Benefit of equal length control arms on all corners

[Saito2 4856]

Not a specific Tamiya question, but what kind of benefits can be realized by using 4 equal length control arms on an off-roader?

The backstory: I've been starting to tinker with my old Stampede 2wd truck, trying to quell its wild wheelie and understeery nature under power. So far, I've Dremel'd the chassis to allow the battery to move forward .75" for better weight balance as well as adding 1:1 wheel weights up front. Its worked so far in reducing understeer and keeping the front end down. One thing that always bothered me was the different length in the front and rear suspension arms. The front and rear wheels have different offsets to compensate for this and keep the wheel track the same front to rear.

Enter RPM's wide front arms. RPM's goal was to widen the whole truck by adding wide front arms and then replacing the rear wheels with the deeper dish "front" wheels giving a wider track front and rear. I would like to consider the opposite. Keep the rear as is (stock width) and then use the wide front arms in conjunction with "rear" wheels. In this scenario, the width of the truck should stay the same as stock, but the arms will all now be equal length. The catch is I would have to lose the 2.2 wheels I currently use for Bigfoot/Monster Jam tires/wheels as they have a larger inner wheel diameter that would allow proper wheel to arm clearance when the wheels are turned. 

Basically, I'm looking at new arms, tie rods, upper links and wheel/tires to achieve all this and I'm wondering if all that investment will give noticeably better handling. I'm looking to make the Stampede less monster truck and more "King Cab". Hopefully I'm making sense.

 

[87lc2 1982]

I don't have much helpful input, but that Stampede looks awesome!  Something about it just speaks to me.

I race my Stampede in our 2wd monster truck class and have actually done pretty well with it.  Mine is a stock truck with 12t 550 power and initially I too had major issues with wheelies all the time, especially on carpet.  Ended up adding a bunch of wheel weights to the front as you did but didn't cure the problem.  Ended up loosening the slipper and its a lot better as far as not lifting the front end all the time.  As far as handling I just chalk it up to being a Stampede and haven't worried about it much, but it's definitely understeery at the limit, especially on power as you said.  Trying to think about it and not sure longer arms would help, but it sounds like a worthwhile endeavor.  I think being 2wd with big tires its always going to push like that but could be wrong.  

Now that I think about it...My truck runs stock front wheels on both front & rear with the fronts having spacers to make the width even.  As far as I remember this did not change or improve the handling much.

Definitely interested to see the results if you move forward with it, wouldn't mind adding some more stability to my truck if there's a solution. 

[svenb 3176]

I believe equal length wishbones help massively with eliminating bump steer.

[svenb 3176]

Also a lot of rwd ford cars were wider at the front than the rear, helped with turn in. Not by a lot say about 10mm each side.

[Saito2 4856]

1 minute ago, svenb said:

Also a lot of rwd ford cars were wider at the front than the rear, helped with turn in. Not by a lot say about 10mm each side.

The old Associated RC10s gained wider front arms through the years (the '89/'91 Worlds Stealth cars having very long front arms) but the Stampede is the opposite, shorter front arms than the rear. That doesn't seem like it would be ideal. At the same time, I don't want to make the investment of moving to equal length arms front and back (as well as losing my current cool orange ones) if the increase in handling dynamics isn't substantial enough. 

[Pablo68 740]

Gotta say, I have no idea. I don't know enough about the theory when it comes to suspension geometry.
As an aside, be careful what you wish for. By changing things you might take away from the truck eactly what it is. I recently installed aftermarket chinese shocks on my Traxxas Slash and I'm seriously considering changing them back cause it doesn't handle like it used to, it handles really well now. No more leaning and wallowing like it used to. Sure it's better now, but I'm not sure I want that.

[BJoe76 392]

What I did with mine is use the XXL RPM arms in the front with their Slash/Stampede 4x4 rear arms and bearing carriers in the back then rear offset 2.8” Prolines all around as well as put in a SAVOX low profile 125oz/in metal gear servo, which allowed me to put the Rx on top of the servo and it has a now vintage New Era aluminum front bulkhead. I’m also using a the offset pivot balls for the Revo on the tie rods to level those out, which seems to have eliminated much of the bump steer that these things have.

Here’s the tie rod assembly I have on it to better explain the pivot balls, also kinda shows the long arm set up with the front offset Proline wheels all around.

 

then here’s the servo and Rx arrangement 

 

Finally, here’s the width of it with the front offset Proline Trenchers on Desperado wheels vs their pre mounted belted Trenchers on their wheels with the replaceable hex section, using the rear offset parts all around.

