Widening front track

I've noticed my '69 and a few other C3s that it looks like the front track width is a bit too narrow for the bodywork. I think the car would look better, and should handle better, with a slightly wider track. IIRC, the stock trackwidth for my vintage is something like 58 3/4 inches, and mine is currently 59 1/4 inches, a quarter inch wider than spec'd on each side (I copied it exactly from the original frame/suspension to minimize the variables in the suspension swap). Anyway, it looks like I could widen the track width another 1/2 to 1 inch (1/4 to 1/2 inch on each side). This should help the handling, and also help reduce the scrub radius a bit with the track wheels and adapters I'm using. I'm planning on welding up an aluminum crossmember in the future, but I might hedge my bet by making the LCA mounts adjustable so that if the wider width doesn't work out I can narrow it back to where I'm at today. There are a few items that concern me with changing the width: bumpsteer, possible tire rubbing, and the best way to make the adjustable area strong enough to not cause me worry on track days.
The bumpsteer issue is the biggy for me. I don't currently know if it's possible to easily change the pivot points on the steering rack. It's a custom width rack, so I'm hoping there might be something in its construction that allowed the manufacturer to make it the width I specified while using a somewhat common (short) ram assembly.

I have 10" rims with 3" backspace. Tubular a arms, long Howe ball joints on top, bump steer block, r&p, but otherwise stock geometry. It was fun around town, haven't driven much since moving. A wider rim might be easier than multi positions.

69427 - Sounds like you're taking the best approach - build the new section.
rtj - Did you experience any fender rub with your set up? Was the steering heavier due to significant change in scrub/kingpan axis?

BTW - wouldn't the bump steer be more affected by angles and "planes" of motion, rather than length?

Sounds like another cool thread on your metal fab!

Cheers - Jim

phantomjock said:

69427 - Sounds like you're taking the best approach - build the new section.
rtj - Did you experience any fender rub with your set up? Was the steering heavier due to significant change in scrub/kingpan axis?

BTW - wouldn't the bump steer be more affected by angles and "planes" of motion, rather than length?

Sounds like another cool thread on your metal fab!

Cheers - Jim

Click to expand...

I hacked a few inches off the fenders and screwed on some l88 flares.

Seemed like every change yielded improvements to the handling. But it's not a Z06 by any means.

69427 said:

I've noticed my '69 and a few other C3s that it looks like the front track width is a bit too narrow for the bodywork. I think the car would look better, and should handle better, with a slightly wider track. IIRC, the stock trackwidth for my vintage is something like 58 3/4 inches, and mine is currently 59 1/4 inches, a quarter inch wider than spec'd on each side (I copied it exactly from the original frame/suspension to minimize the variables in the suspension swap). Anyway, it looks like I could widen the track width another 1/2 to 1 inch (1/4 to 1/2 inch on each side). This should help the handling, and also help reduce the scrub radius a bit with the track wheels and adapters I'm using. I'm planning on welding up an aluminum crossmember in the future, but I might hedge my bet by making the LCA mounts adjustable so that if the wider width doesn't work out I can narrow it back to where I'm at today. There are a few items that concern me with changing the width: bumpsteer, possible tire rubbing, and the best way to make the adjustable area strong enough to not cause me worry on track days.
The bumpsteer issue is the biggy for me. I don't currently know if it's possible to easily change the pivot points on the steering rack. It's a custom width rack, so I'm hoping there might be something in its construction that allowed the manufacturer to make it the width I specified while using a somewhat common (short) ram assembly.

Click to expand...

I guess it's time for me to take some pictures and post them. I just completely rebuilt the front suspension on my C-1. The objective was to develop a high travel (4.5 inches of compression at the wheel), low roll, clearance for 315s and perhaps 335s on 12 inch rims, high caster gain with minimal camber gain during compression (caster causes camber gain "the good way" on both the inside and outside tires during turn; too much camber on compression with very wide tires hurts braking); minimal roll center migration with a 1.5 inch roll center at full compression, good Ackermann, minimal bump steer with the wheels straight ahead and some dynamic toe out with bump and wheel turn.

