TT MTA4 S50 conversion

[j.f.s]

Now, this is why I like the RCM Forums; you get high quality answers

Krawlin, thanks for an easy to understand explanation. I do have the Hudy book in PDF format so I'll start reading tonight

[j.f.s]

Here's a short video. May not tell much but I did notice I had some binding in the rear arms so I fixed that last night.

That damn camera really need great lighting to show it's full potential so there's a bit of noise in the video. 1080p available if played from the tube website.

[YOUTUBE]h8NO4DCWmyA[/YOUTUBE]

[Overdriven]

Krawlin: I've never had an MGT, I just noticed the geometry had been changed quite a bit with the knuckles and bumpsteer was mentioned. After seeing the video, altering the geometry any further probably wouldn't help things. Good point about roll center btw, got caught up in the other elements and forgot about that aspect of it.

[Krawlin]

Sorry if I came off as rude in that post, wasn't trying to be or imply that you were supposed to know that. But I agree that any more separation at the suspension arms, or changing the roll center, would not do much good without making quite a few other changes or truly knowing what effects it will have on the truck's handling.

However, I noticed in the video that the front end had very minimal bump steer, in my opinion not enough to be noticeable when driving a truck like this, but the rear seemed to have enough bump steer that it could clearly be seen in the video, and rear bump steer is not good thing, that can cause some very strange handling characteristics.

J.F.S, I would suggest focusing on getting rid of the rear bump steer first, as you can always "drive through" the front bump steer by correcting with the controls, but since you can't steer the rear wheels you can't as effectively correct any strange things the rear end does while driving like you can with the front end. Unfortunately due to your modifications, even if RPM offered their true-track kit (basically converts to a lower H arm and eliminates the toe link) like they have for the Revo and Maxx trucks, it wouldn't really be able to work for you as it would revert back to stock width and geometry, so you'll just have to work on getting the toe link set up right.

Maybe it would it be possible to do a lockout system similar to what the crawler guys do on steering axles in the rear of their crawlers. If all of the planes were correct, you could in theory run a short linkage from the steering arm on the knuckle to one of the now unused shock mounting tabs on the suspension arm. I just roughly mocked this up on my MGT with a short steering linkage from a 1/8 buggy and with the stock MGT knuckle it seems like it could work, I'll actually bolt it up and post a picture soon.



I'd also like to bring up ackerman angle, the MGT is known for having terrible steering geometry, where it will turn the inside wheel to full lock and barely turn the outside wheel, a common fix for this is a longer drag link which spaces the mounting points of the turnbuckles at the drag link farther apart, which greatly helps in getting the outside wheel to turn in more. Obviously you want the wheel on the inside of a corner to turn in more than the outside wheel, but the longer drag link makes the ackerman angle less extreme and more like how it should be.

I am curious as to what the Revo knuckles did to the ackerman angle, if they helped or made things worse. I know you used the stock length aluminum drag link with your steering, but do the Revo knuckles have a longer arm than the MGT knuckles? On my MGT, stock knuckles, I used an OFNA Ultra MBX Comp 1/8 buggy drag link modded to work on the MGT (just drilled a couple holes in the right places), and it improved my ackerman angle quite a bit over stock, and actually helped out on the bump steer.
With the steering at full lock (left or right, take your pick), at the inside wheel I only get very slight toe in (1 degree at the most) at the halfway point in the suspension stroke and as the suspension continues to full bump (full compression) the inside wheel toes back out to where it would be at full droop, and for the outside wheel the exact same thing happens with the wheels still pointing in the same direction, slight toe in at the halfway point, about 1 degree, then at full bump the wheel goes back to where it was at full droop. Again though, this is with a longer than stock drag link, but stock knuckles, so J.F.S's system may work just fine.


When you check for bump steer I would do so throughout the steering range, as this is where it's going to be noticed when driving, and here's why.....

....First, a scenario where bump steer will typically not be noticed when driving -

- Lets say you are going in a straight line, hit a jump, fly through the air and you land with the wheels pointing straight ahead, and you will continue on after the landing in a straight line. As you land, the suspension compresses, now since the suspension is exactly the same on both sides of the truck, both sides do the same thing. If you have bump steer, the wheels on both sides of the truck react the same, be it toeing in or toeing out (or both) throughout the suspension up stroke, but because you are going in a straight line and the suspension is the same on both sides and thus reacts the same, all this bump steer does in a straight line is scrub speed due to the wheels toeing in or out, which is unlikely to be noticed.

