Front axle angled down causes torque twist?

[devuser]

Hello,

I've having some problems with my SCX10.

Initially i've created some diy links for the power ram body (~12.3 wheelbase), 4 link front, everything worked ok.

In order to mount the new bright jeep body I had to extend the wheelbase. To do that, i've modified only the upper front links and now the front axle is tipped down.

I also added the rc4wd 100mm king shocks.

The problem i'm having is that the left drivers side tips left when accelerating, even a little. Is this caused because of the wrong axle angle or is it because of the softer shocks (i've set them up so that the whole body comes down ~1 - 1.5 cm when left on it's weight).

I've tried stiffening the front left shock without almost any effect...

The whole truck weights around 8 lb.

Any ideas on what I should do to fix that?

I'm going to post some pics and video when I get home to better explain my problem.

Thanks for the help!!!

[allanon1965]

For starters, lengthen the lowers to get the axle back to the proper angle...you should have lengthened the uppers and lowers by the same amount...possibly doing the rear only or splitting the total length equally between front and rear....

[devuser]

Originally Posted by allanon1965

For starters, lengthen the lowers to get the axle back to the proper angle...you should have lengthened the uppers and lowers by the same amount...possibly doing the rear only or splitting the total length equally between front and rear....

i've chose the easiest solution hoping that it would work, but it seems that it didn't

The optimal solution is to have the front links length equal to the respective rear links, right?

Let's hope the traxxas rod ends threads hold another modification

Thanks

[Bmanthei]

From what ive seen on the different stock scx10 models. The rear is where all of the length adjustment is done. When i bought my aluminum parts i had to find the correct length for the rear but the front were all the same length.

[allanon1965]

the easiest thing would have been to lengthen the lowers first, but i usually get the fronts set where they need to be while making sure the servo isnt pushed too far forward and hitting the crossmember, then I set the rear where it needs to be. very seldom is the front and rear lengths the same... a few pics would help alot to see whats going on...

[1967Typhoon]

Originally Posted by Bmanthei

From what ive seen on the different stock scx10 models. The rear is where all of the length adjustment is done. When i bought my aluminum parts i had to find the correct length for the rear but the front were all the same length.

Yup, usually you want to tinker with the rear axle to get the length you need. If you were shortening it greatly I could see you doing the front and the rear. But shortening the front only probably isn't the best route. As far as it adding torque twist, that part doesn't make much since. Are you SURE it didn't have torque twist before? The only solid mods I know of to remove torque twist are to mirror the transmission, or try to tune it out somewhat with suspension setup.

[QuesoDelDiablo]

The torque twist may have just become more obvious with the new shocks you added. This is why I prefer to only make one change to my truck at a time, it allows more opportunity to diagnose oddities.

[devuser]

Here are some images/videos with my setup and problem:

Video 1:
https://youtu.be/h_zwiEiRCSQ
https://youtu.be/tZedtLNpGw8

Video 2: (drive shafts chatter?)
https://youtu.be/eHhuxfQ-bHU

Front axle side view:


Front axle frontal view:


Front left shock setup:


Front right shock setup:


Rear shocks setup:


Rear lower links: ~13.5 cm - including the rod ends


Front lower links: ~13.8 cm - including the rod ends


Lateral view:

[QuesoDelDiablo]

RE: Video 1

That's just torque twist, almost every SCX I've seen does that to some extent. You can mirror the tranny to get rid of most of it, or if you increase the preload on the left rear shock that should help. I don't like mirrored tranny's because you have to delete the spur gear cover.

It looks more obvious because of your soft shocks but it's not specifically hindering performance - it just looks ugly.

I'm not sure if that axle rotation is a problem or not; it probably is. The pinion angle is good by having it rotated that way, but if you don't clock the C hubs it also alters your steering geometry.

RE: Video 2

Looks like maybe your driveshafts are out of phase. Did you align the little marks when you assembled them?

[1967Typhoon]

Originally Posted by QuesoDelDiablo

I'm not sure if that axle rotation is a problem or not; it probably is. The pinion angle is good by having it rotated that way, but if you don't clock the C hubs it also alters your steering geometry.

These cheapo 11 degree Chubs allow you to clock the axle and still have positive caster angle. I HIGHLY recommend these. They are great, regardless of price.

[JohnRobHolmes]

You probably raised the roll center by adding a heavier body on top. Test this by removing the body and seeing if the twist goes away.

You will need to raise your upper links on both axle and chassis to get the roll center back to your CG. This will screw with your antisquat, it might cause more problems than it solves. Alternatively, add weight very low to your chassis (skidplate height) and this will lower your CG towards the roll center. Since the body is further away from the roll center than the skidplate, you will need to add more than the body weight to counterbalance. Anything will help, but you will probably need 2 to 3 times the body weight to negate the twist it causes.

[devuser]

Originally Posted by 1967Typhoon

These cheapo 11 degree Chubs allow you to clock the axle and still have positive caster angle. I HIGHLY recommend these. They are great, regardless of price.

I don't know what clocking the c hubs means

Caster angle - i've just read some things

I'm going to create new links for the front so that the axle has the initial RTR angle which should be perpendicular on the ground and not tilted forward as it currently is..

[devuser]

Originally Posted by JohnRobHolmes

You probably raised the roll center by adding a heavier body on top. Test this by removing the body and seeing if the twist goes away.

You will need to raise your upper links on both axle and chassis to get the roll center back to your CG. This will screw with your antisquat, it might cause more problems than it solves. Alternatively, add weight very low to your chassis (skidplate height) and this will lower your CG towards the roll center. Since the body is further away from the roll center than the skidplate, you will need to add more than the body weight to counterbalance. Anything will help, but you will probably need 2 to 3 times the body weight to negate the twist it causes.

If i remove the body, i think it goes away a little and it isn't as visible as with the body on.

Haven't weighed the body yet but I think it has around 0.5 kg. the whole truck has ~3.5 - 4 kg.

From reading your post and looking at the truck it seems indeed that the body it very up high and combined with it's weight and bad links sizing, it's causing that ugly left tip...

I'm going to recreate all upper/lower links, equal sizes for front and back to fit the new bright body wheelbase and correct axle position.

Would it help if I add a shock lowering kit? The stock shocks were ~90mm and the rc4wd ones are 100mm.