Suspension pickups - race build 1978 X1/9

[Faza128]

I am moving to an x1/9 full track build after a break from building my last 128 - it’s been a long time since I looked at x1/9 chassis set-ups - can somebody bring me up to speed with best practice on relocating suspension pickups (or not) for the front? I and assuming the HP racers out there have a good recipe? Car will be on 15” wheels and slicks and our rule book is open (no space frame/tube frame on this build) so things can be moved…
Any advice would be very helpful!

Thanks
Warwick

 

[myronx19]

I am moving to an x1/9 full track build after a break from building my last 128 - it’s been a long time since I looked at x1/9 chassis set-ups - can somebody bring me up to speed with best practice on relocating suspension pickups (or not) for the front? I and assuming the HP racers out there have a good recipe? Car will be on 15” wheels and slicks and our rule book is open (no space frame/tube frame on this build) so things can be moved…
Any advice would be very helpful!

Thanks
Warwick

I would suggest finding the PBS Project X1/9 book. It documents what to do for the front suspension - including drawings for the parts. I've done it - but careful placement of the welded in components is critical in order to get a good alignment after.

I also raised my steering rack as well but I used a different anti-roll bar setup. One thing I did was cut out the frunk to gain access to the steering rack from above - this helped for easier alignment - oh and camber plates too!

You could google "PBS Project X1/9 pdf" and you'll find a few links that have it available for download, but I suggest finding a physical copy. I have one and it was very valuable for my race car build.

Cheers!

 

[Faza128]

Thanks - I have that already I guess I am just querying if current x1/9 racers are still applying that…

 

[Ulix]

Steve Hoelscher keeps writing that relocating the pickup points is outdated tech and no longer state of the art, IIRC.
I wouldn‘t start cutting today.

 

[GregS]

I have seen several references to the PBS suspension mods being a bad way to go.
https://xwebforums.com/forum/index.php?threads/project-x1-9-book-by-pbs.10045/

Car set-up - road race X

Mike, if I think there is a driving issue we'll get into that but at this point I am not assuming anything. On the tire subject. The cantilever slick is a very old tire and while Hoosier had updated it a couple of times in the last 10 years there has been no real development on it in at least...

xwebforums.com

I kept the original suspension arm pickup points on my track X, but I made some changes for rear bump steer and relocated front strut top mount towards the centre of the car after having a play with some suspension software program to help roll centre effects.

 

[MikeHynes]

The PBS book was published a long time ago. It shows you how to lower the body to gain a lower COG, etc. One problem with the book's implementation is that the the pickup points for the front radius rods were not raised. I have a car with the PBS moods and I can't see/feel that that (not raising the front strut rod pickup points) is a problem. But who knows? Maybe it is?
However some of the fastest SCCA racers made much more extensive mods to the front suspension. One of the better know racers was Bob Boig. Bertone supported him. I can't give you details about the mods "he" made but in general he lengthened the front lower control arms so that the inboard pickup points were in the centerline of the car. He used a steering rack that also moved the inboard tie rod pickup points inward a similar amount. These mods resulted in better front end geometry. But, they also required a rerouting of the under body coolant tubes because the lower control arms were now mounted where the coolant tubes ran before. Bob wasn't the only racer to make such changes, there are others, but the mods I've described above are generally referred to as "Boig" mods. You might try to find someone who has done the "Boig" mods, they might be able to point you in the right direction?

 

[Faza128]

Thanks Mike and Greg - looks like I am going to spend a bit of time on the set up software front! I was bit dubious about the PBS mods so will run some simulations and see what I come up with - running 15”s and slightly taller rubber will require some thought to get everything within a good window but I will start with the stock locations and play around with roll centres and scrub radius and try and dial the bump steer out

wish me luck

 

[MikeHynes]

Generally the PBS mods for the rear have not been a problem. One issue to consider is shock travel. If you lower the body you may run out of room to contain the top of the struts within the body - mostly up front.

 

[Steve Thomas]

Thanks Mike and Greg - looks like I am going to spend a bit of time on the set up software front! I was bit dubious about the PBS mods so will run some simulations and see what I come up with - running 15”s and slightly taller rubber will require some thought to get everything within a good window but I will start with the stock locations and play around with roll centres and scrub radius and try and dial the bump steer out

wish me luck

Exactly - no substitute for first principles. Tyres first as everything else is all about giving them what they need to work best. No fan of the PBS stuff other than as historic interest.

