of start to lose me. I put in a follow up call to the company that I purchased the camber plates from, since they also sell a "jig" for opening up the strut tower openings, which again, I thought that if the shop is going to have the struts out for the camber plates and springs, why not opening these up a bit since it appeared that with the suggested "street/performance" suspension settings, the strut post gets very close to the edge of the stock opening. Just wanted to make sure that I had to make the opening larger...hate cutting up a car if I don't have to. Anyway, I mentioned the bump stop kit install to this gentleman who said that I didn't need it, and that this was just away to sell more product. He said they work well on the Fox and SN 94 platforms, but that the S550 doesn't require them unless one is lowering the car 3 or 4 inches or more. The shop doing the work who races the S197 platform echos the same...don't need them sentiment for my application.

So, I sent the bump stop kit back after the call for an RA number led to an emotional rejection of why I was returning the kit, when asked who was recommending the return! I did return the kit yesterday, figuring that if it was determined that I did need it, it would be an easier retrofit after the other more invasive work was completed. Does anyone have any actual experience with this scenario? If so, any advice?

Thanks in advance!

 

[BmacIL]

The company that is steering you towards progressive rate springs only wants to sell you parts, and they have a history of doing that in spite of what the optimum setup would be for you and your wallet. You want a linear rate spring. There are several other options that are great for 95% road use that are quite competent on track. Feel free to reach out if you want some advice.

The only reason you need bumpsteer correction is if you do front geometry correction. If you stick with OEM-spec control arm points, you do not need those.

 

[JDee]

Interested to hear what people say about this, since I basically have already done exactly what you are doing and did not buy into the bump steer kit either. Particularly like to hear from people who have hard before/after data.

My car works well without bump steer kit, very predictable handling and nothing weird goes on. I would expect that if this was a real thing it would be exhibiting some oddness/unpredictability and it doesn't. Also runs times within 6 seconds of a full on GT4 at my favorite track, that's with a stock engine, so I don't think it's doing too badly.

My alignment/mechanic guy, who trained on high end Euro cars so he gets working to very many decimal places, feels there is not enough toe change to worry about. My focus is always on getting all the rubber/slop out of the suspension and driveline and I think that's a goal that is more important.

One other thought, I wouldn't call square 19X11's/305's an extreme fitment, it's very common and easily done these days. There are guys running 12s and 13s up front, now that's getting extreme!

 

[Grant 302]

I don’t think you need it on the S550. AFAIK, nobody makes a Double Tall Ball Joint Kit. And I don’t think you can, because one is inverted(?)

Anyway, unless you find you have a need to fine tune bumpsteer, you don’t need them and made the right choice to send them back.

FWIW, I absolutely hate those sorts of sales tactics. Completely unethical.

 

[Mark Bedel]

The company that is steering you towards progressive rate springs only wants to sell you parts, and they have a history of doing that in spite of what the optimum setup would be for you and your wallet. You want a linear rate spring. There are several other options that are great for 95% road use that are quite competent on track. Feel free to reach out if you want some advice.

The only reason you need bumpsteer correction is if you do front geometry correction. If you stick with OEM-spec control arm points, you do not need those.

Brian, sure...I'm all ears regarding a better Spring option...

 

[Mark Bedel]

Thanks gents for reaffirming what other's had inferred.

 

[boardkat]

Addressing only the benefit of bumpsteer correction: it makes a difference, if you find yourself driving around it.
In my case, I always noticed that after hitting a bump in competition, I was forced to immediately correct steering to remain on the line I desired - this was happening due to toe change under varying levels of compression - bumpsteer! How much? I measured!







Word of caution: if you have plans within a few hours of tackling this, be prepared to cancel them. Getting this setup, measuring, making the necessary spacer changes, remeasuring (rinse, repeat, MANY times) is a very time consuming process! But what I can say is that I took out 6mm (~3/16") of total toe OUT on the stock setup (prior to installing the link/kit), to within 1mm (~1/32") of total toe IN from static ride height -> 1.5" compression. FWIW, I'm only lowered ~1.5" from stock up front.

You don't know what you don't measure!