[Honza 354]

8 hours ago, svenb said:

I believe equal length wishbones help massively with eliminating bump steer.

Bump steer is caused by bad ratio of bottom arm, top link and steering link. 

For example, LA arms on TL-01 reduce bump steer, because the arms get close to the length of the steering links - the links stay the same. However, longer arms of the TL-01B version are still shorter than the steering links, so bump steer is still present.

Equal length front and rear arms help on touring cars, they keep geometry changes and damper progression in similar rate on both ends of the car. I'm not sure whether they have same effect on an offroad car, since ratio of length between front and rear is smaller.

[Saito2 4856]

1 hour ago, Honza said:

they keep geometry changes and damper progression in similar rate on both ends of the car. I'm not sure whether they have same effect on an offroad car,

This is what I figured and where I'm at. In the end, its just a Stampede so much of this might just be a thought experiment but its fun to learn nevertheless.

[Gebbly 125]

Thats a great looking car with pristine paint and decal job. Love the detail of the netting over the windows. I guess with modifications like these as well as :

10 hours ago, Saito2 said:

if the increase in handling dynamics isn't substantial enough.

we have to be careful it doesnt actually go in the other direction and make things worse. I know in 1:1 cars I sometimes see modifications people have made which "look cool" but have actually made performance worse. You'll have to find some way of testing before and after the changes. Maybe film going round a piece of track? It would be interesting to actually see the difference in performance caused by the changes.

 

A slight aside...

11 hours ago, Saito2 said:

adding 1:1 wheel weights up front

You need @Willy iine to make you a miniature wheel balancing machine  Actually, is that something missing from Willy's workshop? A wheel balancing machine and one of the pneumatic machines that take off and put on tires?

[Willy iine 12665]

15 hours ago, Gebbly said:

A slight aside...

You need @Willy iine to make you a miniature wheel balancing machine  Actually, is that something missing from Willy's workshop? A wheel balancing machine and one of the pneumatic machines that take off and put on tires?

I have not made a tire mounter, but Willy does have a balancer..  

..anyway, back to suspension talk.. 

[El Gecko 1156]

1 hour ago, Willy iine said:

..anyway, back to suspension talk.. 

Still, we never tire of seeing your creations!

re: suspension talk, I don't know that equal length wishbones would have a significant effect on handling, but I suspect not.

What I do know for sure is that when the amount of suspension travel is similar in both the front and the back, it's easier to tune the suspension for different behaviors.

And I suspect the additional width of the RPM kit is what makes it overall handle better than stock. Not necessarily that the wishbones are equal. If you put the narrower offset wheels on all four corners instead of the wide ones, I believe it would handle exactly like a stock truck.

[SlideWRX 450]

I don't know that 1:1 vehicle dynamics experts that worry about equal length front to rear.  It just isn't a concern; with the front suspension doing the steering and the rear 'just' following along, One end or the other doing the driving usually, or the the power distribution front to rear is usually not 50/50, the requirements are different enough that they don't worry about matching the designs.  They have different space/packaging constraints than RC though.  Longer arms on the bottom are still better for suspensions with long travel though, so your idea is still a good one. Will it show up as a noticeable improvement?  ... I don't know.  They have made the arms as long as they can on rc race buggies though.

If you are having the understeer on relatively flat smooth surfaces, I don't think the longer arm is going to change much.  It is generally the right conceptual direction, and I love tinkering with my RCs, so I would say go for it anyway!   It could come down to adjusting the suspension you have.  Ultimately the Stampede is a tall truck, with a high center of gravity to specifically encourage wheelies.  Any bit of lowering will reduce wheelies and keep a little more weight on the front tires when on the power.  Traxxas makes anti-squat wedges for that platform (wait, for the nitro version - does it have the same suspension/diff mounting?)  - That should help reduce wheelies.  Making the rear springs (or shocks) stiffer should help - maybe try adding spacers before going to an actual stiffer spring while trying things out.  Are sway bars available for this? a rear swaybar is an option.

[SlideWRX 450]

On 12/2/2022 at 12:58 AM, BJoe76 said:

... as well as put in a SAVOX low profile 125oz/in metal gear servo, which allowed me to put the Rx on top of the servo ...

then here’s the servo and Rx arrangement 

I did that same thing on my DT-03 buggy, trying to use what I had to move the weight forward a bit.