Long Travel Fork on Short-Travel Frame?

My short-travel frame with 107mm of rear travel is spec'd for 69deg headtube angle with 505mm axle-to-crown fork-- about 120mm travel.

However, I selected a fork based on characteristics: unobtrusive single-crown, modern stiff Boost spacing, robust tapered steerer and 38mm stanchions, 200mm direct-mount brake, bolt-on fender, low weight and cost. The resulting fork is offered in 150mm-190mm travel.

I made an Excel sheet to illustrate the differences.

At 15% sag, the spec'd fork would sag to 487mm axle-to-crown, have 18mm of rebound travel and 102mm of compression travel.
While I've included a 100mm fork similar to spec, as well as a double-crown USD fork, I would point to the 190mm travel RockShox Zeb.
I know the general concern with longer travel forks is greater pedal pump (not relevant on a throttle e-bike), and slackening the headtube angle and thus upseting the steering geometry. However, the spec'd 487mm axle-to-crown (and thus HTA) could be matched with the 190mm fork by sagging down 90mm, leaving 100mm of compression travel. However, the softer spring rate would give greater mechanical traction.
Yes, in rebound, the 190mm travel fork would still rebound out to 577mm instead of 505mm, and thus slacken the headtube angle. However, this would only seem to occur when (completely) unweighing the front end, as during hard acceleration (wheelie), in which case steering inputs are marginal to begin with. In fact, having greater rebound travel should allow the front end to maintain traction and steering with greater rearward weight transfer.

If I've already "paid the price" for long-travel in terms of weight and cost to order to satisfy other requirements, why limit that travel?

fatty said:

However, the spec'd 487mm axle-to-crown (and thus HTA) could be matched with the 190mm fork by sagging down 90mm, leaving 100mm of compression travel.

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If you set sag to 47% (90mm/190mm) then your spring rate will be far too soft in the front and you'll blow through the remaining 100mm just by braking hard or leaning forwards. It'll be like riding a boat.

serious_sam said:

If you set sag to 47% (90mm/190mm) then your spring rate will be far too soft in the front and you'll blow through the remaining 100mm just by braking hard or leaning forwards. It'll be like riding a boat.

Click to expand...

Thanks. I was worried about brake dive, but hoped it could be mitigated by using volume spacers to reduce air volume and thus ramp up spring rate under compression. I planned to start without any and work up to max, but perhaps I should start with the max and test down?

fatty said:

Thanks. I was worried about brake dive, but hoped it could be mitigated by using "Bottomless Tokens" to reduce air volume and thus ramp up spring rate under compression. I planned to start without any and work up, but perhaps I should start with the maximum and test down?

Click to expand...

You could pack it out to reduce the volume, but it's kinda diminishing returns.

Another thing to consider, when you need to lift the front wheel while riding, for whatever reason, that added sag will make it a real pita. Say bye bye to hopping the bike over obstacles.

I may have missed it but didn't see mention of what frame you are using. I have seen headtubes that can not handle the stress of the additional length, as it can basically act as a giant lever, so take that into consideration as well especially if its aluminum

What I did in similar situation with my bikes was stiffen up the rear shock, resulting in restoring some of that change in angle at the head tube for riding when the suspension is loaded only by my weight.

Not sure if this mattered for the angle all that much, but the tilt back was making me bottom out my rear shock more, so that tuning did help with that.

Yep, the rear can be lifted to compensate the extra travel of the front. That is especially pertinent when we are going to ride on smaller wheels than those that a frame was designed for.

If limiting travel with sag, brake dive can be tempered with if the fork has a gate tuning option or more commonly, hi-low compression gate adjustment.

serious_sam said:

You could pack it out to reduce the volume, but it's kinda diminishing returns.

Another thing to consider, when you need to lift the front wheel while riding, for whatever reason, that added sag will make it a real pita. Say bye bye to hopping the bike over obstacles.

Click to expand...

I'll start out with max volume spacers, drop the rear to 33% sag, and plan on lowering CG height.

Good point about hopping, but since the use profile will be that of a light moped/motorcycle, I'd expect to unweight the front end and use the long, soft suspension to ride over obstacles anyway, rather than hopping over them.

Manbeer said:

I may have missed it but didn't see mention of what frame you are using. I have seen headtubes that can not handle the stress of the additional length, as it can basically act as a giant lever, so take that into consideration as well especially if its aluminum

Click to expand...

