Targa Truck '71 GMC build - The first truck in Targa Newfoundland.

[Ron Sutton]

What's your take on the long wheelbase effects on autocross, road course, and street comfort? My S10 is also a regular cab longbed, with wheelbase at 118". My understanding is that this is certainly a disadvantage (versus a shorter wheelbase) for autocross, and probably not a big issue for road courses. The only other issue I ran into was a long driveshaft that ended up requiring carbon fiber to get me to acceptable critical RPMs for use with 4.10 gears.

From wikipedia, it looks like your wheelbase is 127", so I would think you also ran into a driveshaft issue. I'm curious how you solved it and whether you have any points of reference from autocross or track days with shorter wheelbase vehicles.

Hi Jerome,

When Mark & I started working on his truck design package, I asked him if there were any "sacred cows" ... and one of them was the long bed & therefore the long wheelbase. Does it present a challenge? Sure it does. In the two columns of "advantage versus disadvantage" it puts a check mark in the disadvantage column. But we're building cool, unique, individualized vehicles here ... which is what is fun ... or we'll all be building F1 cars.

I told Mark, "no worries. We'll just build some rear steer tunability into the 4-link."

So when Mark's new truck chassis is being built, the lower 4-link bars will be adjustable for neutral rear steer, positive rear steer & negative rear steer. For high speed runs & road courses the truck will be set for neutral rear steer ... or negative rear steer which actually increases rear tire grip as the truck rolls to its max roll angle. If Mark decides to take it drag racing, setting it for negative rear steer helps the launch.

For street driving ... frankly he can leave it where ever as long as the speeds aren't high. But setting them neutral is usually best. When driven hard, at or near the limits of tire grip ... all vehicles have rear steer due to the tire slip angle. This can vary from 5° to 9° depending on tires. For autocross, Mark can adjust the lower links up in the front to add whatever degree of additional rear steer he finds works best.

None of this makes the truck shorter. You still have to get it through the cones. But you can make the truck turn better, by making the rear have more rear steer angle. (Think fork lift).

[Ron Sutton]

Jerome - I'll reply soon. I have an opinion on that. Kinda hoping some of the pros see your question and jump in.

It's late. Work has been crazy this week. However, the interior is mostly gutted to start the 'Cage fab work tomorrow. I'm getting pros do do this as safety stuff needs to be done by accomplished fabricators.

It feels like I just put the Benz seats in, but their time is up after 2ish years. Never imagined I'd be going to full containment race seats, but there it is. And after my track day on Sunday - I need seats the tie me safely. I was bending my steering wheel holding myself in place. Still have to sort out harnesses.

Attachment 95222

Nice ... safer ... seats. Good call.

[bovey]

What's your take on the long wheelbase effects on autocross, road course, and street comfort? My S10 is also a regular cab longbed, with wheelbase at 118". My understanding is that this is certainly a disadvantage (versus a shorter wheelbase) for autocross, and probably not a big issue for road courses. The only other issue I ran into was a long driveshaft that ended up requiring carbon fiber to get me to acceptable critical RPMs for use with 4.10 gears.

From wikipedia, it looks like your wheelbase is 127", so I would think you also ran into a driveshaft issue. I'm curious how you solved it and whether you have any points of reference from autocross or track days with shorter wheelbase vehicles.

First. Thanks Ron for giving an opinion on this.

Here is my 2 cents.

A long wheelbase for autocross is less than ideal - I race with Miatas, S2000s etc and the tracks are tight. I have to take completely different lines to squeeze my way through. Crazy, crazy fun - but it's a massive uphill battle to be competitive. I'm doing this for fun, so I don't care about being #1. But I have every plan on putting up a good fight, again for fun.

The biggest thing with autoXing a long wheelbase is trusting and using your peripheral vision. I have to wait for the cones to blip out of my vision before turning or I'll clip the cones. It took me a while to figure this out - but I do have far less cone scuffs on box around the rear wheels now.

On the track, it's stable. I do have to watch my lines a bit. I haven't experienced anything that I'd call a drawback, even the weight didn't really seem like a big deal - 600HP make the truck do what I want. Again, it's stable, dives into corners better than you'd ever imagine.

On the street it rides fine - it's a truck, it rides better than it once did and with the new ridetech shocks - I like it even more.