When I got through, the new front crossmember looked a little like a steel version of a C6 Corvette cradle. My supension pick-up points ended up much like a C6 -- long swing arm (to the instant center), with caster gain created by the non-horizontal (front to rear) angle of the lower control arms and the anti-dive angle of the uppers. The set-up uses longer control arms (which required a shorter rack), but I didn't gain much track width because as I widened the wheels and adjusted the back-spacing to bring the scrub near zero, most of the tread width gained was inboard and the track width (center of tread to center of tread) remained about the same.

Every pick-up now is fully adjustable using slugs. The rack is also on slugs so I can move it back or forward to adjust Ackermann. The biggest issue with the low scrub, high travel, deep backspace combo was clearance with -- everything. It took special high clearance upper and lower control arms, a new sway bar configuration, and moving the inside fender panels a mile. Nothing a chainsaw couldn't handle.

Have you looked at the clevis-end racks (Woodward and Sweet)? You can probably adjust the rack length (lengthed) by custom building different cleavises. The clevis is nice because it gives you 1 inch of vertical adjustment for bumpsteer corrections. I ended up using an 18.25 inch rack and moved it up vertically to get the proper tie rod length. That was the shortest rack I could get that would give me a full 6 inches of travel that was needed with fairly long steering arms to get 30 degrees of wheel turn. With adjustable Howe outer tie rod ends I was able to achieve the correct rack-to-steering arm geometry without building new steering arms.

Her is one "not-so-good" picture showing the clearance requirements for a 9 inch backspace wheel with zero scrub.

Okay, here are a few construction pictures of my new C-1 suspension -- no metal finish work done yet, so a little rough. Note that the crossmember started life as a C-3 unit. The lower control arm pick-ups come off of the crossmember and the rears are then tied to the frame rail. The front pick-ups are tied to the sway bar tube. The whole thing is adjustable for: static camber, static caster, anti-dive, roll center, caster gain, camber gain, bumpsteer, and Ackermann. The basic geometry is fairly close to a C6, but modified to handle more compression travel without going off the charts. Hope this helps.

Hey Pappy, I admire what you're doing there. :thumbs:

I'm still at the steep part of the suspension geometry learning curve. I welded up some new UCA mounts over the winter to give me some more caster range and a bit more camber gain. The track day last week was a bit better in the tight corners, and I'm still looking for more improvement if I can find it. I've also been wanting to measure my Ackerman for quite a while, and I've finally scrounged up some stuff to help me do that.

69427 said:

Hey Pappy, I admire what you're doing there. :thumbs:

I'm still at the steep part of the suspension geometry learning curve. I welded up some new UCA mounts over the winter to give me some more caster range and a bit more camber gain. The track day last week was a bit better in the tight corners, and I'm still looking for more improvement if I can find it. I've also been wanting to measure my Ackerman for quite a while, and I've finally scrounged up some stuff to help me do that.

Click to expand...

Ackerman works in conjuction with bumpsteer with the wheels turned to give you "dynamic toe". You would like just a little toe out change with the wheels turned 20 degrees or so. One thing I have notice with a couple of "very successful, semi-stock" C-2s and C-3s is that they are spacing the front lower control arm pivot down which causes the control arm to slope up to the rear -- adding a small element of anti-dive, but most importantly, adding to caster gain during suspension compression. If you look at the C5/C6 Corvette dimensions, you will see they are doing the same thing. The accepted optimum dynamic caster (static plus caster gain with compression) is 1.5 to 2 degrees greater than the KPI (Ron Sutton - the Guru). When the wheels are turned, caster produces negative camber on the outside tire and positive camber on the inside -- all good. Camber gain, by itself helps the outside tire, but hurts the inside. Keep us posted on your changes.

Pappy

Seems like there's always something silly that holds up progress. I got my old set of front wheel turntables cleaned up and regreased, and slid under the front wheels to check the ackerman. Sure as shit I can't find my stuff to actually measure the wheel angles. Looks like I'll have to make a stop in town tomorrow and see what I can find to get the job done.
I've already got about half the parts cut for the new crossmember, and I'd like to start welding on it, but I can't until I know if there's any geometry changes I might want to make.

Keep us updated. I'm curious to see your plan.

mfain said:

Keep us updated. I'm curious to see your plan.

Click to expand...