 

[myronx19]

Bob Boig

My friend owns Bob's FP car. It's near Toronto, ON Canada. The suspension is radically different from stock - even the inclination of the strut tower is modified (Bilstein inverted struts from memory). The steering rack is a center steer type offering really long tie rods (which of course help to minimize bump steer). The control arms are custom and much longer (the pick up from the center of the car both front and rear).

It's a sweet machine - does low '30s at Mosport. The fastest I've lapped Mosport is 1:42 in my race X1/9. I've been a passenger in this thing (on a lapping day) but unfortunately never got to drive it. I'll get some suspension pictures next time I see it - it's retired from racing and will be on display at their service garage in the future. Paul Stolze owns Bob's GP car which is also quite nice!

One of the things that raising the suspension offers is restoring the arc of the control arm so that you don't get wicked negative camber when that corner is loaded up. There is only so low that you can go with stock suspension. Other points like changing the strut's lower mounting points are useful so that you never bottom out the struts (internally). The PBS book is not useless at all - the radius rod modification is useful for racing (rod end).

A good book to rear would be Carrol Smith's Engineer-to Win and Tune to Win - it covers a lot of suspension theory. But I still think the PBS book is handy. I noticed major improvements in lap time after I did the modifications to my car (I raced it for several years without suspension mods and then with mods) - I made my own camber plates and used different struts, played around with spring rates. The car handled quite well. The PBS book also has drawings and know how on using spherical bearings for the control arms rather than rubber bushings - even if you keep the original pickup points, it's still handy to have.

 

[myronx19]

rear bump steer

Is the X1/9 affected by rear bump steer? The tie rod end is fixed to the control arm and moves with it. Assuming the geometry and arc are near stock, it shouldn't really steer when in jounce (much anyway). What did you do back there if you don't mind me asking? (after reviewing the video, it does look like there is some minor toe change!)

Brian Staley made a cool video of the rear suspension:

and front!

 

[Dr.Jeff]

First let me say I do not know a ton about suspension design. But I'm curious about the X's rear suspension. How is it affected by the stock A-arm mounting points being on different pivot axis lines? It would articulate much better if they were on the same axis. Also why does the rear radius rod have a bushing where it mounts to the A-arm? For that matter it really doesn't need a ball joint on the other end either. There's no movement of the rod independent of the A-arm and the upright can only travel in one plane? It can be made adjustable for toe without all those sloppy joints.

I have similar questions about the stock front suspension design, but the rear seems the worst in my view. I understand the whole affair was a cheap adaptation from a FWD design. But the items I questioned could just as easily been done very differently and with less production cost....makes no sense to me.

 

[RXEFP19]

I have driven both, the PBS mods works good up to a point, if you don't lower it to much ( I own the PBS X1/9 in the project X1/9 book). I also have the Bob Boig Design Suspension in my FP X1/9 with upgrades (double adjustable Konis etc..) and it is amazing, in my prime we had the track record at Road Atlanta when they redesigned the track in the early 2000.

 

[GregS]

Is the X1/9 affected by rear bump steer? The tie rod end is fixed to the control arm and moves with it. Assuming the geometry and arc are near stock, it shouldn't really steer when in jounce (much anyway). What did you do back there if you don't mind me asking? (after reviewing the video, it does look like there is some minor toe change!)

Brian Staley made a cool video of the rear suspension:

and front!

I was having trouble with snap oversteer on fast corners and it was suggested it may be caused by Bump Steer.
Measurements showed that I was getting some rear bump steer. With 50mm of bump travel there was 1mm toe out, and 46mm droop there was 2mm toe in. The car was sitting nearly 50mm lower than standard on shortened springs, but I was surprised how much toe change there was considering that the rear toe link inner end is bolted to the suspension arm, not the car body. Is this entirely due to the lowering, or is it normal for an X1/9?
A CAD drawing showed lowering the outer tie rod end 5mm would almost cure the bump steer, but lowering it in an X1/9 is easier said than done, as the rod end axis is horizontal rather than vertical. One way to lower it is by bending the arm on the wheel hub, but this is a very solid forging/casting, and I didnât fancy trying that. The option I took was buy a Chromoly rod end, machine a taper to fit the wheel hub arm with a hole through it at an angle for a high tensile bolt, this angle would mean the bolt head is now 6 mm lower than the axis of the taper. Another way would be to machine a tapered shaft to fit in place of the tie rod ball joint and have a ½ inch diameter parallel shaft on the other end for the rod end, with the axis 6 mm offset. As there appears to be no universal rod ends with an M16 x 1mm left hand thread, I used

a ½ inch UNF RH rod end, and screwed that into a plug that fits the M16 LH in the toe link. I was very fiddly PITA work, but doable. May not be legal for road use, but this is essentially a track car.
Checking now showed that 35mm of bump now gives 0.2mm toe in, 35mm droop gives 0.1mm toe out. (less susp. Travel with stiffer springs) These both worked to reduce the tendency to oversteer.

