Gus

I drive a 964 Narrow body and am contemplating moving the front suspension setting to the outer or wide body position. I followed Jim Hall back in the 60's and 70's and he was a supporter of a wider front track over the rear track.
When they developed the 917, Porsche increased the front track width to improve handling with the increased HP. So there is validity to having a wider front track over the rear. My question is - "Has any one today experimented with a wider front track??" If so, what have you found?? My interest is in the handling changes that would come about if you made the front fracker wider than rear in a rear engine car (911) vs the changes to a mid-engine ((Cayman).
Here is some interesting info that is a quick read-

A wider front track effects handling in a number of ways. Assuming that all other things remain the same...

1) A wider track will make the springs feel weaker since you will be using a longer lever on them. So a wider front track will make the front suspension feel softer, promoting a reduction in understeer.

2) A wider track on one end of the car rather then the other will affect the way that load is transfered when cornering. When Porsche developed the 917/10 out of the 917K, they had a number of handling issues, one of which was power oversteer (going from 600 to 1000 HP -- duh!!! ). When they developed the 917/30, they actually reduced the rear track some. So the changes looked like this...

Track Front/Rear (rear as a % of Front)
917K 1564 mm / 1584 mm ( 101.3%)
917/10 1620 /1638 ( 101.1%)
917/30 1670 / 1564 ( 93.7%) (Frt is 106 mm wider)

The result was that when cornering, the load would be transfered to the outside front wheel sooner then the outside rear wheel since the front wheels had a wider track and thus were traversing a longer distance for a given amount of body roll. This will tend to reduce oversteer.

3) A wider track will lower the roll center at that end of the car, thus making it softer in roll. This is because the car's CG will not change, and thus the vertical distance between the CG and the roll center at that end of the car will increase, kind of like using a torque-wrench with a longer handle that pivots at the roll center. So a wider track at the front will make the front softer in roll which will once again tend to reduce understeer.

4) A wider track will tend to reduce the load transfer to the outside when cornering, which in general will increase cornering power since you'll be able to get more traction from the inside tires. So once again, increasing the track at the front will tend to reduce understeer.

So in general, making the track wider will increase cornering power -- but -- it is important to maintain an optimum ratio between the front and rear tracks as a means to managing the load transfer from the front to the back, and thus the resulting balance between the two ends of the car.

Now, widening the track using spacers also does a number of other things to the geometry...
1) It increases the front scrub radius, which can increase the steering effort and affect the feel and kickback.

2) It can increase the load on the bearings since the load will be cantelivered further out then the suspension designer may have intended.

3) Without working out the geometry, I'd also be concerned about causing the suspension to pick up a tendency to change track under bump/rebound which can cause all sorts of sudden strangeness in the handling as the suspension loading is changed.

959fan

seems like you're talking about wider track via. longer A arm. a better way to me seems to be retain the full stock geometry, but physically move the mounting points out entirely. that way you will retain most of the original feel, but gain the benefits of wider stance/track. i'm would imagine that is how 917 is done.

are you traction challenged up front? why are you considering this? seems like you can greatly increase rim/tire width before having to mess with the geometry.

Honda FWD race cars are indeed wider front track than the rear. actually i'm building my Honda with goal to break Street FWD record next year. i will be running 285 mm width tire up front and rear will be probably 255 mm or 235 mm wide... that's all it needs since weight distribution is about 62/38%...

Gus

On the 964 there are 2 suspension mount points for the font "A" arms. For the narrow body the inside mounts are used and for the wide/turbo body the outside mount points are used. If you use the outside points you need to reverse the upper strut mounts to keep all geometry the same. What you are gaming is about a 3 inch wider front stance, which should give you better handling and reduced over steer which is what I am looking for
But, if you do this you need to change to wide body front fenders and nose as the wheels will no longer fit in the wheel well. I am running 235/18 on front on an 8 inch rim. This is the max I can fit right now, going to wider settings would allow me to go larger..

959fan

ahh do you have any pix of this? i'm not super familiar with porsche susp so this is something new me. sounds very cool. all 964 have this feature? or are all 993, 996, 997, 991 are also like this? since there are always narrow and wide body for each generation of 911's...