Well, it's not a fancy plan. I'm just trying to find any areas I can improve on while retaining the stock C4 suspension parts (upper and lower control arms). I'm real happy with how the rear suspension is working after some toe-control and spring changes, so I'm concentrating on the front again. I've been trying to digest some of Sutton's advice, and incorporate it if possible. He's got a different "perspective" on geometry issues compared to some authors who's work I've read. For the most part, I'm darn happy with the way my car feels (and decelerates) at maximum braking, and the high speed corners are very acceptable (to me), but I've got the common problem of a mid-corner push in the slower speed corners. That's the primary complaint I have right now, and I'm hoping I can make a little improvement on that issue.
I'm tire width limited due to my stock fenders, so if I can improve the geometry a touch, and take a little weight out in the process, then that ought to help make up for things a touch.

69427 said:

mfain said:

Keep us updated. I'm curious to see your plan.

Click to expand...

Well, it's not a fancy plan. I'm just trying to find any areas I can improve on while retaining the stock C4 suspension parts (upper and lower control arms). I'm real happy with how the rear suspension is working after some toe-control and spring changes, so I'm concentrating on the front again. I've been trying to digest some of Sutton's advice, and incorporate it if possible. He's got a different "perspective" on geometry issues compared to some authors who's work I've read. For the most part, I'm darn happy with the way my car feels (and decelerates) at maximum braking, and the high speed corners are very acceptable (to me), but I've got the common problem of a mid-corner push in the slower speed corners. That's the primary complaint I have right now, and I'm hoping I can make a little improvement on that issue.
I'm tire width limited due to my stock fenders, so if I can improve the geometry a touch, and take a little weight out in the process, then that ought to help make up for things a touch.

Click to expand...

Have you considered maybe adding late shark flares to the body?? that's what p/owners did to my car....I have measured early shark and later shark fenders from the corners to the end of the flare, and even being a '72, the thing definitely has late shark body style, so my 255/50/17 in front and 275/50/17 in rear fit fine, and have never rubbed....

:thumbs:

Here's something that might help with the mid-corner push. At last year's Goodguy's autocross finals, Ron Sutton gave me this discussion. He said that the cars entered the corners in dive (compressed) due to entry braking, and the result was the dive/roll attitude that provided the best tire slip angles (based on their suspension geometry) for corner entry. He noted, however, that some cars were losing the dive attitude prior to mid-corner which was causing a push as some of the camber gain was being lost. His suggested fix was to use slow rebound shock valving to "pin" the nose of the car down until later into the turn. He even suggested that, at a given track, he looks at the average time from corner entry to mid corner for the most troublesome corners and sets the shock valving to keep the nose down for that many seconds. Don't know if that will help you, but I thought I would pass it on.

Ackerman

Well, after a bunch of frustrating setup work I finally got some data. (the measuring concept is simple, but the actual work is a bit more involved). According to Sutton he was recommending about 35* on the inside wheel when the outside wheel is turned 25*. That's a 1.4 ratio. I didn't have the car centered on the lift rails so I was only able to turn the outside wheel 21* and still have room to set the angle gauge on the rail and measure things. The inside wheel was turned 24*, for a ratio of 1.13. So, it looks like I could use some more ackerman.

Now the difficult part. I've admitted my ignorance on this forum multiple times before (and requested help and answers), but this is the first time I've been genuinely embarrassed that I can't figure out the answer by myself (the question is which direction do I move the rack to increase the ackerman). I've stared at the tie rod end areas on the spindle knuckles ('96 Corvette suspension), and I can't see a clear angle difference between the left and the right sides. From appearance it looks like there can't be any reasonable ackerman action with this suspension. I'm 99.9% sure that this is a mistaken reading of the steering link geometry. So, please help me out understanding the arm placements and which way to move the rack to increase the ackerman.

