Front end vibration

[Jeff t]

I think I have read every thread on the search looking for help but I’m Struggling to solve an issue with vibration when braking. Set up is a PM single disk anti dive, BGM dampers and I think progressive springs. I have checked every nut and bolt, had the headset off and nipped up the steering nut. Changed the angle of the anti dive up and down and put the dampers on the softest setting. Also had a DTI on the disk and it runs true.
Also done the bit rocking back and forward with the brake on and no play between the headset and leg shield, the stand on blocks no play in the front forks when I try and move the wheel.
The only area I can think of is the front end is pretty hard when I push down on the forks could stiff springs be an issue, as it’s a pain of a job thought I would get opinions first.

Oh and It’s a new build so always been like this any thoughts welcome.

[Warkton Tornado No.1]

I would suspect the springs, especially nowadays with no certainty of components remade to OEM standards. The best springs I ever fitted were genuine OEM TV175 (progressive springs) which literally allowed hands-off riding on such terrible surfaces as Three Sisters. Unfortunately, I have never found any remade sets that have come close as far as matching them. That includes a pair from the chap that almost claims to have invented the wheel....

However, TBH although my personal preference is not to have anti-dive - I have tried it but prefer the fact that 'dive' sharpens up the steering geometry for corners - if it's a set-up you like, soft springs should be better as they only have to perform the function of suspension, not resisting dive under braking.

IMO damping control is important & OEM dampers are very good. That's not to say they cannot be bettered, but the steering damper type are dire. The springs are there to absorb surface irregularities, not to pass the full sensation on to the chassis & rider. The dampers are there to control the springs from oscillation - predominantly to control the rebound.

[hullygully]

my mates gone thru the same brain numbing scenario
he changed bearings & cups & races that many times I'm crying for him.
even just ballies instead of the bearing rings.
he's got Italian GP150 frame & PM Tuning forks, plus external disk & anti-dive

[al pushpak]

I think I have read every thread on the search looking for help but I’m Struggling to solve an issue with vibration when braking. Set up is a PM single disk anti dive, BGM dampers and I think progressive springs. I have checked every nut and bolt, had the headset off and nipped up the steering nut. Changed the angle of the anti dive up and down and put the dampers on the softest setting. Also had a DTI on the disk and it runs true.
Also done the bit rocking back and forward with the brake on and no play between the headset and leg shield, the stand on blocks no play in the front forks when I try and move the wheel.
The only area I can think of is the front end is pretty hard when I push down on the forks could stiff springs be an issue, as it’s a pain of a job thought I would get opinions first.

Oh and It’s a new build so always been like this any thoughts welcome.

Tyre/rims ?

[ULC Soulagent]

It has been said if fitting anti dive then the softness possible springs are needed

[Jeff t]

Thanks for the replies I have not had chance to do much else but will strip down and change the springs, tyre is brand new but it did have to be swapped over as the PM disk has it the other way round. (bought the wheels and tyres before the disk)

[Storkfoot]

Vibrations travel too. Check your engine isn’t touching the frame. Also, what engine mounts do you have and are the engine bolt nuts too tight? Is your rear shock absorber okay too?

Could it be an issue with the disc brake itself?

I have no experience of anti dive but I have had experience of too stiff front springs. In my experience, they result in a very harsh front end and the front tyre skipping across the road rather than riding the undulations.

It has taken me many years to conclude that standard springs with Escort dampers is about as good a front end as you can expect. Such a front end needs to be balanced with a decent rear shock too, say a Prodive/ Chiselspeed one.

The last two Lambrettas I have restored have been a 1962 TV and a 1969 GP. I have reused the original springs in both cases. One has a set of MEC/EUR dampers, the other Escorts. I have a really badly potholed road not far from my house. I have deliberately driven fast across the potholes to test my theory about what constitutes a good front suspension. Both these scooters just rode out the potholes effortlessly.

My TS1 has MB progressive springs and Escort dampers and has done for many years. That scooter is bone shuddering as it travels over the potholes. I would guess that the front tyre spends more time in the air than it does on the ground which is why I tend to get uneven tyre wear at the front.