Gus

Pretty sure 964 and 993 have this set up as front suspensions are the same - tears are different. Not sure about later models.
There are 3 mount positions on the cross bar that allows the lower "A" arm to be moved in or out by one space. Once you move the bottom out then you need to flip the top start mount to move the up position out to match the bottom move. This keeps suspension geometry the same. Will get some pictures and post .

Saidgta

Hi, is there any relation between Adding spacer(or changing Rims Offset) and Bump response to steering wheel?

Apex1

If you are using a few mm's of spacer, you most likely not notice, but if you are using a lot, like I do 18 mm spacers in front with my OEM wheels and 23mm in the rear, I felt it really soften up the spring rate/shock rebound. I added Ohlins R&T coilovers, with a much greater spring rate and better compression/rebound dampinig, which over compensated at the wheel rate, even with the spacers in place, again changing the bump result in the steering. Then I added lighter wheels and tires, which again changed the wheel rate amount. After that I changed my oem LCA's to Tarett Cup LCA's with internal spacers, which are lighter weight, stiffer monoballs inside and outside and a solid thrust puck. These LCA's changed camber as well as dropping my ride hight so my suspension arc was causing my toe to be out of alignment in bump steer droop/ compression to a point I needed Elephant toe control arms with bumpsteer adjustments to get rear toe adjustments to 12 minutes. These new Tarett Cup control arms stiffened up my bump response a lot, so for street driving, I took out compression/rebound rate in my Ohlins shocks to compensate. For the track, I put it back in and then some, so this really worked well to stiffen up the bump in racing but my wife hates that effect for daily driving.

The end point is yes you do feel the result of changing your track using spacers as the lever is lengthened by what ever amount you change your track width, with either internal or external spacers or wheel offsets. The Tarett Cup LCA's can actually push your tires out 20 mm and add -2.0 degrees of camber. The good news is that 20mm does not affect your wheel bearings, like external 20mm spacers would or wheel offset of w reduction in offset of 20mm.

Gus

Brian C in Az

Bump steer is caused by the tie rod angle. You can reduce bump steer by moving the outer rod end up or down relative to the oem mount position.
Remove your springs, assemble the steering and suspension, then measure toe changes as you move the tire up and down through it's range of motion.
For track use, only be concerned about toe changes that occur within 2 inches of normal (track) ride height.
Shim the tie rod end to raise it up 1/4" and repeat the measurements. See if the bump steer increases or decreases. Adjust accordingly.
Bump steer is very important. You don't want your toe changing every time you brake and start to turn.

Apex1

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The lower your car ride height is, the more important bumpsteer becomes, as the attachment points of the LCA's are not the same distance from the wheel upright as the toe link attachment points, so a mismatch on a parallelogram occurs. By slightly changing the angle of the toe arms at the upright fitting, you can get the two arc's to be consistant for 1 in bump and 1 in droop. Attached is an adjustable Elephant rear toe link, the bumpsteer adjustment is spacers before the attachment to the upright or on top of the upright depending on the angle. Good speed shops have an angle tool that makes this adjustment to parallel the toe link to the LCA. They then jack up the car so they get 1 inch droop, measure that , then add weight to 1 inch compression, measure that, change the spacers as necessary. Time consuming at shop hours of over $200/hr.
Bump steer is change in toe due to the vertical displacement of the wheel, such as happens when going over a bump. The change in toe causes the car to turn, and the driver needs to correct for this to maintain a steady trajectory. Bump steer becomes more profound when the car is lowered. Elephant Racing bump steer adjustable tie rods have provision to adjust bump steer, such that bump steer can be minimized reducing the need for driver corrections and making it easier to go fast. Elephant Racing tie rods have an angled head to partly compensate for the changed angle of the tie rod when the car is lowered. This keeps the stud near the center of range of motion, preventing bottoming out.





The bottom toe links are rear, the upper are fronts.

Brian C in Az

We both said the same basic thing. I refered to the angle difference of the tierod vs the lower arm and you refered to it as a parallelogram.

We both agree that the 1" above and below ride height is the only part of the range of suspension motion in reference to bump steer that matters.

I forgot about Elephant Racing's bumpsteer tie rod ends.

Next lesson should be about roll centers since we are talking about lowered suspensions...