 

[CKAL]

I am moving to an x1/9 full track build after a break from building my last 128 - it’s been a long time since I looked at x1/9 chassis set-ups - can somebody bring me up to speed with best practice on relocating suspension pickups (or not) for the front? I and assuming the HP racers out there have a good recipe? Car will be on 15” wheels and slicks and our rule book is open (no space frame/tube frame on this build) so things can be moved…
Any advice would be very helpful!

Thanks
Warwick

Hello Faza128,

If you are only going for a track day car (not full on race) I would use the stock pick up points and mostly stock suspension pieces. How much time and money do you want to spend? For the front add a rod end to the track rod and either a spherical or stiff nylon insert to the control arm ( I made my own nylon inserts). Steering rack should be raised. I was able to buy a set of KSport struts years ago (for Chinese junk they are actually very well made and work excellent). A set of Bilstein struts is the only option nowadays. This limits how low you can go with this set up. Of course camber/caster plates for the top end (spherical bearing here also). For the rear spherical or slightly more pliable urethane inserts in the control arms (struts and camber plates on top). It is a good idea to slightly move the pick up point for the toe adjustment. I have recently fabricated a new rear control arm with rod ends and modified toe control. That will be this years experiment. I have the V6 Mazda powered Fiazda that you can read about in another post. I primarily run at Road America, lap time sub 2:30. It's just a go fast fun time machine for me and I am kind of cautious (don't want to bend it) and I really haven't gotten anywhere near it's limit. So, I couldn't tell you if it has any bad habits with the suspension. Best part is giving terror rides to unsuspecting victims, ahh... passengers.!!

 

[MikeHynes]

Thank you GregS and CKAL - I love hearing about how others have improved the suspension on their Xs.
Here is a pic I grabbed, somewhere, that is interesting to me. I would think that by moving the inboard rear toe link mount to be in the same plane as the control arm pivot point you would have better rear toe control.

Any comments? (Other than the kink in the front leg of the control arm.)

 

[Dr.Jeff]

moving the inboard rear toe link mount to be in the same plane as the control arm pivot point

Not that I would know, but that makes sense to me. On the stock layout those two arms are starting from two different locations....by a little bit. And it seems that would put them on a different arc, therefore they'd have a different length of movement as they travel.

However on the other hand, the toe link is essentially a solid extension from the main A-arm. Therefore does the toe link even "move" relative to the A-arm? So do they actually travel differently? I don't know. That was a question I posted earlier about the way the toe link has a ball joint on one end and a bushing on the other end - does it really need any joints? Once the toe is adjusted it remains set and the toe link should not move (I think).

 

[GregS]

Not that I would know, but that makes sense to me. On the stock layout those two arms are starting from two different locations....by a little bit. And it seems that would put them on a different arc, therefore they'd have a different length of movement as they travel.

However on the other hand, the toe link is essentially a solid extension from the main A-arm. Therefore does the toe link even "move" relative to the A-arm? So do they actually travel differently? I don't know. That was a question I posted earlier about the way the toe link has a ball joint on one end and a bushing on the other end - does it really need any joints? Once the toe is adjusted it remains set and the toe link should not move (I think).

That photo from Mike is an interesting concept. I think the only way to know for sure if it is an improvement would be to model it to see what happens. I didn't think the standard setup would cause toe change either, but alignment measurements showed that it did, at least on my car.

 

[Steve Thomas]

standard setup would cause toe change either

It does but not due to toe adjustment as such. The standard setup results in the inboard arm pivot axis formed by the mounts on the car being non-parallel with the outboard arm pivot axis formed by the primary ball joint and the toe joint.. Toe adjustment works fine - static. But as the suspension articulates, the non-parallel axis effect kicks in. Not really an issue at standard ride height and with standard-ish tyres.

 

[Ulix]

I had read that the rear suspension design has a toe-change built in that results in an increase in toe-in as the suspension compresses so that the oversteer tendency is reduced as the car leans into a turn.

So is this incorrect?
GregS showed above that the rear toes OUT as the suspension compresses.