Do I need a bump steer kit ?

[RS-GT]

Hi,
I will install the spacers under the sterring rack, but what will tell me if I need a complex bump steer kit (the one who replace the rod ends : +300 $) ?

Is there a minimum ride height or what is the maximum angle I should have on the steering rack arms compare to the horizontal plane ?

The car is a narrow 72 with 205/55 and 225/50 on 7R in the back

Thanks !
JD

[Kevin Stewart]

the oly way to do it correctly is a bump steer guage, Kevin

[RS-GT]

What does a bump steer gauge looks like ?

JD

[slodave]

What ride height are you at now? Are you having problems with bumps now?

[Bill Verburg]

Originally Posted by RS-GT

Hi,
I will install the spacers under the sterring rack, but what will tell me if I need a complex bump steer kit (the one who replace the rod ends : +300 $) ?

Is there a minimum ride height or what is the maximum angle I should have on the steering rack arms compare to the horizontal plane ?

The car is a narrow 72 with 205/55 and 225/50 on 7R in the back

Thanks !
JD

Under a static load as on an alignment rack the steering arms should be horizontal

[Kevin Stewart]

a bump steer guage bolts to the hub with the wheel off with the car sitting at ride height. you jack the hub up so it sits at wheel heigth, you then have a dial indicator on the front and rear side of the hub, you move the hub up 2 and below zero 2inches, the indicator should read zero or real close if not going into a corner the toe changes. the cause of bump problems is when the tie rod link is at a different angle then the lower control arm. if you have a bent spindle at the tie rod end area this will cause ackermen problems which is not checked by a bump guage. Kevin

[Gunter]

Originally Posted by RS-GT

Hi,
I will install the spacers under the sterring rack, but what will tell me if I need a complex bump steer kit (the one who replace the rod ends : +300 $) ?

Is there a minimum ride height or what is the maximum angle I should have on the steering rack arms compare to the horizontal plane ?

The car is a narrow 72 with 205/55 and 225/50 on 7R in the back

Thanks !
JD

What height are you aiming for?
Front fender lip to ground how many inches?
Rear fender lip to ground " " " ?

New turbo tie rods are recommended if your stock tie rods are worn.

Bump steer kit are just spacers under the rack and it helps to keep the tie rods horizontal.
When you install the spacers, loosen the U-joint on the steering column located in the smugglers box so you can push the rack up.
Loosen the existing shorter bolts for some slack, then install the longer bolts one at a time to keep the rack in position.

[RS-GT]

I already have the spacer under the rack, but i was talking about this kit :



I didn't decide the final ride height now, I'm still rebuilding and buying new parts to continue my project, I was asking this to understand little more about steering geometry and if I must plan 300 buck on that. So I can understand that this kit is not popular on street ride ?! but maybe just on very low racing car ?

JD

[Plays with cars]

As Bill said: the rule of thumb is tie rods should be horizontal (or very slight dowhill from rack to the wheel hub), if they are running up hill you will have problems.

Bump steer gauge would be ideal, and absolutely required on a car w/ unknown suspension geometry. But the 911 has been around enough and enough people have messed with them, that the rule of thumb is a reliable way to go.

[Kevin Stewart]

i am not sure i agree, the tire diameter-ride height-and rack height all play a part, if your looking to get close or near then a guess with what others have done will be good, but taking50 thousand off can creat a toe in under turning. but a guess or doing something simular to others will better then nothing, raising the rack will get a rough place for the bump, the tie rod end kit will perfect it, I am not really sure you need it unless you plan to track it, and if you do track it and you have some darting problems then would be the time to try it, Kevin

[911st]

My feeling is no, you should do not need more than the rack spacers unless you raise your spindles on the struts. It dose not matter how far you lower you 911.

However, most of these guys know more than me.