[Jeff t]

Thanks I’ll check the points you raise, rear shock is a protech and engine mounts are new think they where BGM. I have took the forks out and everything looks ok springs are definitely stiff though so will replace them even though they are new.
i was trying to measure the forks to make sure they are straight and aligned but not that easy although they do look ok by eye. The only thing I noticed was the tyre didn’t run true when I spun the wheel so will check the disk without the wheel when I put it back together, can’t see anything else.

Cheers

[Storkfoot]

Thanks I’ll check the points you raise, rear shock is a protech and engine mounts are new think they where BGM. I have took the forks out and everything looks ok springs are definitely stiff though so will replace them even though they are new.
i was trying to measure the forks to make sure they are straight and aligned but not that easy although they do look ok by eye. The only thing I noticed was the tyre didn’t run true when I spun the wheel so will check the disk without the wheel when I put it back together, can’t see anything else.

Cheers

Protech, yes

Whilst you are at it, checking the fork links, get someone to push it forward and activate the front brake. You watch what happens to the front wheel. It should stay central but if there is something wrong with the fork links or springs, you should see that the wheel goes down at an angle to the forks themselves.

[Adam_Winstone]

Try some different engine mounts. A pal recently had similar issues at the bars, which was related to vibration travelling through the frame. He checked everything, including having the crank alignment checked by 2 different engineers (both confirmed it as being true), and a change to softer mounts instantly resolved the issue; riding like a completely different bike!

[Rich Oswald]

Thanks I’ll check the points you raise, rear shock is a protech and engine mounts are new think they where BGM. I have took the forks out and everything looks ok springs are definitely stiff though so will replace them even though they are new.
i was trying to measure the forks to make sure they are straight and aligned but not that easy although they do look ok by eye. The only thing I noticed was the tyre didn’t run true when I spun the wheel so will check the disk without the wheel when I put it back together, can’t see anything else.

Cheers

Were the BGM mounts definitely put on the correct sides, it does matter.

Rich'

[Jeff t]

Good shout but yes remember it was a nightmare seeing the mark which identifies them.

[Adam_Winstone]

In the case of the 2 bikes that I know to have suffered badly with vibration, these were correctly fitted to their respective sides. Indeed, it was this specific attention to manufacturing specification that made both owners choose the BGM mounts over other brands.

PS - I love and use a fair bit of BGM stuff, and will continue to recommend it, however, it seems as though these mounts (like the early troublesome Scootopia ones) are too hard.

[Andy B.L.C.]

Just a thought... Sticky's 'bible' has a pic of the fork internal spring mounting points being measured with a tape measure, the reason being, if I understand correctly, is that slack engineering/tolerances result in a difference in spring travel length meaning that the hub is no longer at 90 degrees to the road which will inevitably cause vibration...

[nsaints]

After trying everything I’d consider swapping your BGM silent blocs for SIL
Much to my surprise this is what solved my vibration issues I have been chasing down on my standard GP

I now wonder if all handed silent blocs have this characteristic, with a harder rubber right hand mount where some vibration is an inevitable consequence.

[Jeff t]

Plan now is to rebuild the forks with the new springs when I get them, check the disk runs true before putting the wheel on and then check the other tips on here. If that doesn’t solve it probably then look at the mounts. To recap though the vibration is only when I brake and coming to a stop.

[Adam_Winstone]

Sounds more like wheel judder under bracking load. I think your brake and first first approach is the right one.

Good luck.

[Jeff t]

Stripped down the forks and one of the top fork link buffers is loose looks like the bolts have bottomed out, I am sure it won’t help but could that cause it?

[Andy_S]

What type are the softer engine mounts and where do you get them from ?

[Warkton Tornado No.1]

Stripped down the forks and one of the top fork link buffers is loose looks like the bolts have bottomed out, I am sure it won’t help but could that cause it?

It's not likely as even with the bottomedoutscrews, the top buffers will be pushed into place by the rising fork link & subsequently compressed,

Has tyre pressure been mentioned?