Hi guys, I’m new to the world of road bikes and recently bought myself a Trek Domane Al5 Gen 4 (2024).

I am over the moon with the bike, and it has opened up a whole new world of enjoyment.

That said, I have been dialling in my bike fit recently and was considering lowering my stem. The bike was supplied with a 5mm spacer on top, and 30mm of spacers below, so it sits quite high but with plenty of adjustment, or so I thought.

After some research into the ramifications of stem adjustment on full carbon forks, I realise that their are certain considerations that need to be taken into account, the primary concern being the positioning of the compression plug in relation to the stem bolts.

It is my understanding that the compression plug serves two purposes. It provides an anchor point to preload the headset and it provides internal reinforcement to the steerer tube when the stem is clamped on, preventing the carbon from being crushed/split.

This in mind I messaged Trek and asked if it would be safe to adjust my stem, and run 15mm of spacers on top and 20mm on the bottom. Trek assured me that this would be absolutely fine, so long as there is always a minimum of 5mm spacers on the top.

This in mind I consulted my bikes service manual in order to understand how my headset is assembled and how I would go about adjusting my stem height. This is when I discovered just how short the Bontrager compression plug supplied with my bike is. Even with the stem positioned as it is from factory, at the top of the steerer, I would guess it is slightly too short to provide support all the way between both stem bolts. Therefore, if I lower the stem further without cutting the steerer, the compression plug will in no way be serving to support the inside of the steerer where the stem attaches.

I’ve since reached out to Trek again and have been sent some very conflicting information;

Hi Scott,

Thanks for your message and for bringing this to our attention.

If the compression plug is not long enough to extend to the portion of the steerer being clamped by the stem, it can indeed be a concern. The compression plug is designed to reinforce the steerer tube and ensure that the clamping force from the stem is properly distributed to prevent damage, although the compression plug will still be reinforcing the steerer in the upper section, so there’s a chance your setup will be fine.

I went on to explain I explain that the compression plug supplied with my bike was not long enough to extend the distance between both stem bolts as it is, and that if I lower my stem without cutting the steerer it would provide zero internal reinforcement at the point of attachment of the stem.

Treks reply;

Hi Scott,

Thank you for reaching out and sharing your concerns.

It's understandable to be cautious about your new bike's setup. While the compression plug does provide some support to the steerer tube, it is not documented that it needs to be the full length of the stem bolts. Its primary role is to ensure that the correct preload of the headset system can be applied and also add some structural support to the steerer. The plug supplied should not be considered inadequate.

Given the importance of this component and for your peace of mind, we highly recommend visiting your nearest Trek authorised store when you can.

I am left rather confused and a bit perturbed. I am clearly being sent contradictory information.

I’ve since been on Treks website to try to find an appropriate compression plug, and they only seem to supply the short Bontrager one that appears to be fitted to my bike. It’s not entirely practical for me to visit a Trek retailer with my bike right now, and I had hoped to use it this coming weekend, albeit without adjusting the stem like I intended.

Is my bike safe? Must I cut the steerer if I wish to lower the stem by just a few mm? Should I consider installing a third party compression plug that is longer than that supplied by Trek? Why would Trek supply a brand new bike with an inferior compression plug that is either a) too short, or b) too short to allow for any adjustment in the stem height? Am I being way too cautious?

For some reason i stumbled across articles related to the ring of death and issues with carbon forks, and I can’t deny it has made me somewhat nervous.

Any advice would be massively appreciated!

Fusion Expander Compression Plug (origin8.bike)

Stop obsessing. Set the bar where it needs to be and ride your bike.

If it helps you sleep better consider that no CF seat post uses an internal plug to prevent crushing or splitting when clamped.

Stems are bored to produce round on-size holes which will not distort or crush steerers, with or without the plug inside. It's kinda like how you can't break an egg by squeezing it in your palm.

FWIW I suspect that the plug canard dates to the early days of CF steerers before stem makers realized the need to produce to tighter tolerances.

BTW Trek guarantees their stuff. If they had any concern they surely would have said so.

Where did you buy the bike?

Pedal On, they are an authorized Trek dealer here in the UK

OK thanks, that's reassuring RE carbon seat posts

Just be sure to torque the stem bolts to spec.

Barry

Do this and be done with it.

Normally the Trek dealers offer a free tune up within the first year. Gives them a chance to check over the bike. They should also be willing to lower the bars.

If you have the tools, allen wrenches and torque wrench you could just remove the top cap and loosen the stem. Then move some spacers on top of the stem. Do this and ride a bit to check the bar height is to your liking. Then later unless you have the tools, ask the shop to shorten the steerer tube. You can shorten it but cannot lengthen it. Well you can but that requires a new fork. Enjoy your new ride.

I have a Trek Domane SLR, and I assume the fork or at least steerer tube is the same. As I recall the manual for the bike shows the maximum amount of spacers on top of the stem and I think that happens to be all the spacers that come with the bike.

I asked one of the senior people at my local Trek store this same question and he said it’s not an issue. That the steerer does NOT rely on the plug for support from the stem clamping pressure. He also went on to say that this concern dates back to the early days of carbon use and it’s really not a thing for modern bikes. Of course, he can’t speak for the entire industry.

As for the contradictory advise, not everyone gets the memo and your first response may just someone restating the old wives tale rather than knowing what they are talking about. If your manual is like mine and lists the maximum spacers allowed on top of the stem, I’d trust that. Something in writing in the manual likely has thought behind it from people who really know. Not just some rando tech support guy that may have just started at Trek the same day they responded to your email.

While it’s not your job to educate the Trek tech, I would politely ask him to double check the issue as his advise is contradicted by the manual, if it is. Which it seems you may have already done, but got a reply from a different person. If so, nothing else you can do.

I ran my stem with a full set of spacers above the stem. So quite the chimney. I did this for at least 6 months until I was confident in the position before having it cut down. I still have a little space above, 10 mm or so. I’ve had no trouble with steerer tube.

I thinks it’s obvious given the short plug, that Trek does not expect the plug to support the steerer tube against clamping pressure.

The reason makers strongly suggest limiting spacers above the stem isn't mechanical.

It's medical and legal. They don't want to be blamed if you impale yourself and injure small delicate body parts in a crash.

I can’t post a picture till I have ten posts but this will snap in half (I tried putting a picture here, of it broke)
A picture is worth a thousand words, this is the second time this has happened, you can clearly see it is set up correctly and the front of the steer tube failed. The compression plug no where near the stress point.