I believe it is not the angle of the tie rod relative to the ground but to the A arm. If when in a turn the tie rod and a arm are close to parallel then they will move at the same arcs and not contribute to noticeable bump steer. Again, that is in a turn, not sitting still with the wheels pointed straight.

Lowering the front in effect makes the a arm longer and increases the total at rest neg camber.

In my opinion, lowering a stock suspension on a 911 more effects the front camber curve. That is if to low, there will be very little gain in neg camber with further compression in a turn.

This is not an issue with the rear because of its design. It continues to gain neg camber with compression. 911 Turbos gain neg carber in the back even faster.

Improving the camber curve is the best of several reasons for raising your spindle. This improves the level of camber gain with compression. However, if you do this you will put your self at risk of increased bump steer as the difference in the arc of the a arm and the tie rods will be more acut.

This is when you will most need the extension style bump steer kit.

Agin, these guys know more than me. On my 911's I do as Bill and Smart Racing suggests and keep the a arm at a slight up slope toward the rack or level and never have had any noticeable bump steer.

[Flieger]

It is more the change in toe than the change in camber. Toe change is affected by the relative movements of the tie-rod to the A-arm (different arcs).

If the A-arm is pointing down and the tie rod is level (stock suspension setting), then under compression, the hub/A-arm moves out but the tie rod gets shorter. Therefore, the toe turns out under compression. This is bump-understeer.

If you lower the car: A-arm level, tie rod up, zero toe-alignment at this ride-height.
Under compression, the tie-rod still causes toe-out.

If you add dropped tie-rod pickups, the tie rod will not get shorter at a faster rate than the A-arm. Therefore, less toe-out bump steer.

Theoretically, you could make the tie rods angle down while the A-arm was level. This is bump over-steer. The toe turns inward on the wheel in bump, causing steering input tightening the turn.

Camber effects complicate the severity of the bump steer.

[Bill Verburg]

Originally Posted by 911st

....
I believe it is not the angle of the tie rod relative to the ground but to the A arm.

Yes & No, at static loaded rest the steering arm is presumed to be at the mid point of it's travel range and this is ideally the point were camber and toe changes are linearly correlated to wheel travel, out of this sweet spot the changes in camber and toe are non linearly correlated to wheel travel and cause excessive extraneous steering inputs to be even more undesirable. Ideally the steering arm and lower control arm will always be parallel and move thru the same arc minimizing camber & toe changes(which cause bump steering) but that can't happen because the pivot points for the 2 arms are in different planes both horizontally and vertically. So you can only minimize the difference in arcs over the relatively narrow sweet range of travel, to limit travel you use big springs and stiff shocks, to minimize arc difference you start in the middle of the travel range and start w/ parallel to the ground for the steering arms and as close to that as possible for the A arms.

Here is a '73RS that could stand to be lowered a tad, you can see that the steering and A arms are at different heights and have different pivots so they move in different arcs, a washer type bump steer kit will lower the steering arm pivot to be nearer the plane of the A arm pivot but they will never coincide.

Originally Posted by 911st

If when in a turn the tie rod and a arm are close to parallel then they will move at the same arcs and not contribute to noticeable bump steer. Again, that is in a turn, not sitting still with the wheels pointed straight.

??? bump steer is caused by a vertical suspension movement on one side of the car that cause the wheel on that side toe toe in or out causing a corresponding change in vehicle yaw, in a turn the same effects are still in extent, but now the turn is causing one wheel to move up and the other to move down, one wheel is trying to turn more than the other,the 2 wheels will steer in different arcs because of built in ackerman and because of the camber/toe changes caused by wheel movement

Originally Posted by 911st

Lowering the front in effect makes the a arm longer and increases the total at rest neg camber.

