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  #1  
Old 7th December 2021
tkvalentine tkvalentine is offline
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Default Looking for community help/steering/front end issues

Decided to post here rather than the suspension forum as they are generally dealing with tube front ends on stockish bikes. My issue is more of a bobber front end topic.

So need help/advice/pointers or an ass chewing on the following issue as documented.

Here is the project history:

1. 1998 Sportster frame
2. David Bird hard tail - 4 inch stretch, 1 inch lowered
3. Stock neck rake (unmodified)
4. 2 inch under springer forks
5. 18 inch rear rim/Shinko classic tire
6. 19 inch front rim/Shinko classic tire

And here is the present metrics (as measured on bike):

1. wheelbase 64 inches
2. 30 degree rake
3. 3.5 inch offset (neck center line/rear fork leg to axle center line/rocker axle mount)
4. 13.5 inches to front axle center (front wheel radius)
5. 26.5 inches from top of neck head to rear fork leg rocker pivot
6. The bottom frame rails are level on the standing bike.

I computed the trail prior to building/purchasing hard tail and springer to check
if it would be within the acceptable value 4.5 inches plus or minus some and came
up with 4+ inches so decided to go ahead with obtaining the hard tail and springer.

I checked the trail with an online calculator (RB racing )
and got 3.8 inches of trail.
I did a manual math check with measurements from the standing bike and got 4.8 inches of trail.
I also did a physical check of standing bike (extending centerlines and dropping vertical from axle)
and got 4.3 inches of trail.

So here is the result of my first test rides at low speed <10mph:

1. Front springer suspension is very stiff, little movement at best.
2. Overall steering is very heavy, noticeably more than it should be.
3. Bike seems to want to steer itself left or right, no prominent side tendency seems to start on one
side then as you correct steer is seems to migrate to the other side.
4. Bike has the sensation of wanting to lean/tip to one side or other and is noticeably
hard to correct.
5. Bike has what I call a negative gyroscopic feel, like the wheels are fighting the normal experience.
6. All above happens under 10 mph.
7. Not feeling comfortable going any faster until I sort this out.

I'm a long time forum guy, old army welder, long time rider and this is my 2nd custom project so I understand the physics, terms, mathematics and etc. I think I overlooked something along the way and need second opinions.

Thanks for your input.

And some pics...

Facebook pics
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  #2  
Old 7th December 2021
Fivecats's Avatar
Fivecats Fivecats is offline
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2. Overall steering is very heavy, noticeably more than it should be.
3. Bike seems to want to steer itself left or right, no prominent side tendency seems to start on one
side then as you correct steer is seems to migrate to the other side.

I would say the steering head bearing adjustment is too tight.
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  #3  
Old 7th December 2021
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Hi,

I don't think the length of the hardtail makes much difference to the front end performance if the geometry is all good, the back wheel follows the front wheel.

I agree with the first reply, the front end maybe bolted up to tight, I would also be looking at the front wheel & checking it's in the middle of the forks, is the front brake caliper sitting correct on the forks & the disc brake?
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  #4  
Old 7th December 2021
casual_obser casual_obser is offline
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Default

I'm not knocking you and I'm sure you did the math right, but the description sounds like you may have negative trail instead of positive.
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  #5  
Old 7th December 2021
tkvalentine tkvalentine is offline
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Default

Ugh! Schedules! Just getting to this.

Thanks for the replies/remedies. The trail is OK was measured at 4.3" positive direction (behind the centerline of the head), so close enough to 4.5" stock to not be the problem.

My local pro builder bud whom I have used services in the past and is hugely in the know said to start with the head bearing tightness first. So I think your replies here jive with him.

Will tear down and adjust tomorrow as time is too late today. Will report back when done and tested. Might get snow tomorrow so may have to wait for road testing.
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  #6  
Old 8th December 2021
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rejeanprimeau rejeanprimeau is offline
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Straddle the front wheel between your two legs, shake the handle bar and it will tell you how the front behave. Have someone sit on the bike holding it straight but you, not the sitting helper, hold the handle bar.
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Old 9th December 2021
tkvalentine tkvalentine is offline
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Bingo!

Put the front end in the air and yes steering was really tight/sticky.

Tore down the bars etc. to get to the head. Noticed that it was no longer tight/sticky.

Put the handle bars back on (base flange bolted to rear fork legs) and the steering was again tight/sticky. Hmmm... what's going on here.

Pulled the bars and base mount flange off again and loosened up the crown nut reinstalled the bars and base mount again. This time much looser but I could still tell that mounting the handlebar base was cinching down on the bearings.

Managed to get it loose enough to road test (snow missed us today) and WOW big improvement, took care of 90+% of the problem. I still perceive a slight sticktion in the steering which I will try to dial out but first...

To understand the handlebar mounting base causing the head and bearings et. al. to tighten up when mounted. Here's my thinking, the force applied by the mounting bolts for the handlebar base is pushing the top clamp down or more accurately pulling the whole damned fork assemble up through the top clamp. This in effect is putting immense pressure on the lower bearings/race (the top is fixed by the upper fork clamp and bearing nut). To alleviate this I need to shim between the handle bar mount and the top of the rear fork legs that is is bolted to. This will make clearance so the handlebar mount base will not interact with the top clamp. My handlebar mount base has counter sunk recesses that the top of the rear fork legs fit into. Maybe just mount it upside down?

Thanks all.
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Old 10th December 2021
tkvalentine tkvalentine is offline
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Success! Pretty well dialed in on the steering head/bearing tightness issue.
Had the bike up to 50 (I think the speedo is reading high probably more like 40) in 3rd gear on my short test runs and all seams quite workable. Will continue the increase by 5mph test to bring it up to speed.

Seems like the handlebar base mount was the source of the tightness. I never readjusted the top bearing nut as it was acceptable tightness to start with. I did shim the handlebar base mount (it had countersunk recesses to mate with the rear fork tops) to provide clearance with the top clamp and this solved the extra cinching tightness when mounting the handlebar base.

And snuck in an additional correction with the primary chain adjuster being too tight, backed off an 1/8 turn to loosen and now no more chain chatter. I had this adjusted to start by the FSM specs. Funny how much difference there is between the spec and real world application.

Thanks again.
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  #9  
Old 11th December 2021
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F.W.I.W. there is a method of adjusting the primary chain by sound. After returning from a ride, bike warmed up, set the idle high,(a tachometer helps but not a necessity I like 2,500 r.p.m.) tighten up the adjuster till you hear a “whine” then back it off till the “whine” is gone, then a tiny bit more and lock it down. Been several years now and have enjoyed ease of finding neutral, less clunk going into first and a bit easier shifting. I know my chain life and tensioner assembly are enjoying longer life as well. Your experience may vary.
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