Thanks a lot,
Mike

Mike,

I'm assuming you have a front steer rack, so moving the rack back increases
Ackerman. In addition to moving the rack, the outboard offset (from the ball joint line) of the outer tie rod end pivot contributes to Ackerman. There is an excellent, short, very clear explanation in the technical section of the Woodward Precision Steering catalog -- beginning just past page 64, I think. Here's something to consider -- you desire some toe out with the wheels turned, but this "dynamic toe" is not necessarily all Ackerman. It is a combination of Ackerman and bump steer. Bumpsteer may be, but is probably not the same with the wheels turned as the bumpsteer you measure with the wheels straight ahead (the steering arm swings in an arc that has a vertical component, so the tie rod angle with respect to the control arms changes a little when the wheels are turned). To find out how much dynamic toe out you actually have when turning, you need to set the car at your turn-in dive and roll angle, then measure the turn angles of the wheels like you did when you checked the Ackerman. As you lower the car with the wheels turned, you will see a change in the toe angle from that measured at ride height. Are you sure you read Sutton's recommendation right? That sounds like a lot more dynamic toe than I am used to. - I use 3-5 degrees, but it is a road race car and not an autocrosser that requires more toe out to help turn-in on tight corners. The C5/C6 Corvette uses very little Ackerman (zero desired).

Here's the link. The Ackerman discussion starts on page 72. http://woodwardsteering.eu/images/basic rack and pinion tech.pdf

mfain said:

Mike,

I'm assuming you have a front steer rack, so moving the rack back increases
Ackerman. In addition to moving the rack, the outboard offset (from the ball joint line) of the outer tie rod end pivot contributes to Ackerman. There is an excellent, short, very clear explanation in the technical section of the Woodward Precision Steering catalog -- beginning just past page 64, I think. Here's something to consider -- you desire some toe out with the wheels turned, but this "dynamic toe" is not necessarily all Ackerman. It is a combination of Ackerman and bump steer. Bumpsteer may be, but is probably not the same with the wheels turned as the bumpsteer you measure with the wheels straight ahead (the steering arm swings in an arc that has a vertical component, so the tie rod angle with respect to the control arms changes a little when the wheels are turned). To find out how much dynamic toe out you actually have when turning, you need to set the car at your turn-in dive and roll angle, then measure the turn angles of the wheels like you did when you checked the Ackerman. As you lower the car with the wheels turned, you will see a change in the toe angle from that measured at ride height. Are you sure you read Sutton's recommendation right? That sounds like a lot more dynamic toe than I am used to. - I use 3-5 degrees, but it is a road race car and not an autocrosser that requires more toe out to help turn-in on tight corners. The C5/C6 Corvette uses very little Ackerman (zero desired).

Here's the link. The Ackerman discussion starts on page 72. http://woodwardsteering.eu/images/basic rack and pinion tech.pdf

Click to expand...

Thanks, I'll give the link a read.

I went and looked at Sutton's post again. I misremembered his numbers. They were 25* outside tire and 30* (not 35*) on the inside tire.
I don't have any significant bumpsteer. When I was first assembling this suspension several years ago I spent a lot of time figuring out what width custom rack I was going to need, and getting it mounted at the right height. At the time I was under the impression that bumpsteer was undesirable and I put in a lot of effort to get close to zero.
This is the main reason I made all the suspension attachment points/brackets bolt-in pieces. I needed a manageable way to deal with any of my fabrication/design mistakes or changes in suspension geometry "philosophy".

Mike,

A point I was trying to make is that zero bumpsteer with the wheels pointed straight ahead WILL NOT be zero bumpsteer with the wheels turned 25 degrees. Because of caster, when you turn the wheels, the end of the steering arm will get closer to the ground, but the ball joints will stay roughly where they were (vertically). This changes the tie rod end to rack pivot orientation with respect to the line to the instant center (swing arm). If you do a bumpsteer measurement with the wheels turned 25 degrees (vice straight ahead) you will probably find some toe out bump with compression -- which is good. That means you may need less Ackermann to achieve the desired total (dynamic) toe out. I would turn the wheels 25 degrees and do the bumpsteer measurement (especially compression to your predicted corner entry dive angle) and see how much toe out you have. You may not need any more Ackermann. With my high travel suspension at full compression, I get almost 4 degrees of toe out (@ 25 degrees turn), of which about half is due to Ackermann and the other half is bumpsteer. I have less than .25 degrees of bumpsteer at full compression with the wheels straight ahead. Moving the rack back 1/2 inch adds another 3 degrees of Ackermann (toe out). Hope this makes sense. It took me a while to think "dynamically - what the suspension is doing in dive and roll" rather than statically.



Pappy