Front downtube hitting tire when shocks compress

[jkallay]

If the suspension fork on my conversion takes a good hit, the front downtube will hit the tire, bringing the front of the bike to a sudden stop (which is obviously dangerous). I've tightened up the suspension, which mitigates the problem without solving it. The options I've considered are 1) put some kind of spacer on one of the shocks such that the fork bottoms out early, before the point where the tube hits the wheel. 2) Replace the fork with a rigid one.

Suggestions?

 

[Cruzbiker]

No suggestion yet.
Since it is a conversion, did you flip the fork around? Which size is the rim?
Pictures kind of help understand what is happening...

 

[Charles.Plager]

No suggestions except to post pictures. Maybe we'll see something.

 

[jkallay]

Compressed front shock

• jkallay
• Sep 9, 2015

Here's a photo. I used bar clamps to compress the shocks until the front downtube hits the wheel. That happens after about 1.75" of compression, so the shocks still have a fair bit of travel left in them beyond this point.

UPDATE: image is now public. It should be self-evident that the fork is turned around. The rim is a standard mountain bike 26" x 1.XX" (originally held 26" x 1.95" tires). Note also that the adjustable downtube is at maximum extension due to my height. I will post another picture of the whole bike.

 

[SamP]

"You do not have permission to view media within this album."

BTW, You never did say what size rims you've got, nor comfirmed you turned the fork around.

 

[SamP]

Now it's obvious. Now that I think about it, the telescoping front tube (official name for the thing you're calling the downtube) hitting the tire is the ultimate limit on TFT extension on my bike too; the only time that happens is when I'm doing maintenance stuff to my bike, my normal extension is pretty short and I'd actually like it to be longer to improve the angle for the front derailleur.

I think you're going to have to try to find ways to reduce the extension, probably by moving the seat back or your position on the seat. A layback seat post might help you move backward on the seat--though you'll end up pretty reclined. More radically, you might need a larger framed bike with a longer top tube, also to reduce the TFT extension.

With the TFT extended that far, don't you have shifting problems with your front derailleur?

 

[Cruzbiker]

As SamP said, the best option is to move your seat backwards.
You could also try smaller wheels, but I don't like this approach.
A fork without suspension will solve the problem, but you might miss the front suspension.
A rather radical approach is to angle the TFT (have a good welder do the job) as this drawing shows:

ScreenHunter_001

• Cruzbiker
• Sep 10, 2015

ScreenHunter_001

• Cruzbiker
• Sep 10, 2015

 

[Cruzbiker]

In case you don't want to go without front suspension, another option is to get a suspension fork with 30 mm travel.

 

[jkallay]

Now it's obvious. Now that I think about it, the telescoping front tube (official name for the thing you're calling the downtube) hitting the tire is the ultimate limit on TFT extension on my bike too; the only time that happens is when I'm doing maintenance stuff to my bike, my normal extension is pretty short and I'd actually like it to be longer to improve the angle for the front derailleur.

I think you're going to have to try to find ways to reduce the extension, probably by moving the seat back or your position on the seat. A layback seat post might help you move backward on the seat--though you'll end up pretty reclined. More radically, you might need a larger framed bike with a longer top tube, also to reduce the TFT extension.

With the TFT extended that far, don't you have shifting problems with your front derailleur?

Thanks. I posted the update about the extension of the TFT after looking at the brag board and noticing the commonality of the conversions having tons of clearance off the tire and short front tubes; playing with the length of the front tube makes the relationship between the two quite obvious.

@SamP, what connection do you notice between the TFT length and shifting performance? I'm not seeing it.

The one thing I have observed is that the bracket on the fork effectively can add an inch or more to the length of the chain stay, depending on how deep into the dropout (that is, away from the bottom bracket) the wheel has to go to align the rim with the brake shoes in their lowest (that is, toward the hub) position. That extra inch renders the chain too short to fit around the largest gears I happen to have (30T on the cassette and 42T on the chainring). Sure, that's cross-chaining, but that extra inch surprised me.

 

[SamP]

The front derailleur is designed for a specific range of chainstay angles, typically around 60-70 degrees. With my short TFT extension, the angle probably is around 80 degrees and getting shifting to work really well is a finicky adjustment. Your chainstay angle is probably out of spec on the low side.

Chainstay angle: angle between tube the derailleur is mounted on and the line between the bottom bracket axle to cassette (hub) axle. Normally on DF bikes it's basically the chainstay, but on Sofrider and Conversion Kit the chainstay isn't on the bottom bracket axle or hub axle.

[a clarification and fixed a few typos]

 

[hurri47]

Before embarking on any bike surgery, I'd consider the proposition you are just too tall for that bike (unless you can find a way to move the seat pan aft - a lot).

If the frame won't permit enough seat location adjustment, is there any way to move the clamp at the steerer tube up at all? Like about three inches? Maybe with a steerer extension? That would have the possibly undesirable effect of lengthening the boom even more. If you are near the maximum now, forget I said anything.

-Dan

 

[SamP]

That's a good point Dan. Putting some 2-3 inches of spacers between the headset and the steerer extension tube might be enough--if the steerer tube on the fork has enough length.

 

[Doug Burton]

Integrating some of the previous posts, I'd do this:
1. The bike fits you now. Measure the distance from where the upper and lower seat shells bolt together to the center of the bottom bracket axle.
2. Reduce the length of the adjustable front tube until the derailleur "chainstay angle" is between 60 & 70 degrees. Preferably 63 - 67 degrees, as this is what most front derailleurs are designed for.
3. Now see how far you have to move the seat back to regain the original seat-to-bottom bracket dimension. If it can't be done, your frame is too short. Life will get easier if you get a longer frame.

Be careful with spacers. You can run out of fork steerer tube length and the steering column tube can detach from the steerer tube if you're pulling hard up a hill.