A 3kg dirt jump frame. The tapered headtube is very robust.

I've heard of such concerns, but closer attention must be paid to geometry:
Hucked to flat, a longer fork will have rebounded fully and thus greatly slacken HTA, meaning the force vector (vertical) is even further off-axis from the fork upon landing. Thus, the torque increase on the headtube is compounded by not only the longer lever, but also by the off-axis force.
However, when commuting, while a longer fork at similar sag% will still slacken HTA, that slackening puts the fork closer to the axis of force vector (horizontal) caused by road bumps. Combined with the ability to run softer spring with longer travel, I would argue this actually reduces the torque on the headtube.
In short, steep HTA = on axis for jumps, slack HTA = on axis for bumps

Therefore, I think this concern is greatly valid for MTB, but not valid for commuting.

In any case, I plan to compensate for the longer travel by running more sag.

dogman dan said:

What I did in similar situation with my bikes was stiffen up the rear shock, resulting in restoring some of that change in angle at the head tube for riding when the suspension is loaded only by my weight.

Not sure if this mattered for the angle all that much, but the tilt back was making me bottom out my rear shock more, so that tuning did help with that.

Click to expand...

MadRhino said:

Yep, the rear can be lifted to compensate the extra travel of the front. That is especially pertinent when we are going to ride on smaller wheels than those that a frame was designed for.

Click to expand...

I know this is a self-induced problem, and I do appreciate the brainstorms, but raising the rear ride height just introduces other handling problems, it doesn't solve them. Not only do you sacrifice what little (10-15%?) rebound travel you had (= no dip compliance), but the increased CG height increases longitudinal load transfer, worsening brake dive. Assuming an air spring, the increased spring rate to raise the rear would also reduce compliance and mechanical traction. A coil spring to separate ride height and spring rate would only resolve that last disadvantage.

Good discussion, but I still think sagging the front to maintain steering geometry is the best approach, at least on paper...

MadRhino said:

If limiting travel with sag, brake dive can be tempered with if the fork has a gate tuning option or more commonly, hi-low compression gate adjustment.

Click to expand...

I agree that increased compression damping would be desirable.
Unfortunately, to meet the budget target, the fork mentioned does not provide compression damping adjustment. But maybe increasing fork oil viscosity?

Lifting the rear is commonly done with dropouts positioning. The use of smaller wheels diameter is common and does bring back some of the height to the original frame design. Some frames that I have built are designed with dropout height and shock travel ratio adjustments, many have an excentric BB to tune crank positioning. I usually add a Cane Creek Angleset headset to fine tune the front.

I just realized that switching from 26" to 27.5" increases wheel+tire diameter by ~41mm, and thus ride height by ~21mm.

I think I need to add this 21mm to the axle-to-crown length, thus requiring even more sag to restore ride height.
Restricting sag to 50% raises effective a-c at ride height from 487mm to 503mm = 16mm, slackening HTA less than one degree.

MadRhino said:

Lifting the rear is commonly...

Click to expand...

If anything, I'll be dropping the rear from 15% sag to 33% sag.
I'll have to accept slacker HTA.

fatty said:

Manbeer said:

I may have missed it but didn't see mention of what frame you are using. I have seen headtubes that can not handle the stress of the additional length, as it can basically act as a giant lever, so take that into consideration as well especially if its aluminum

Click to expand...

A 3kg dirt jump frame. The tapered headtube is very robust.

I've heard of such concerns, but closer attention must be paid to geometry:
Hucked to flat, a longer fork will have rebounded fully and thus greatly slacken HTA, meaning the force vector (vertical) is even further off-axis from the fork upon landing. Thus, the torque increase on the headtube is compounded by not only the longer lever, but also by the off-axis force.
However, when commuting, while longer fork at similar sag will still slacken HTA, that slackening puts the fork closer to the axis of force vector (horizontal) caused road bumps. Combined with the ability to run softer spring with longer travel, I would argue this actually reduces the torque on the headtube.
In short, steep HTA = on axis for jumps, slack HTA = on axis for bumps

Therefore, I think this concern is greatly valid for MTB, but not valid for commuting.

In any case, I plan to compensate for the longer travel by running more sag.

Click to expand...

Makes total sense, I was originally thinking of crossing a log on a trail or bombing downhill over rocks but save for slamming curbs and giant potholes it sounds like a moot point. also, a DJ frame will def be overbuilt