As for the driveshaft - I have a 1 piece chromolly 4" drive shaft that I spin to huge rpm without issue. Just have a a good shop make it and don't skip on the u-joints. I also had to be very careful with pinion angle AND be mindful of clearancing the centre frame cross member. I have had some issues with parts touching that I never imagined could touch.

My biggest comment on building an untypical car/truck would be:

--- What do you want out of this?

Do you want to win?
--- Build something more common and has the proper proportions and CG.

Do you love the truck and want to race it?
---This is my camp - do whatever you want. People told me it would not work, but I proved them wrong long ago.

Do you have loads of money and/or time?
--- Build whatever you want.

Do you have limited funds and/or time?
--- Build something more common. There is nothing wrong with building a common car, it's cheaper and easier.

Build process vs. track time.
--- If you are the type of person who loves to bench race and build, so be it have at it. I'm not that guy, I hammer on my builds to spend as little time in the garage and as much time on the track.

In reality, figure out what you want out of the truck/racing - do that. Think before you build. Time and money go by quickly, so be sure.

Fair?

[bovey]

The truck is mid-fabrication at the chassis shop. Going for a fitting tomorrow. Again, this is something I do not have the talent to do, and it needs to be safe. Fortunately, I have a great shop on the case who are efficient and fair priced.

One of the purchases I was on the fence about was wheels. I pulled the trigger on some NewGen wheels. Jon has been great and I love that he changes the design to suit you use, putting a bit more material is stressed areas of the wheel.

However, out of all the things I have to do - the colour of the wheels was hard for me to pick. I know, dumb. But I was a designer for years, so stuff like this kinda bugs me when it's wrong. So, I hired someone to do a few quick comps. Totally worth it. These are rough, just to pick a wheel colour. FYI - the wheels are 18x10 with 295/35s Rivals (planned tire)



I do like the murdered out look, but that doesn't suit me personally. Oddly, I love the goldy/bronze/tan colour. It's almost impossible to be original with this year of truck, but this does make me happy.

And it will not be this low with my current set-up, it looks more bad-@ss than expected.

[Ron Sutton]

Cool. How long until the cage is in ?

[bovey]

Cool. How long until the cage is in ?

I'll have progress photos tomorrow. I'm going to visit tomorrow. Should be done next week. Aiming to hit the track on the 24th.

[brawls43]

So you're going with #7? I think that's the look I like best too, personally. Polished lip just doesn't fit the look. And murdered out, meh. Always looks good, but done lots.

[jimi_cat]

seem many a flat tire in nfld. cause usually being broken road ,potholes ,apexing country road corners etc. may want to consider more side wall . although saying that i ran 35"s on my vette with no issues

[snopro]

They all look so good, how does one choose lol. I like the look of #1 though.

[bovey]

So you're going with #7? I think that's the look I like best too, personally. Polished lip just doesn't fit the look. And murdered out, meh. Always looks good, but done lots.

Thanks! I'm liking #7 too. 2 I like, because it tied in the little chrome I have left. Will share the final result when they come in! Gotta have a little suspense here and there.

seem many a flat tire in nfld. cause usually being broken road ,potholes ,apexing country road corners etc. may want to consider more side wall . although saying that i ran 35"s on my vette with no issues

Yes, I've heard many stories about people having to harsh of a suspension set-up or low profile tires. I'm going to start with 35s, and if they fit fine I may step up to a 40 series for the race - just to play safe.

They all look so good, how does one choose lol. I like the look of #1 though.

Thanks! #1 is cool - but I'm not sure how long it would be before I'm tired of that look.

More to come! I was out getting my seat fitted and checking out the roll cage progress yesterday. It looks awesome.

[keithq69]

Hope the fitting went well.
Did you get any pictures you want to share or are you holding off for more progress?

[srh3trinity]

I like a bronze look wheel too.

[bovey]

Roll Cage Progress.

It's not done, but everything looks great.






Fortunately, it wasn't to hard to make the cage work for both my Targa Newfoundland tech and my ECTA Land Speed Racing tech. I was going to do fancier door bars. But the opening is small enough that a diagonal bar is best. It also leave expansion room should I need to "x" it in the future.

Went out to test fit seats. Everything barely fits. Make sure you measure race seats, I did and it's still tight. Of course you may not need to incorporate a child's seat seat. But most race seats are around 21-24" wide, they are bigger than you think. These trucks are 65" door to door. Add a Roll Cage. Some room to move and boom - no space in the middle.