I agree w/ the latter but the former is misleading, the A arm has max Y axis displacement when it is parallel to the ground, the arcs can only move horizontally inboard from there, The effect is to shorten the effective length of the arm at the lower end of the steering axis, the steering arm is doing a similar thing in a slightly different arc but if both start horizontal then you have a better chance of staying in the linear ranges for camber/toe changes

Originally Posted by 911st

In my opinion, lowering a stock suspension on a 911 more effects the front camber curve. That is if to low, there will be very little gain in neg camber with further compression in a turn.

camber curves are typically S shaped w/ a relatively linear portion at the middle of the S that is where you want your ride height to stay in the linear middle portion of the curve

Originally Posted by 911st

Improving the camber curve is the best of several reasons for raising your spindle. This improves the level of camber gain with compression. However, if you do this you will put your self at risk of increased bump steer as the difference in the arc of the a arm and the tie rods will be more acute.

The spindle is raised to maintain full shock travel, at the same time it allows the A arm to start in a more horizontal position on a lowered car again, starting at horizontal is good because you stay in the sweet spot for camber/toe changes more of the time

You will always have some bump steer, it is in the nature of the suspnsion design used, all you can do is minimise it.

Kevin's suggestion actually measuring is good( and very interesting to be able to see) but unnecessary.

If you stay at RoW ride height, or ~ 25.5" at the fenders w/ stock 16" wheells and tires then the factory specs are great, they did all the work.

the lower you go from there the more other "stuff" needs to be done, go to ~ 25" and the washers are needed, go to 24.5" and raised spindles and Steering arm kits are needed

[kwikt 911]

So getting back to the rear suspension For example .... going around a sweeper or slalom, does the rear suspension gain more toe in or toe out while being under load or being compressed????

Kevin

[911st]

Wow Bill, if that suspention picture was part of a calander I would buy it.

Again, I am not an expert and I learn a lot from these exchanges.

Adding stiffer springs helps to.

Limit compression and you limit toe change or bump steer. Limit compression and you lessen body lean or loss of effective camber.

For most the rack spacers with stiffer springs and an eye toward how low one goes seems very sufficient.

I am pretty interested in understanding camber up front at present. I wonder if we are getting the work out of our front tires that we can.

Toyo R888 say they like -2.5 to -5 deg of camber. Lets say our goal is -2.5.

In the rear we can get -2.5 at rest on a euro height car with out problem but not much more without mods or a bunch of toe in. On top of this when the rear suspension is compressed we gain negative camber which is good.

However, as the body leans this takes away from our total of effective camber. I think with about every .9” of lean or angle we are loosing a deg of effective camber. Thus, stiffer springs will limit sway and effective camber loss.

With the front strut system, the max negative camber I think is going to be with the a arm parellel to the ground. What it is at rest is not as important as where it is in the turn. Past level to the ground we may star to loose some camber. Resting angle is only a reference. A stock car might need a bit positive a arm lean. A very stiff one might go lower.

As Bill notes, the gain and loss in camber up front on strut system has a flat point where there is not much camber change. The way we set up our cars when lowered seems to put us right at this point with a stock strut. Thus, we will not typically gain any camber with compression and if to low will actually loose some.

Raising the axel on the strut (increasing the a arm angle) puts us back to a point on the S curve where we gain camber again or at a faster rate which should be helpful.

On a stock lowered 911 is it tough to get more that -1.25 camber up front. We can do things like de-cambered ball joints and strut top plates that increase the amount of camber up front. Using these methods we can get the same -2.5 deg at rest neg camber in the front. I suspect however, that unless we set the front at more neg camber than the rear we will not have the same effective camber up front as in the back in a turn as we will not have any camber gain with compression.

Also, our cars weigh a lot less up front. To get the most of our contact patch we might benefit from a little less air pressure. With less air pressure should come a need for a bit more camber.

When I was at my last auto cross I notice a lot of the hot shoes with big sticky tires had what looked like much more negative camber in the front than the back. this was not consistant with how most seem to set up there cars or how I thought it was done. Now I think I understand why they are doing this.

Thus, rising the spindle and de-cambering the axel relative to the strut could be a solid opportunity to increase total effective camber.