 

[racer47]

Addressing only the benefit of bumpsteer correction: it makes a difference......
You don't know what you don't measure!

I agree. Adjusting / correcting bump steer helps. Going from bad bump to good will make the car feel more stable and more confidence inspiring. But simply buying a bump steer kit doesn't mean that it will get installed correctly. You need to do the work to get it right. I just rechecked mine a week ago after some changes. The typical recommendation is zero to 0.015" bump out per inch of compression travel from static ride height. I would buy or borrow a bump steer gauge first and see for yourself, then decide if you need the kit or not. I'm also not a fan of variable spring rates but yrmv, sometimes they are fine.

 

[Grant 302]

Addressing only the benefit of bumpsteer correction: it makes a difference, if you find yourself driving around it.
In my case, I always noticed that after hitting a bump in competition, I was forced to immediately correct steering to remain on the line I desired - this was happening due to toe change under varying levels of compression - bumpsteer! How much? I measured!

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Word of caution: if you have plans within a few hours of tackling this, be prepared to cancel them. Getting this setup, measuring, making the necessary spacer changes, remeasuring (rinse, repeat, MANY times) is a very time consuming process! But what I can say is that I took out 6mm (~3/16") of total toe OUT on the stock setup (prior to installing the link/kit), to within 1mm (~1/32") of total toe IN from static ride height -> 1.5" compression. FWIW, I'm only lowered ~1.5" from stock up front.

You don't know what you don't measure!

True. Nice to hear some actual S550 measurements. Off the cuff that does sound better than OEM S197. Even from your 1.5” drop reference point.

My opinion is that this is perfectly acceptable for street going or dual purpose cars that aren’t setup with static toe out. And that helps with turn in.

Do you know how much you adjusted the toe back in when you dropped the front 1.5”?

 

[boardkat]

True. Nice to hear some actual S550 measurements. Off the cuff that does sound better than OEM S197. Even from your 1.5” drop reference point.

My opinion is that this is perfectly acceptable for street going or dual purpose cars that aren’t setup with static toe out. And that helps with turn in.

Do you know how much you adjusted the toe back in when you dropped the front 1.5”?

Yep, I agree - I already have a hint of static toe-out, and don't need the extra turn-in help (especially when it comes at the cost of unpredictable behavior over bumps), but the car is definitely not a DD anymore!
No idea how much I adjusted toe after initially dropping, that was waaaaaay too long ago

 

[Grant 302]

For those interested in the subject, one reason that the S550 has less of a problem (compared to S197) in this regard is because the double ball joint setup. It creates a longer ‘virtual’ arm. The longer the arm, the less horizontal change under compression and droop.

 

[JDee]

Question I have is it bump steer making the car twitchy on a bump or is it just the tire loading and unloading going up, over and down the bump? Are the two things related? How do you tell one from the other?
I get you can measure it at static in the garage, but the car is not static on the track, everything is very dynamic and a tire at or near the adhesion limit is going to do something when loaded/unloaded.

I'm not trying to deny bump steer is a thing, back in the 60s/70s I hung around with a team that built and raced F. Fords and Sports Racers and bump steer was huge with them, they were always checking it every weekend. But that's a totally different thing than our cars.

I wonder what else is going on there in that bump scenario? Bumps do not upset my car badly with my (terrible and awful) progressive springs which I like the hell right out of. Perfect DP springs, though now that the car is no longer street driven I will probably be looking at something else.

 

[BmacIL]

Bump steer creates slip angle which means it generates lateral force, and not always in the direction you want.

 

[Mark Bedel]

The company that is steering you towards progressive rate springs only wants to sell you parts, and they have a history of doing that in spite of what the optimum setup would be for you and your wallet. You want a linear rate spring. There are several other options that are great for 95% road use that are quite competent on track. Feel free to reach out if you want some advice.

The only reason you need bumpsteer correction is if you do front geometry correction. If you stick with OEM-spec control arm points, you do not need those.

Hi Brian, I would enjoy hearing what other spring options there are for my intended use. Thanks, Mark

 

[racer47]

Question I have is it bump steer making the car twitchy on a bump or is it just the tire loading and unloading going up...