Have had to work with tech, like a lot. With the truck being heavier, they wanted larger bar and gussets. We are still in discussions about gussets - they want them on the rear main hoop for added role over protection. No biggie. We just have to do it. The Corbeau Seats are my street seats. We are making mounts for swapping out the Momo Containments seats for track days.





The floor is padded with 6"x6" steel plate sandwich both top and bottom. It's bolted together so I can pull the body off when I do Phase 2 with Ron Sutton and my chassis builder. The pads on the bottom are tied directly to the frame.



My little guy seems to like the new jungle gym. Overall, very happy. The guys have build it so it can be added too as I further the build. Eventually, it will reach into the engine bay and I'll put a couple extra bars in the bed for rigidity.



More soon.

[keithq69]

Looks great Mark.
When's the next track day?

[snopro]

does tech not require any door bars?
Looks great, clean welds, did your fabricator TIG it all?

[jerome]

Hi Jerome,

So when Mark's new truck chassis is being built, the lower 4-link bars will be adjustable for neutral rear steer, positive rear steer & negative rear steer. For high speed runs & road courses the truck will be set for neutral rear steer ... or negative rear steer which actually increases rear tire grip as the truck rolls to its max roll angle. If Mark decides to take it drag racing, setting it for negative rear steer helps the launch.

For street driving ... frankly he can leave it where ever as long as the speeds aren't high. But setting them neutral is usually best. When driven hard, at or near the limits of tire grip ... all vehicles have rear steer due to the tire slip angle. This can vary from 5° to 9° depending on tires. For autocross, Mark can adjust the lower links up in the front to add whatever degree of additional rear steer he finds works best.

None of this makes the truck shorter. You still have to get it through the cones. But you can make the truck turn better, by making the rear have more rear steer angle. (Think fork lift).

Ron and Mark,

Thanks for the thoughtful comments on wheelbase. Any consideration for a 3-link with converging lower links and a watts link or panhard bar? That would allow you to adjust roll steer somewhat independently of anti-squat. Roll steer axis determined by the line drawn between the watts/panhard height and the height of the lower link convergence point in side view. Anti-squat determined same as 4-link. Instead of a 4 link where increasing antisquat (angling lower bars up towards chassis) necessarily increases roll oversteer, a 3 link would let you change the roll steer axis to always have neutral steer by setting the lateral location device height at the same height as the convergence point of the lower links.

In any event, your comments were very helpful for my own build, as I had not thought about setting up the chassis for roll oversteer for autocross to get the effective wheelbase shorter. I'll have to make sure I have enough holes in the brackets to do that.

Jerome

[Ron Sutton]

Nice cage Mark !

[Ron Sutton]

Hi Jerome,

Ron and Mark,

Thanks for the thoughtful comments on wheelbase.
You're welcome. I've had to race long wheelbase cars many times.

Any consideration for a 3-link with converging lower links and a watts link or panhard bar?
Did I consider a 3-link? Yes. For road course, autocross & even oval track, an offset 3-link is the best rear suspension design for a solid axle car (or truck).

My preferred design is the offset 3-link because of the zero torque steer & great tunability. The only concern with a 3-link is all the "lift & pull" forces put on that single top link. For Mark's goal of running his somewhat heavy truck in the Targa, my concern was durability with a 3-link. A four link splits the forces & loads in two, effectively doubling the strength-to-force ratio of the upper link(s).

The two primary draw backs of the 4-link, in comparison to the 3-link are:
A. The 4-link will bind at a lower angle of housing articulation than a 3-link.
B. Typically, you're building a 4-link symmetrical, so you do have have 6-12% torque steer under hard acceleration, just like with a centered 3-link. Only an offset 3-link neutralizes the torque steer with no side effects.

All suspension designs have issues, or cons, if you will. You have to decide what your priorities are & where to make sacrifices. In consideration of these issues (A&B) ... the evaluation went like this:
A. Since Mark's new chassis will run a modern high-travel/low-roll suspension strategy ... the rear suspension will never come close to the articulation limits, so any chance of bind is nil.
B. Purely a choice of prioritizing safety over dealing with about 9% torque steer (typical for 9" Ford).