The down side is we are more likely to hang the inside front wheel in the air with the loss in travel extension that comes with the mod. This will make us more llikely to lift the inside front sooner. With this the sway bar rate is taken out of the fronts effective spring rate. Not sure how this effects us if at all.

Seems we are balancing trade offs and there are some inherent quirks in the basic design of a 911’s suspension.

This takes us back to the real world of what is proven work.

For me, I have not had any noticeable bump steer with a lowered 911 running a rack spacer kit and I suspect going so low that it might impart this would cost needed front camber.

[Steve@Rennsport]

Great information here!!!

I would only add that we always check bump steer while aligning a car. Its easy to watch this as the car is raised and lowered through 2" of bump & droop from targeted ride height.

Corrections are made either using rack spacers or a proper bump steer kit, depending on what suspension components are used and ride height. As Bill said, you cannot eliminate it but you can minimize its effects and its well worth taking the time to do so.

[Steve@Rennsport]

Originally Posted by kwikt 911

So getting back to the rear suspension For example .... going around a sweeper or slalom, does the rear suspension gain more toe in or toe out while being under load or being compressed????

Kevin

The rear suspension (semi-trailing arms) gains toe-in as its compressed (loaded).

[Kevin Stewart]

Bill all cars need some positive bump gain, usualy like 10-20 thou, I can tell you from doing the bump on many cars that less then a 16th of an inch can change 20-30 thou, the bump shims come in 1/2, 3/8, 1/4. 1/8, so it is common to have to grind an 1/8 shim down to get the correct bump. Is this needed on a street car, i would say depends on the response of the car, if the car is darty or acting weird when turning and the owner has put the suggested bumps shims in place on the rack. If the suggested rack shims are in place and it drives well then no, I have not had time to do my lowered 911 and yes it can be a handfull, i have done other 911's for guys and was amazed at the difference, i was recently getting my amg alignment looked at at mbz and was amazed that the amgs have everything set up to do bump, (just slide)

it comes to this each 911 is a little different when it was built, the bump spacers under the rack should work, expect to pay up to 6 hours of labor if some one has to grind shims, even the picture suggest its easy to add or remove a shim,yes it is , but you will have to grind to get it perfect...

[Bill Verburg]

Originally Posted by Kevin Stewart

Bill all cars need some positive bump gain, usualy like 10-20 thou, I can tell you from doing the bump on many cars that less then a 16th of an inch can change 20-30 thou, the bump shims come in 1/2, 3/8, 1/4. 1/8, so it is common to have to grind an 1/8 shim down to get the correct bump. Is this needed on a street car, i would say depends on the response of the car, if the car is darty or acting weird when turning and the owner has put the suggested bumps shims in place on the rack. If the suggested rack shims are in place and it drives well then no, I have not had time to do my lowered 911 and yes it can be a handfull, i have done other 911's for guys and was amazed at the difference, i was recently getting my amg alignment looked at at mbz and was amazed that the amgs have everything set up to do bump, (just slide)

it comes to this each 911 is a little different when it was built, the bump spacers under the rack should work, expect to pay up to 6 hours of labor if some one has to grind shims, even the picture suggest its easy to add or remove a shim,yes it is , but you will have to grind to get it perfect...

If a 911 is darty it's far more likely that the initial(static) toe is going positive. A/Xers love it that way, for street or track it's not much fun, most prefer a little toe in to 0 or toe out. Same thing in the back, a car that is darty there is most likely running too little static toe.

Sure you can pay someone $100/hr to get the shims perfect, but is it really worth it? Not to me.
If you have the time, tools and patieience you can do it yourself but that lets out a lot more people(besides cheapskates like me) for one reason or another.

[dshepp806]

I read this thread and quickly see there's much for me to learn on this subject.....

Helluva' class here at Pelican University.

Thanks for providing these opinions...way cool.

Best,

Doyle