Its not only for bumps. It also affects cornering on smooth tracks. Everyone specs bump out because its more stable. You turn and the outside wheel compresses. If it bumps in, that adds to the steering. If it bumps out, it takes a little away. Usually anytime get more than you want, its more unstable. A little less than you want is typically more comfortable and predictable.

My Longacre bump steer gauge in the picture is old. I've measured many cars. I don't remember any of them as being good before fixing them. It's very likely yours could be improved as well. Maybe a little, maybe a lot. Boardkat measured 3/16" bump out, that's a ton and not uncommon.

How To Measure Bump Steer And Its Effect On Toe Changes

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[JDee]

Its not only for bumps. It also affects cornering on smooth tracks. Everyone specs bump out because its more stable. You turn and the outside wheel compresses. If it bumps in, that adds to the steering. If it bumps out, it takes a little away. Usually anytime get more than you want, its more unstable. A little less than you want is typically more comfortable and predictable.

My Longacre bump steer gauge in the picture is old. I've measured many cars. I don't remember any of them as being good before fixing them. It's very likely yours could be improved as well. Maybe a little, maybe a lot. Boardkat measured 3/16" bump out, that's a ton and not uncommon.

Thanks for the explanation, easy to understand. I always run toe out in the front end, usually 1/8"-3/16" total but this spring I had it set at 1/4" total. I know it's a lot but wanted to try something different and it came from a source who has given me good tips in the past. Now I just need a track day to try it out.....

 

[TCactus04]

That's interesting. So the takeaway from this on bumpsteer kits is....If running stock to 0 toe, then bumpsteer kit is pretty useless. However if some degree of static toe out is already employed, controlling the amount of toe out added in compression could be a good idea.
Is it more about compression during a corner or actually hitting a bump?
Seems to me that cornering aspect would be more useful, and just people think of actual bumps when hearing "bumpsteer". If I'm running toe out and turn right, weight is transferred to the left front, and compresses. Then if the toe out increases on that side as result of the compression the tire is turned less, and that's the tire with the most traction.
Also any insight into situations where the car is using different sized wheel and tire heights, wouldn't that change the geometry of the control arms?

 

[racer47]

Bump steer is important regardless of static toe setting. And yes "bump" steer is a misleading name.

Oval track late models have different size LF vs RF, diff a arm lengths, only go left and yet bump steer is still important on both sides.

 

[JAJ]

There is, on another forum nearby, a thread comparing the 2020 GT500 front knuckle, the 2020 GT350R front knuckle and the original GT350 front knuckle. It's pretty clear that the 2020 knuckle will have different bump steer characteristics than the earlier knuckle because not only is the steering arm connected further forward, it's also lower down relative to the hub. Lower down is what bump steer kits do.

 

[captdistraction]

Everyone else has covered this so very well in the thread; I don't have much to add about bumpsteer on S550 specifically (I have not noted a significant amount on either of the S550 cars I've owned), however I did want to add with bumpsteer kits in general there's a very important aspect that's rarely discussed: Validating the bumpsteer kit at the intended setting does not have bind anywhere in its range of motion (steering maximums and max compression and droop of the suspension).

Not all the kits are made the same; some do not provide the proper amount of misalignment in the spherical via the flanking spacers. Others use tapered spacers to accomodate the range; however almost none mention that you should check this.

I've seen some wild failures at the track of mustang front-end suspension (on S197 primarily) that fell into one of two categories: tall ball joint failures, and bumpsteer kit failures. The bumpsteer kit failures usually were at the tapered stud, and you could find evidence/witness of the spherical body interfering with the flanking spacers and that force either bends the spherical, or snaps the hardened stud (or both). I've had a set from a particular parts pusher bend the sphericals myself (thankfully caught before anything broke). I've also seen a S550 kit from the same brand pulled off a S550 racecar that had a similar issue. Then 3 more failures at the track of another popular brand.

So look at the kits if you use them to see if they will provide the range, check the range as best you can, and keep an eye on it. I feel they become consumables when tracked, and should be replaced on a schedule (cheaper than replacing a bumper/fender/hood/splitter, etc)