As far as considering a converging 3-link? No ... because the lower converging links cause positive rear steer that can not be simply tuned out. When you run the lower links parallel to the chassis, you have the options of ...
1. Running the lower links uphill (going forward) for positive rear steer.
2. Running the lower links level for neutral rear steer.
3. Running the lower links downhill (going forward) for negative rear steer.

Mark is going to run this truck at a lot of different venues. For autocross, positive rear steer will help the truck turn better. But for road courses with high speed corners, positive rear steer is dangerous ... because the faster you go the more it loosens up the rear end ... and therefore is not desired. For road courses, you want to run either neutral or negative rear steer. Negative rear steer actually adds grip to the rear tires on high speed corners.

With the converging 3-link, you always have rear steer ... unless ... and there is always an "unless or exception" ... unless you lower the front pivot points down super low. But that messes up the instant center location & anti-squat percentage. So I do not utilize, nor recommend, converging 3-link designs unless it is a dedicated autocross vehicle. Even then I don't use that design, because I create the same rear steer as a converging 3-link ... by simply adjusting the angle of the lower links on a parallel 3-link or 4-link.

That would allow you to adjust roll steer somewhat independently of anti-squat.
That's not accurate.

Roll steer axis determined by the line drawn between the watts/panhard height and the height of the lower link convergence point in side view. Anti-squat determined same as 4-link. Instead of a 4 link where increasing antisquat (angling lower bars up towards chassis) necessarily increases roll oversteer
We don't typically adjust the anti-squat of 3-links & 4-links with the lower bars. We use the lower links to define the degree of rear steer ... and adjust the upper link angles to define the instant center location & anti-squat percentage. There is MUCH more to the traction equation than anti-squat percentage. In fact it is only 1 of a dozen tuning items to the rear suspension geometry. But I leave it at that unless someone wants to understand rear suspension dynamics better.

, a 3 link would let you change the roll steer axis to always have neutral steer by setting the lateral location device height at the same height as the convergence point of the lower links.
That's not exactly accurate, but more importantly, we need the lateral location device ... watts link or panhard bar ... to define the roll center for tire loading purposes. You need it to be adjustable ... as a tuning device to balance the car (or truck's) handling to be neutral ... without any concern of affecting roll steer. Having the Watt's link or panhard bar adjustable is one the best track tuning tools.

In any event, your comments were very helpful for my own build
Cool.

, as I had not thought about setting up the chassis for roll oversteer for autocross to get the effective wheelbase shorter. I'll have to make sure I have enough holes in the brackets to do that.
Cool. Let us know how it goes.

Jerome

Thanks for chiming in Jerome !

[jerome]

From Race Car Vehicle Dynamics: the roll axis is defined by the convergence point of the lower links and the lateral location device.

[Ron Sutton]

I think you may be confusing roll axis with roll steer (rear steer). The roll axis (the pivot points & resulting pivot angle) defines how the rear end housing will articulate in the chassis during body roll from cornering g-forces. Roll steer is the relation of the left rear & right axle/hub centerlines (or tire tread centerline) as the housing articulates in the chassis.

As the body rolls during cornering, if the outside tire CL moves rearward (effectively lengthening the wheelbase on that side) ... and the inside tire CL moves forward (effectively shortening the wheelbase on that side) ... that is positive rear steer ... as both rear tires are angled toward the outside of the corner (relative to the chassis CL).

As the body rolls during cornering, if the outside tire CL moves forward (effectively shortening the wheelbase on that side) ... and the inside tire CL moves rearward (effectively lengthening the wheelbase on that side) ... that is negative rear steer ... as both rear tires are angled toward the inside of the corner (relative to the chassis CL).

As the body rolls during cornering, if the outside tire CL does not move forward or rearward ... and the inside tire CL does not move forward or rearward ... that is neutral rear steer ... as both rear tires stay parallel to the chassis CL.

During body/chassis roll, from cornering, a converging 3-link, with the lower links level, causes the housing to rotate in a way that creates some positive rear steer. If the lower links are running uphill (towards the front) it would increase the rear steer. The rear steer will reduce if the lower links are ran downhill (towards the front) but it takes quite a bit & typically messes with achieving the optimum I/C & anti-squat.

Positive rear steer is a benefit on autocross cars … but when I am hired to help the occasional road race team running a converging 3-link … it throws another challenge at us & we have to do extra things to achieve rear grip on high speed sweeping corners.