Front end hops under hard braking

[Tom LaJuett]

I built my first bucket this year, and I'm having front end issues. It goes down the rd perfectly and steers great, but if I'm carrying any speed, say 70 mph or better, when I hit the brakes, the front end bounces violently from side to side. The wheels bounce 3 or 4 inches in the air from one side to the other. I custom built the frame, as well as the front shock mounts, which I think may be where the problem is. The shock mounts are on top of the frame and the shocks extend straight down to the top of the axle. If you measure from center of shock to center of shock they are 18" apart. I think what I need to do is build new mounts and widen the shocks so they are out closer to the wheels, but I"m not 100% sure. Has anyone else experienced this issue?

 

[2old2fast]

We need pictures!! but, yes , if the shocks are only 18" apart, that's probably the cause. You should have the shocks mounted as close to the unsprung weight [wheels ,tires .brakes] as possible,
dave

 

[ellis8500]

With shocks only 9 in off centerline, they are not really "there" as far as the suspension dynamics are concerned. As stated above they HAVE to be at least at the ends of the springs to have any affect due to the characteristics of a leaf spring. Where they are now they can only move fractions of an inch up or down because a stiff leave spring barely moves at all near the centerline. You can get shock mount spring shackle plates from RPM; use your existing top mounts with longer shocks and probably solve your issue. You can also get new shock mounts from RPM. Send him a pic of your front end and he'll tell you what you need. His prices and service are the best there is so ya won't go wrong talking to him.

 

[putz]

I would try going on gravel and adjust your proportioning Sounds like you are doing to much braking with narrow front tires and not enough with larger rear. If this where to happen on wet roads it could get UGLY.Try this first easy no money spent.

 

[409T]

I would try going on gravel and adjust your proportioning Sounds like you are doing to much braking with narrow front tires and not enough with larger rear.

Yes.

Also check everything that keeps the axle in place when you apply the brakes. Spring perch bolts, radius rod connections, and so on. Loose or broken brake pad or shoe? Loose or broken caliper mounts? It certainly wouldn't hurt to move those shocks outward, but at 70 mph broken or loose suspension or brake parts will ruin your day a little faster than poor shocks.

 

[ellis8500]

Just another thought here, especially for the new builder; suspension geometry is not something to experiment with unless you are ready to make the necessary changes to get it right. By enlarge, most of the T-bucket style hot rods have very similar to identical front suspension design. Noted builders like Spirit and RPM use the traditional, proven beyond doubt, design. The reason they do is because it works well and is a safe design. The shock loads experienced in just normal driving are much more than we might imagine. Lets say at 60 MPH you hit a bump that rises your front wheel 1 inch in 1/2 inch of travel. The impulse you have on your suspension is about 5-1/2 G's. If your normal front wheel patch weight is 500 lbs, this becomes a 2,750 upward "Wack" on your suspension. That's more than the weight of the car suddenly being felt by 1/2 of your front suspension. Poor geometry and insufficient spring dampening can lead to wild, sometimes catastrophic oscillations; what we call "Bouncing". So for you Newbies, stick with what works, ask lots of questions before you depart from the known to the unknown just for the sake of making it look different than anything else you've seen.

 

[Ma_n_Pa T]

If you have Disc brakes up front, check your wheel bearing preload, loose wheel bearings are known to start wheel movement during braking, check the whole front end as 409 stated, a loose part is one of the biggest cause of front end problems, also check that you have not preloaded your radius rods bothe front and rear when you installed them, they must be equal to each other or they will act as a torsion bar. Although I believe in shocks on all cars, they are there to dampen our road feel, they should not be used to cover up a problem.

Jerry

 

[T-Test]

Rotors could be warped if discs up front.

 

[ellis8500]

Wheels bouncing 3 to 4 inches in the air is a spring oscillating problem. The forward shifting weight during braking compresses the spring and then it bounces back. It continues to compress and bounce because it is not dampened sufficiently. This same thing would happen if you ran a normal car without front shocks. I've seen it to a lesser degree on some cars people were driving. They start to stop and the front end starts bouncing up and down. These were much heavier cars than our buckets. It is certainly wise to check all of the other things mentioned here; but, I'm guessing it's a suspension geometry problem. Contact Josh at Spirit and Ron Pope. Send them good pics of your front end and see what they have to say. These guys build cars for a living and can probably steer you in the right direction. Josh and Ron will help you even if it's not one of their cars. That's the kind of guys they.

 

[Tom LaJuett]

 

[Tom LaJuett]

 

[Tom LaJuett]

 

[Tom LaJuett]

I actually bought most of my parts through Josh @ Spirit, he was gonna send a pic of one of their front ends, so I can see how they do it. I appreciate everyones input, and I posted a couple pics so you guys can see what's going on, if nothing jumps out @ you guys, I'll probably fab up some new mounts and move the shocks as far out as I can get them. PS, the shocks are pretty far down in the stroke, maybe a shorter shock??

 

[EX JUNK]

I am no expert, but in your first picture the shocks see to be further apart at the top then at the bottom. Perhaps it is only camera angle. Another thing that seems odd is that I believe the upper and lower mountings should be rotated 90 degrees. As they are, I believe that they would hinder proper shock movement as they would seem to want to bind. The rubber bushing on the mounting points are absorbing a lot of the movement and not the shock.

This is only my opinion. I would hope that someone with a greater insight would chime.

Jim

 

[T-Test]

You could have a bad axle also as I see it is not a new one. I had one that was worn out at the king pin hole, not the king pin bushings.
Pull the spindles off and stick king pin [is it new] in hole and see if it has any play. Should be a snug[ push in by hand without too much resistance]. Mine was worn 20 thousanths, and would cause tires and wheels to shake[up and down as well as side to side] at different speeds.
Bruce

 

[ruggs]

I think X-junk got this one right. the shock mounts are 90 degrees out to allow them to work properly. A little wider stance at the lower end may also help....JMO.........ruggs

 

[Tom LaJuett]

Hey everyone, thanks for all of your input, I appreciate it! I'm gonna start by making sure the kingpins are tight & then I'll tilt the upper mounts out & move the bottom one's out as far as I can get them& turn them 90 degrees.

 

[roadmonster]

Get your wheels balanced first. The oscillation is caused by various frequencies harmonizing and the easiest one to control is wheel balance. But you must mount the shocks for maximum travel to solve the problem. If your tie rod is long and heavy it will also aggravate things.

 

[Rogers23T]

Dude did any of you guys think about the shocks needing to be on the 15 degree angle? Straight up and down has got to be a huge issue there must be that angle for proper ride. Look at any famous T bucket the front shocks are at an angle.

 

[donsrods]

Dude did any of you guys think about the shocks needing to be on the 15 degree angle? Straight up and down has got to be a huge issue there must be that angle for proper ride. Look at any famous T bucket the front shocks are at an angle.

Yeah, I held back from saying anything, but the shocks are not mounted correctly at all. To control a wheel the shock needs to be as close as possible to that wheel, and on an angle. Where they are mounted and how they are mounted they are doing no good at all. I would cut them off and mount them further out on the bottom and angled in a little at the top.

There are a lot of opinions as to the exact angle shocks should be mounted at, some say 15 degrees some say more. I have had them both ways. My 27 has the shocks mounted at more of an angle than some feel is right but I did that on purpose and they have been working great for over 20 years the way they are. I say on purpose because I like the shock to also control some side to side movement, and again, it must work because I can corner the car really hard and it tracks great. Lots of ways to build a hot rod and there is no one right way to do it.

Don