Now, two scenarios where bump steer will typically be noticed when driving, both include weight transfer but one is much more extreme than the other-

- Lets say you are about to hit a jump that is just ahead of a corner. You plan to land with your wheels turned slightly after landing the jump. So you hit the jump, fly through the air, and you are approaching flat ground with your wheels turned, ready to dart into the corner. The corner is going to the left, so you land with your wheels pointed at nearly full lock to the left, but it's not a perfect landing, you land first on the front right wheel. Now as soon as the tire touches the ground the suspension starts to react by compressing, and as noted earlier, bump steer occurs and changes throughout the suspension stroke, so as the front right suspension compresses, going through its entire range of motion with the wheels turned to the left, the wheel in this instance will toe out at a pretty extreme angle of 5 degrees, but most of the truck's weight is on that front right corner as it lands, so naturally the front right tire dictates where the truck goes because it has the most traction and largest contact patch at that moment, but it's just toed out at 5 degress because the truck has some pretty bad bump steer and that was the first corner to have it's suspension go through its range of motion. So now even though you are inputting left steering, your steering geometry says no and sends the truck in the direction that the first wheel to make contact with the ground is pointing, which would be more like straight ahead to the mid point of the corner, and the outside barrier of the corner. This means the truck did not react as you wanted it to, it reacted as the steering geometry wanted it to. This is very common in offroad vehicles with bump steer, landing from a jump with the wheels turned and the truck wants to veer away from the corner (in extreme cases), or turn less than the steering angle at full droop.


(See next post, had to edit because the post was actually too long! )

[Krawlin]

Scenario two is much less confusing than my rambling sentences in scenario 1. In scenario two, you are going full speed ahead down a straight away on a track and about to enter a large, high speed sweeping corner, and again the corner goes to the left. Assume you are driving a car with a high center of gravity and soft suspension. As you slow down slightly to enter the corner and begin to input steering, the vehicle (and thus obviously the suspension) react, and you begin to turn through the corner. As soon as you input steering, the suspension reacts from the weight wanting to continue in the same direction, so the weight transfers to the outside (right side of the vehicle) and the chassis rolls (vehicle leans to the outside of the corner). The suspension is reacting to this by the car's left side (inside) suspension drooping and the car's right side (outside) suspension compressing. In this case, the bump steer would be toe out as the suspension compresses. So the as the weight shifts, the suspension reacts as the chassis rolls, and at the same time that the right side (outside) suspension is compressing, the outside front wheel is toeing out. This causes the car to under-steer, as the outside tires have the most traction, and the front outside wheel/tire toes out as a result of the suspension compressing and making bump steer show its dark side. You have to basic walk a fine line between losing it/spinning out and compensating for this bump steer, which is basically over correcting if the bump steer were not there.




So the point of all of that? Well, not much point other than just to spread the knowledge, because really we are just dealing with a monster truck, not an F1 car, and I think the only bump steer you will likely notice any ill effects from would be the rear bump steer, as you can't correct that by direct input to the rear wheels as you go. There are at least 2 digits worth of things you have to factor in here, and trying to make them all right would be like trying to polish a turd, because its just an MT and its suspension and tires are going to be numbing enough to the feel of the truck that you shouldn't notice much difference.

If you so much as adjust the camber on simple geometry pillow ball suspension like the MGT has you can affect bump steer at the same time, as well as camber rise, because you have to lengthen or shorten the arm (threading the pillow ball in or out of the arm counts as changing the length of you arm, as the pillow ball is 1 of the suspension's pivot points), and lengthening or shortening the arm changes the height difference between the upper and lower arm which affects camber rise, and changing the camber will affect the kingpin inclination angle which affects how bump steer changes throughout the suspension stroke at any given steering angle because kingpin inclination angle causes the arm on the knuckle to swing in an arc, which alters the plane of the steering linkage in relation to the lower arm throughout the steering range of motion and suspension's range of motion, etc., and things like this should be given some thought when trying to eliminate bump steer, and realistically unless you can completely redesign the suspension and steering geometry, you may not be able to get rid of all of the bump steer, in many cases though you could work with it as a tuning tool or a driving aide (Remember, bump steer is not always bad).

The simplest way to reduce bump steer to where it does not affect the vehicle or can even be noticed when cycling the suspension on the bench, is to play around with the height of the link at either end, now in the rear you can't adjust that since the bolt goes through the turnbuckle horizontally, but you can change the height of it at the knuckle, and in your setup you can shorten or lengthen the link and change it's mounting point on those CF link mounts you have to fine tune it. I will try out that lockout setup on my MGT now and post a picture later tonight and let you know if it works well on the bench (my MGT is in the roller and for sale stage, can't really run it).

Enough rambling, just thought I would throw some information out there since changes to suspension and steering geometry on our RC cars are often not given the thought that is given to changing the suspension and steering geometry on a 1:1 real vehicle, and 1 change can affect so many other things at the same time and not always in a good way, and even though we aren't dealing with an F1 car, the same things still apply. If you have a question about it, just ask, I'm no expert but I'll try to help as best I can, as I've had alot of experience to say that I've "been there done that", especially with trying to improve suspension/steering geometry, I own two Mugen MBX5 series cars (5R and 5T), so I know all about bump steer issues.



Just some food for thought.

[Krawlin]

Any updates to this or did I kill the thread with all the suspension/steering geometry talk?

[coolhandcountry]

I think you may have temporarily stunned it.

[j.f.s]

lol, no worries. Family, weather and a persistent flu has kept me from tinkering with the truck.

I've connected all the electronics and calibrated the ESC to the RX but that's about it. Still have to set the slipper and set the torque control. Then there's the crazy weather preventing driving with rain and storm winds and temps in the 50°F range. We usually have around 20 inches of snow and temps around -4°F from the middle of November.